Carbapenem-Resistant Acinetobacter baumannii: Virulence Factors, Molecular Epidemiology, and Latest Updates in Treatment Options

Abstract

Acinetobacter baumannii is a Gram-negative, non-motile pathogen commonly associated with healthcare settings. It is capable of causing severe infections, particularly in immunocompromised and critically ill individuals, and is linked to poor clinical outcomes. Infections caused by carbapenem-resistant A. baumannii (CRAB) represent a major public health concern due to limited treatment options and high resistance rates. Several virulence determinants contribute to CRAB’s pathogenicity, including capsular exopolysaccharide (CPS), lipopolysaccharide (LPS), lipooligosaccharide (LOS), efflux pumps, outer membrane proteins (OMPs), pili, metal acquisition systems, two-component regulatory systems (TCSs), and secretion systems (SSs). The dominant resistance mechanism in CRAB involves the production of carbapenemases, most notably oxacillinase-23 (OXA-23) and metallo-β-lactamases (MBLs) such as Verona integron-encoded MBL (VIM) and New Delhi MBL (NDM). Accurate identification of these resistance mechanisms is crucial for guiding effective antimicrobial therapy. Potential treatment options include older agents like polymyxins, ampicillin–sulbactam, high-dose carbapenems, tigecycline, and minocycline, along with newer antimicrobials such as eravacycline, cefiderocol, and aztreonam–avibactam. This review aims to explore the virulence mechanisms and molecular pathogenesis of CRAB, while also presenting recent developments in its epidemiology and available antimicrobial therapies.

1. Introduction

Acinetobacter baumannii is a Gram-negative, non-motile pathogen frequently encountered in healthcare environments, where it is responsible for severe infections, particularly in critically ill or immunocompromised patients [1]. Its presence in intensive care units (ICUs) is of particular concern, as carbapenem-resistant A. baumannii (CRAB) has been linked to higher rates of mortality and morbidity, as well as substantial healthcare costs [1,2,3]. CRAB emergence is primarily driven by cross-transmission, often resulting from lapses in infection control measures—such as poor hand hygiene—and extensive use of antimicrobials, which fosters selective pressure for resistant strains [4,5]. The 2024 World Health Organization (WHO) Bacterial Priority Pathogens List (BPPL) designates CRAB, third-generation cephalosporin-resistant Enterobacterales, carbapenem-resistant Enterobacterales (CRE), and rifampicin-resistant Mycobacterium tuberculosis as pathogens of critical priority [6]. The burden of CRAB is especially severe in endemic areas [7], with Greece reporting the highest prevalence among EU countries, reaching a resistance rate of 95.3% in 2023 [8]. CRAB isolates are commonly categorized into three resistance levels: multidrug-resistant (MDR), defined as non-susceptibility to at least one agent in three antimicrobial classes; extensively drug-resistant (XDR), defined as non-susceptibility to at least one agent in all but two classes; and pandrug-resistant (PDR), characterized by non-susceptibility to all agents across all tested categories [9,10,11,12,13]. Recent studies show that most CRAB isolates remain susceptible to only one or two available antibiotics, placing them in the XDR category. In some cases, these strains further progress to PDR status, leaving no viable treatment options. The persistence and spread of XDR/PDR A. baumannii within healthcare environments are linked to elevated mortality rates and substantial infection control challenges, highlighting the urgent need for innovative and effective antimicrobial approaches [14,15,16,17]. Thus, a comprehensive understanding of both the resistance mechanisms at the molecular level and the epidemiological trends of CRAB is critical for developing and selecting effective treatment strategies [18]. Accordingly, this review aims to highlight the virulence traits of CRAB and provide updated insights into its molecular epidemiology and current therapeutic options.

2. Genomic Pool, Virulence Factors, and Molecular Pathogenesis

The genome of A. baumannii consists of a core (persistent) component, present in 95–99% of strains, which encodes essential functions such as cell division, metabolism, and energy production [19,20,21]. In contrast, the accessory (shell) genome, found in 15–35% of isolates, contains genes related to virulence and antimicrobial resistance, often acquired via mobile genetic elements (MGEs) like plasmids, transposons (Tns), and integrons (Ints) [20,21,22,23,24]. CRAB exhibits an open pan-genome, enriched with insertion sequences (ISs), Ints, and Tns embedded in genomic islands (GIs), facilitating the acquisition of new genes [25,26]. Antimicrobial Resistance Genes (ARGs) are distributed across both the core and shell genome, frequently mobilized by ISs and other elements through integrase and transposase activity [11,12,21,24,27,28].

Notably, ISAba-type ISs, approximately 1 kb in size, play a central role in activating and disseminating carbapenemase genes, significantly contributing to the genomic plasticity and multidrug resistance seen in CRAB strains [21,24,26,29,30].

The development of antimicrobial resistance in A. baumannii and CRAB strains is largely driven by horizontal gene transfer (HGT), alongside notable contributions from mutations and the MGEs, particularly those of the ISAba family. Such genomic changes can influence the expression of constitutive genes involved in enzyme synthesis, efflux pump function, and outer membrane protein (OMP) modification, thereby equipping the bacteria with resistance to a wide spectrum of antimicrobial agents [21,24,31].

The GI AbaR1 plays a pivotal role in acquiring a wide array of resistance determinants through its multiple class 1 Ints in A. baumannii and CRAB strains. These Ints carry gene arrays capable of expressing a diversity of effective enzymes, e.g., β-lactamases, aminoglycoside-modifying enzymes, and efflux systems. Expression of these enzymes results in the appearance of resistance in CRAB superbugs to not only numerous antimicrobial agents (such as sulfonamides, chloramphenicol, trimethoprim, etc.) but also a variety of heavy metals and some other chemicals. In addition to AbaR1, the proliferation of ARGs in A. baumannii and CRAB strains is further facilitated by plasmid-mediated HGT [24,31,32].

Plasmids serve as major mediators of HGT, playing a central role in the dissemination of carbapenem-resistant genes (CRGs), particularly within CRAB populations. It is known that the bla_OXA-58 gene is frequently plasmid-borne, and studies have reported instances where bla_NDM-1 and bla_OXA-58 coexist on the same large plasmid [31,33,34]. Although uncommon in A. baumannii, metallo-β-lactamase (MBL) genes—such as bla_NDM, bla_VIM, and bla_IMP—as well as Class A β-lactamase genes—such as bla_KPC and bla_GES—have been identified on plasmids. These plasmid-borne genes have been reported in A. baumannii, e.g., CRAB strains [35]. In a study, Ramoul et al. documented the co-existence of bla_NDM-1 alongside bla_OXA-23 or bla_OXA-58 in clinical A. baumannii isolates obtained from northeastern Algeria [31,36].

In a comparative analysis, Wang et al. [37] investigated the MGEs of two strains comprising 2023-AB023 and 2023-AB033. Strain 2023-AB023 harbored six plasmids, while strain 2023-AB033 carried a subset of four of these. At the chromosomal level, both strains shared five common MGEs, e.g., ISAba8, ISAlw1, ISAba21, ISAba26, and ISAba14.

They [37] also found that the ARGs of bla_OXA-23 and bla_NDM-1 were located on separate plasmids in each strain, while all other ARGs were situated on the chromosome. In addition, the bla_NDM-1-carrying plasmid was linked to the ISAba125, which encodes the IS3 family transposase ISAba45.

CRAB expresses a wide array of virulence factors, categorized into nine major types: capsular polysaccharide (CPS), lipopolysaccharide (LPS), lipooligosaccharide (LOS), efflux systems, OMPs, pili, metal ion acquisition systems, two-component regulatory systems (TCSs), and secretion systems (SSs) [12,21,38]. Most of the genes responsible for these virulence traits are disseminated via MGEs—such as ISs, Tns, and Ints—frequently carried on plasmids [39,40].

2.1. Capsular Exopolysaccharide (CPS)

The high-molecular-weight polysaccharide capsule is essential for immune evasion and environmental persistence. It supports growth in ascitic fluid, resists complement killing, and is critical for virulence in pulmonary infection models. A. baumannii exhibits phase variation between highly capsulated (virulent) and low-capsulated (avirulent) forms. This switch is regulated by TetR-family transcription factors, affecting susceptibility to antimicrobials, lysozyme, disinfectants, and oxidative stress. Genes like wzc and small RNAs such as SrvS influence this phenotypic shift [41,42,43,44].

The capsular type (K-type) is a key determinant of A. baumannii and CRAB virulence, as the capsule provides vital defense against environmental challenges, nutrient limitation, and host immune responses. Driven by its crucial protective role and intense selective pressures, the capsule exhibits remarkable structural diversity, with over 200 distinct K loci identified to date [45,46,47,48]. Unlike KL2, the most prevalent K-type worldwide, many other K-types display regional distribution patterns. For instance, KL2 and KL7 have been reported in CRAB strains from Portugal, KL7 and KL28 in China, and KL9 in Canada. In Taiwan, a wider diversity has been observed, including KL2, KL10, KL14, KL22, and KL52 [46,48,49,50,51].

2.2. Lipopolysaccharide (LPS)

LPS is a key outer membrane component of Gram-negative bacteria, consisting of lipid A, a core oligosaccharide, and the O-antigen. In A. baumannii, LPS contributes to immune evasion and triggers host inflammatory responses [52,53,54,55,56,57]. Lipid A, the toxic portion, anchors the LPS and modulates membrane permeability and antibiotic resistance via enzymatic modifications during biosynthesis. Its hepta-acylated form is predominant in A. baumannii, supporting membrane integrity through divalent cation bridges and negative charge interactions. Specific sugar compositions and structural variations, such as mutations in lipid A biosynthesis genes (e.g., lpxABCD), have been linked to resistance against polymyxins [38,55,58].

2.3. Lipooligosaccharide (LOS)

A. baumannii expresses various surface structures—including adhesins, capsular polysaccharides, glycoproteins, and LOS—that contribute to its virulence. Unlike many Gram-negative bacteria, A. baumannii lacks O-antigen and instead produces LOS with a hepta-acylated lipid A linked to an oligosaccharide core. This structure triggers innate immune responses via Toll-like receptor 4 (TLR4) and enhances resistance to antimicrobial peptides and desiccation. Under cold stress, the bacterium modifies lipid A using the LpxS enzyme to maintain membrane fluidity [41,59,60,61].

Wang et al. [37] found that all CRAB strains they examined possessed a core set of nine virulence genes, which are crucial for adherence (ompA), drug efflux (AdeFGH), and iron sequestration (barA). A notable exception was a group of six strains involving 2019-AB015, 2019-AB016, 2019-AB019, 2019-AB055, 2023-AB023, and 2019-AB023, which were missing the biofilm gene bap and the quorum-sensing genes abaI/R. [37]. The isolates showed significant genetic diversity, with 12 different KL types and three lipooligosaccharide outer core (OCL) types. The most common combinations were KL2-OCL1, KL9-OCL1, and KL3-OCL1. Interestingly, the KL30 locus was present only in the isolates collected in 2023.

2.4. Efflux Pumps

A. baumannii employs broad-spectrum resistance–nodulation–division (RND) efflux pumps to expel diverse antimicrobials. The main characterized systems—AdeABC, AdeFGH, and AdeIJK—consist of an inner membrane transporter, a membrane fusion protein, and an outer membrane channel. The AdeABC pump, regulated by the adeRS two-component system, is strongly associated with carbapenem resistance, especially when adeA, adeB, and adeC are overexpressed. Notably, carbapenem resistance can occur even in the absence of regulatory mutations, suggesting multifactorial gene interactions. Synergism between AdeABC overexpression and carbapenemase (e.g., OXA enzymes) further amplifies resistance [62,63,64,65,66,67,68].

2.5. Outer Membrane Proteins (OMPs)

OMPs are crucial for A. baumannii pathogenicity and antibiotic resistance. Among them, OmpA is the most dominant, aiding in drug resistance, adherence to epithelial cells, and biofilm formation. It interacts with host fibronectin to facilitate cell attachment and invasion, while also contributing to immune evasion, serum resistance, apoptosis induction, and outer membrane vesicle (OMV) production. OmpA’s role in modulating host immune responses, particularly by promoting Th1 differentiation via dendritic cell activation, further enhances the bacterium’s persistence and virulence [69,70,71,72,73,74,75,76].

Indeed, OMVs play multifaceted roles in bacterial physiology, with a significant contribution to cellular detoxification. By facilitating the removal of toxic periplasmic substances, OMVs alleviate envelope stress and support bacterial fitness. This detoxification capability has been observed, for example, in non-native bacterial hosts expressing Class B β-lactamases such as VIM-2 and SPM-1 [77,78].

2.6. Pili

A. baumannii’s persistence in healthcare settings is largely due to its strong adhesion capacity. Adhesion to host or abiotic surfaces is a crucial first step in infection. This process is facilitated by surface structures like pili and fimbriae, which enable firm attachment and biofilm formation. Fimbrial adhesins promote motility and surface colonization, while non-fimbrial adhesins bind host extracellular matrix components like fibronectin and collagen. These structures also aid in plasmid transfer, contributing to the spread of antibiotic resistance [79,80,81,82,83,84,85,86,87].

Adhesins are key virulence factors that help bacteria resist mechanical stress from bodily fluids. In A. baumannii, they include pili proteins, autotransporters, and other surface enzymes, each with distinct receptor affinities. Their interactions with host molecules—such as integrins, selectins, and TLRs—trigger immune responses and enable bacterial survival, inflammation, and biofilm stability. Targeting these adhesins could support new infection-control strategies [79,88,89,90,91,92,93]. All in all, A. baumannii possesses various types of pili, including chaperone-usher (CUP), curli, type IV, type V, and conjugative type IV secretion pili. Among them, CUP pili are the most common and play a key role in surface attachment during infection [10,87,94,95,96].

2.7. Metal Ion Uptake Systems

A. baumannii uses siderophores like acinetobactin to acquire iron under host-imposed limitations. Transporters BauA and BauE import iron–siderophore complexes, with Oxa binding iron directly and Isox aiding under stress. Genes like bauB and bauC drive siderophore synthesis, while the Fur regulator suppresses this process in iron-rich conditions to maintain homeostasis [97,98,99,100].

Calprotectin, released by neutrophils, restricts zinc and manganese during infection, indirectly exacerbating A. baumannii’s iron limitation. Alongside host iron-binding proteins like transferrin and lactoferrin, this creates a nutrient-starved environment. In response, A. baumannii enhances siderophore production and activates metal transporters and enzymes like ZrlA to sustain survival under metal stress [97,101,102,103]. Moreover, zinc is vital for A. baumannii, particularly for activating enzymes like MBLs. Overexpression of znuABC enhances zinc uptake from the environment, supporting bacterial survival and resistance [104].

2.8. Two-Component Systems (TCSs)

TCSs consist of a membrane-bound sensor and a cytoplasmic response regulator. They enable bacteria to adapt to changing environmental conditions. Acinetobacter baumannii possesses up to 20 distinct TCSs, which play key roles in its resistance to antibiotics, virulence, and pathogenicity by modulating gene expression in response to environmental stimuli [38,105]. TCSs like BfmRS, PmrAB, and GacSA, along with stringent response regulators such as ppGpp and DksA, play crucial roles in controlling virulence-related genes in Acinetobacter baumannii. The expression of these genes and associated virulence factors is highly complex. Notably, PmrAB and BfmRS influence the production of outer membrane vesicles (OMVs), and the levels of AbOmpA within these OMVs can fluctuate based on culture conditions or regulatory input from TCSs [74,106,107,108,109,110].

2.9. Secretion Systems (SSs)

Although six secretion systems are well-known in Gram-negative bacteria, their functions in A. baumannii are still under investigation. These systems contribute to virulence and antibiotic resistance. With growing resistance, carbapenems are becoming less effective against MDR strains. As a result, CRAB is a WHO-designated critical threat. Targeting secretion systems may offer new therapeutic and vaccine strategies against drug-resistant A. baumannii [21,111,112].

3. Antimicrobial Resistance Mechanisms in CRAB

Multiple strategies are employed to resist antimicrobial agents (Figure 1), including:

(1)

The production of enzymes like β-lactamases and aminoglycoside-modifying enzymes, which are normally acquired [113,114,115].

(2)

Reduced cell membrane permeability due to the loss of OMPs or porins, which are normally acquired [115,116,117].

(3)

Overexpression/mutation of efflux pumps, belonging to six known multidrug resistance (MDR) families, MATE, ABC, PACE, MFS, SMR, and RND, which are normally acquired [115,118].

(4)

Structural modifications at antibiotic target sites, which are normally acquired [115,119].

3.1. Ambler Classification of β-Lactamases

Although β-lactamases were initially classified by Bush et al. [120] based on their substrate and inhibitor profiles, the molecular classification proposed by Ambler is now more widely accepted. Ambler’s system relies on amino acid sequence analysis to establish a molecular phylogeny and divides β-lactamases into four classes: A, B, C, and D [121,122].

Class A includes serine-based β-lactamases such as SHV (sulfhydryl variable), TEM (temoniera), CTX-M (cefotaxime-hydrolyzing), PER (Pseudomonas extended-resistant), GES (Guiana extended-spectrum), VEB (Vietnamese extended-spectrum), IBC (integron-borne cephalosporinase), SFO (from Serratia fonticola), BES (Brazil extended-spectrum), BEL (Belgium extended-spectrum), and TLA (Tlahuicas Indians). Some, like SHV, TEM, and CTX-M, can function as extended-spectrum β-lactamases (ESBLs) following minor genetic changes. This class also includes inhibitor-resistant variants such as IRTs (inhibitor-resistant TEMs), IRSs (inhibitor-resistant SHVs), and KPCs (Klebsiella pneumoniae carbapenemases) [122,123,124,125]. Notably, KPC-producing strains serve as long-term reservoirs, facilitating the spread of MGEs carrying the bla_KPC gene [126].

Class B β-lactamases, known as MBLs, are carbapenemases that can hydrolyze nearly all β-lactams except aztreonam. Their enzymatic activity depends on zinc ions (Zn²⁺). The most common MBLs include Verona integron-encoded MBL (VIM) and New Delhi MBL (NDM), followed by others such as imipenemase (IMP), German imipenemase (GIM), São Paulo MBL (SPM), Seoul imipenemase (SIM), Australian imipenemase (AIM), Dutch imipenemase (DIM), and the more recently identified Tripoli MBL (TMB) and Florence imipenemase (FIM) [122,127,128,129,130,131,132,133,134,135]. MBLs are further categorized into three subgroups, B1, B2, and B3 [121,122].

Class C β-lactamases, also referred to as cephalosporinases or AmpC β-lactamases, are serine-based enzymes capable of hydrolyzing a wide range of β-lactam antibiotics. Their expression can be either constitutive or inducible, and they may be encoded chromosomally or on plasmids—plasmid-encoded variants are commonly known as AmpC-like β-lactamases. Based on Jacoby’s classification, these enzymes are divided into specific subclasses according to their molecular structure and functional properties [122,136]. At present, ADC-68 is the sole Class C β-lactamase identified in A. baumannii that has been experimentally verified to possess carbapenemase activity [115,122,137].

Class D β-lactamases, known as oxacillinases (OXA), are also serine-based enzymes. Their hydrolytic activity varies by subtype, with certain subtypes functioning as carbapenemases and contributing to broad-spectrum resistance and multidrug-resistant phenotypes. Most OXA-type carbapenemases have been identified in CRAB, including the recently emerging OXA-48 variant [122,138,139,140]. The majority of OXA-type carbapenemase genes are known as acquired carbapenemase genes (ACGs) in CRAB strains [115,122].

A. baumannii naturally harbors the bla_OXA-51 gene, which typically exhibits low carbapenemase activity. However, its expression can be markedly enhanced by the presence of upstream IS elements [115].

According to an in silico analysis conducted by Capodimonte et al. [78] in a Dry Lab [141], enzymes belonging to Classes A, B, and C exhibited a very low incidence of lipobox sequences. Conversely, Class D β-lactamases, primarily composed of OXA types, showed a significantly higher prevalence, with roughly 60% possessing a lipobox sequence, leading to the inference of their likely function as membrane-associated lipoproteins. Indeed, these predicted OXA β-lactamase lipoproteins were exclusively detected within Acinetobacter spp. [78]. Moreover, Capodimonte et al. [78] demonstrated that OXA β-lactamases are membrane-associated, a characteristic that facilitates their incorporation into OMVs, akin to NDM-1. This tendency is further supported by predictions indicating that the major carbapenem-hydrolyzing Class D β-lactamases (CHDLs)—including the chromosomally encoded OXA-51-like and the acquired OXA-23-like, OXA-58-like, and OXA-24/40-like variants—function as lipoproteins [78]. Furthermore, according to Capodimonte et al. [78], soluble OXA enzymes are not expected to exist in Acinetobacter spp. While lipidated Class A and B enzymes are prevalent across many bacterial hosts, the direct association of protein lipidation with a specific host like Acinetobacter is a characteristic unique to Class D enzymes [78].

Migliaccio et al. [142] conducted a phylogenomic study involving 837 Acinetobacter isolates representing 72 different species. Using the Pasteur Multilocus Sequence Typing (MLST) scheme, they analyzed phylogenetic relationships and compared them with genome-based and ribosomal MLST (rMLST) phylogenies within the A. baumannii group. Additionally, the study aimed to identify ARGs across the Acinetobacter genomes [142]. ARGs associated with at least three different antibiotic classes were detected in 91 isolates spanning 17 distinct Acinetobacter spp. Moreover, a class D oxacillinase—an enzyme inherently found in various Acinetobacter spp.—was detected in 503 isolates spanning 35 different species within the genus [142]. Moreover, Migliaccio et al. identified class D oxacillinases in 503 isolates across 35 Acinetobacter spp. A total of 94 class D β-lactamase genes from 11 bla_OXA families—including bla_{OXA-211, -134, -214, -294, -51, -213, -274, -286, -58, -40}, and _-23—were detected. Within the A. baumannii group, A. baumannii and A. pittii carried intrinsic bla_OXA-51 and bla_OXA-213, respectively, while A. nosocomialis and A. seifertii lacked intrinsic class D β-lactamases.

Carbapenem resistance in A. baumannii has increased globally, largely due to acquired class D β-lactamases—bla_OXA-23, bla_OXA-40, and bla_OXA-58. Among them, bla_OXA-23, originally identified in Scotland and likely derived from A. radioresistens, is the most widespread. Outbreaks of OXA-23-producing strains have been reported in various countries, from Eastern Europe and West Asia to Southeast Asia and South America [143,144,145,146].

The spread of bla_OXA-23 is linked to Tns, e.g., Tn2006, Tn2007, and Tn2008. Tn2006 includes two ISAba1 elements, while Tn2008 has one. Tn2007 carries a single ISAba4. In some strains reported from the Arab states of the Persian Gulf region, the gene is related to one ISAba1 [143,147,148,149,150,151,152].

According to Papadopoulou et al.’s report [17], all examined A. baumannii isolates possessed an intrinsic, chromosomally located bla_OXA-51-like gene. In 97.7% of the isolates, a bla_OXA-23-like gene was present. Those isolates harboring bla_OXA-23-like demonstrated resistance to carbapenems. In these strains, an ISAba1 element was situated upstream of the gene’s promoter region, a genetic arrangement that may enhance OXA-23 expression [17,153,154].

Accordingly, Mugnier et al. [143] investigated factors contributing to the global spread of the bla_OXA-23 gene in A. baumannii. They analyzed 20 OXA-23-producing CRAB strains from various regions (including Asia and Europe) using pulsed-field gel electrophoresis and MLST. Eight distinct sequence types were identified, including four novel ones. The majority of isolates were associated with two dominant European clonal lineages [143].

3.2. Reduced Cell Membrane Permeability Due to the Loss of OMPs

OMP loss contributes secondarily to the development of CRAB. Studies have linked the loss of the CarO porin to imipenem resistance [155,156]. Additionally, overexpression of iron-regulated OMPs under iron-limiting conditions may further enhance carbapenem resistance [157]. The connection between the absence of the 29-kilodalton Omp and reduced susceptibility to imipenem has also been recognized for years [158].

3.3. Overexpression of Efflux Pumps

Efflux pump overexpression also plays a secondary role in the emergence of CRAB. Mutant-based experimental models have highlighted the significant contribution of the AdeABC efflux pump to carbapenem resistance [159]. Additionally, efflux pumps belonging to the RND family have been linked to imipenem resistance in A. baumannii [160].

3.4. Structural Modifications at Antibiotic Target Sites

Scientific evidence on target site modifications contributing to antimicrobial resistance in A. baumannii remains limited. The most commonly reported mechanism involves reduced binding affinity of penicillin-binding protein 2 (PBP2) for imipenem [161]. These alterations are typically caused by genetic mutations at specific loci or by post-transcriptional modifications of certain proteins [162].

4. Trends in Molecular Epidemiology

The earliest reports of CRAB emerged as isolated cases several years ago [163,164]. Initially, the carbapenem resistance was linked to MBLs and OXA-type enzymes [164]. However, in the early stages of CRAB’s genetic evolution, some strains exhibited low-level resistance to carbapenems despite the absence of detectable carbapenemase production [165].

4.1. OXA-Type Carbapenemases

The chromosomally encoded bla_OXA-51-like gene, first identified in A. baumannii in the early 1990s, is considered intrinsic to this species and is not inherently linked to carbapenem resistance. However, overexpression of this gene—often driven by upstream ISs such as ISAba1 and, less commonly, ISAba9 or ISAba19—can lead to low-level resistance [166,167,168,169]. Variants like OXA-65, OXA-66, and OXA-69, also part of the OXA-51 family, are commonly detected and possess carbapenemase activity [170]. Over the years, numerous other bla_OXA-51-like alleles such as bla_OXA-64, bla_OXA-94, bla_OXA-365, bla_OXA-68, bla_OXA-90, bla_OXA-132, bla_OXA-79, bla_OXA-82, bla_OXA-92, and bla_OXA-131 have emerged [171,172,173], along with the recently identified bla_OXA-1117 and bla_OXA-1118 [173]. Although this gene was once widely used as a diagnostic marker for A. baumannii, it is no longer considered sufficient for accurate species identification [169].

Until around 2010, OXA-58 was the dominant OXA-type carbapenemase in CRAB isolates, largely due to its higher enzymatic activity compared to intrinsic OXA-51. The bla_OXA-58-like gene is typically flanked by two ISAba3-like elements, a structure first described in France [170,174,175,176,177].

OXA-23, although initially detected earlier, gained prominence after 2010 and gradually replaced OXA-58 as the most common carbapenemase in CRAB due to its superior hydrolytic efficiency [178,179,180,181]. The bla_OXA-23-like gene remains dominant and is often found adjacent to ISAba1. In some endemic regions, strains co-harboring both bla_OXA-23-like and bla_OXA-58-like have been reported [182,183,184,185,186,187]. The OXA-24 variant—sharing approximately 60% identity with OXA-23—is more frequently seen in Spain, Portugal, and China [188,189,190].

Domingues et al. [48] reported a temporal shift in the prevalence of bla_OXA-like genes, with bla_OXA-40-like detected in 2005, subsequently replaced by bla_OXA-23 between 2006 and 2019. This trend reflects the broader historical pattern in Portugal, where bla_OXA-58 was predominant from 2002 to 2004, followed by bla_OXA-40 (2002–2006), and later bla_OXA-23 from 2006 onward. These shifts highlight the central role of Class D β-lactamases as the predominant carbapenemases in CRAB clinical isolates—a pattern also observed in reports from the United States and multiple Asian healthcare settings [46,48,189,191].

Mavroidi et al. [192] documented the emergence of A. baumannii isolates belonging to sequence type 3LST ST101, producing bla_OXA-23 and classified as international clone II, which exhibited resistance to both tigecycline and colistin in a Greek hospital over a three-year span. According to the authors’ report [192], this represented the first reported occurrence of such isolates in Greece. The identification of two bla_OXA-23-producing strains resistant to both agents is particularly alarming, highlighting the organism’s expanding ability to develop diverse antibiotic resistance mechanisms [17,192]. CRAB recruits intrinsic OXA-51-like enzymes as well as acquired forms such as OXA-23-like, OXA-40-like, and OXA-58-like for most strains worldwide [48].

The OXA-23 gene, a prominent member of the OXA family, is recognized as the most prevalent carbapenem resistance mechanism worldwide [45,48].

Other notable OXA-type carbapenemases include OXA-49 and OXA-73 (prevalent in China), and OXA-143 and OXA-231, primarily reported in Brazil [166].

A. baumannii isolates like CRAB strains display a complex resistance profile, with OXA-type carbapenemases representing the primary mechanism underlying resistance to carbapenems and other β-lactams. Additionally, target site modifications and efflux pump activity often contribute to resistance against key therapeutic agents, including polymyxins, cefiderocol, and other β-lactams [31].

4.2. Metallo-β-Lactamases (MBLs)

MBLs, particularly VIM enzymes, are the most frequently detected among CRAB strains. In endemic regions such as Greece, VIM-1 and VIM-4 are the most common [193,194,195,196], whereas VIM-2 predominates in South Korea [197,198]. The bla_VIM genes are consistently integrated within class 1 Ints [193,194]. Although rare, other variants like VIM-35 have also been identified [199]. Additional MBLs, including IMP-1 and SIM-1, have been reported in countries such as South Korea, Iran, and Morocco [131,200,201,202,203]. SPM-producing CRAB strains are also notably present in Morocco [203,204]. Recently, strains co-expressing VIM-2 and OXA-23 were identified in Tunisia [205].

Although VIM-producing CRAB strains have been predominant, the global emergence of NDM-1 and NDM-2 producers began after 2012 [206,207,208]. NDM-1, in particular, has spread extensively, leading to multiple severe outbreaks over the following years and remains widespread today [209,210]. The bla_NDM-1 gene is typically preceded by the insertion sequence ISAba125 [211,212]. Additional NDM variants have also been reported, including NDM-6 in Spain [213], NDM-9 in France (2023) [214], and NDM-5 in Thailand during the same year [215]. CRAB strains co-producing NDM-1 and OXA-58 have been identified [35,216], though the most commonly observed pattern is the coexistence of NDM-1 and OXA-23 in the same isolates [217,218,219,220,221].

In a study by Wang et al. [37], the resistome of 125 CRAB isolates was analyzed, revealing 47 unique ARGs. These genes conferred resistance to four major antibiotic classes of aminoglycosides, sulfonamides, tetracyclines, and β-lactams. A key finding was the presence of carbapenemases in all CRAB isolates.

Wang et al. [37] found that 26 ARGs were highly prevalent, being detected in more than half of the strains. The most common were AdeFGH, bla_OXA-66, and bla_OXA-23, respectively. Temporal patterns such as bla_OXA-51, bla_OXA-217, and bla_OXA-374 were found only in 2019 isolates, while bla_NDM-1 and bla_OXA-91 were unique to 2023 isolates. No resistance genes of mcr-1 (colistin) or tet(X) (tigecycline) were detected by Wang et al. [37]. However, two isolates from 2023 (e.g., 2023-AB023 and 2023-AB033) were identified by Wang et al. [37], harboring two different carbapenemase genes and a total of seven resistance genes, including bla_OXA-23, bla_OXA-91, and bla_NDM-1.

4.3. Klebsiella pneumoniae Carbapenemases (KPCs)

KPC-type carbapenemases are relatively uncommon in CRAB isolates. The bla_KPC gene was first identified in A. baumannii strains from Puerto Rico in 2009, involving variants such as KPC-2, KPC-3, KPC-4, and KPC-10 [11,222]. However, its presence remained largely confined to that region in the following years [223]. Since then, only sporadic cases have been reported globally—for instance, a KPC-3-producing CRAB isolate in Portugal [224] and a recent case from Tunisia involving a strain co-producing OXA-23 and KPC-2 [225].

4.4. Sequence Types (STs)

The emergence of resistant Acinetobacter calcoaceticus–baumannii (Acb) strains has prompted widespread epidemiological research. MLST is now the standard tool due to its strong reproducibility, global consistency, and ability to trace evolutionary patterns. As an advantage, its use of standardized ST nomenclatures enables effective tracking of major clonal lineages [166,226,227]. In A. baumannii, MLST’s effectiveness is limited by the existence of two main schemes: Oxford and Pasteur. Though both classify similar strains, comparing their accuracy, resolution, and phylogenetic consistency remains necessary [226,228,229,230]. Molecular studies have identified major clonal lineages in A. baumannii, now known globally as International Clones (IC) I–III. In the Pasteur MLST scheme, these align with clonal complexes CC1 (ST1), CC2 (ST2), and CC3 (ST3). Other common sequence types include ST10, ST15, ST25, ST32, ST78, and ST79 [166,171,226,230,231,232,233]. The Oxford scheme provides higher resolution but is prone to recombination and technical issues, especially in the gpi gene, part of the capsule operon. As gpi affects capsule type and virulence, some studies recommend using this scheme for tracking capsular variation [226,234,235,236,237].

The updated epidemiology of A. baumannii sequence types, isolates, and genomes is available on the AcinetobacterPubMLST website (https://pubmlst.org/organisms/acinetobacter-baumannii, accessed on 27 June 2025) [238].

In detail, 2779 Pasteur STs and 3415 Oxford STs are available at https://pubmlst.org/bigsdb?db=pubmlst_abaumannii_seqdef&page=query (accessed on 27 June 2025). Also, 12570 genomes assigned to validated Acinetobacter spp. are available at https://pubmlst.org/bigsdb?db=pubmlst_abaumannii_isolates&page=query&genomes=1 (accessed on 27 June 2025) [238].

Before ST2 became dominant, ST1 was the most common sequence type [239]. The rise of ST2 has been linked to the spread of OXA-23-producing CRAB [178]. VIM-1 and VIM-4 enzymes have been associated with ST2 and ST1, respectively [166]. International Clones IC1 and IC2 correspond to ST1 and ST2, including their single-locus variants [240].

NDM-1-producing CRAB strains are typically assigned to ST25 [241]. Less frequently reported types include ST1407, ST164, and ST85 [242,243,244,245]. Other NDM variants have been linked to various sequence types: NDM-9 with ST25, NDM-5 with ST19, and NDM-2 with ST103 [214,246,247]. In recent years, a concerning trend has emerged with CRAB strains co-producing NDM-1 and OXA-23, most commonly associated with ST2 [211,221], though ST1 has also been identified in some cases [220].

5. Trends in Antimicrobial Treatment

A range of older and newly developed antimicrobials show potential as treatment options for CRAB infections. However, some of the recently introduced agents lack activity against these resistant strains [11,248,249].

5.1. Colistin—Polymyxins

Colistin (PubChem CID: 44144393, Figure 2a) is an old antimicrobial discovered in 1949 and classified as a polymyxin (specifically polymyxin E). Its clinical use was largely discontinued in the 1980s due to severe side effects, particularly nephrotoxicity [250].

The resurgence of CRAB and other MDR bacteria in recent years has led to the renewed use of colistin as a key therapeutic option [263].

Both colistin and polymyxin B have been employed to treat hospital-acquired pneumonia (HAP), ventilator-associated pneumonia (VAP), and bloodstream infections caused by CRAB [249]. However, colistin has been associated with a higher risk of nephrotoxicity compared to polymyxin B [264].

There are different reports from different studies. In an in vitro study performed by Vardakas et al. [265], colistin and other polymyxins have shown strong synergistic effects when combined with carbapenems, rifampicin, or vancomycin, which may contribute to improved survival outcomes [265]. Some clinical studies have found no significant difference in treatment failure rates between colistin monotherapy and combination therapies such as colistin–meropenem or colistin–rifampicin [266,267], while in an investigation conducted by Dickstein et al. [268], they studied 266 A. baumannii cases and found lower adjusted mortality in colistin-resistant infections compared to colistin-susceptible ones. Colistin-resistant patients had better baseline health and needed less ventilation [268]. Among colistin-resistant cases, colistin–meropenem therapy led to higher mortality than colistin alone [268]. Colistin resistance, based on broth microdilution (BMD) testing, was linked to improved outcomes, with monotherapy outperforming combination treatment [268].

A randomized controlled trial (RCT) comparing colistin with ampicillin–sulbactam for treating CRAB strains susceptible to ampicillin–sulbactam found no significant differences in mortality, clinical outcomes, or microbiological failure [269]. However, another study reported higher rates of all-cause mortality and microbiological failure with colistin. These findings suggest a possible advantage of ampicillin–sulbactam over polymyxins, though the overall quality of evidence remains low [270].

Colistin has demonstrated notable synergistic activity with levofloxacin in treating VAP caused by CRAB [271]. Given the emergence of colistin-resistant CRAB strains [192] and the risk of resistance development and treatment failure with colistin monotherapy [272,273], it is recommended that colistin be used as part of combination therapy for severe CRAB infections [274].

5.2. Ampicillin–Sulbactam

Ampicillin–sulbactam (PubChem CID: 18541918, Figure 2b) is another potential option for treating HAP/VAP caused by CRAB, although the supporting evidence is limited or conditional [249]. As aforementioned, it has shown a strong synergistic effect when combined with levofloxacin in managing CRAB-related VAP [271]. In the treatment of A. baumannii infections, the antibacterial activity of ampicillin–sulbactam is primarily attributed to its sulbactam component, with ampicillin functioning mainly as a carrier. Based on this understanding, the 2023 guidelines designated ampicillin–sulbactam (6–9 g of sulbactam daily), in combination with at least one other agent, as the preferred therapy for CRAB [275]. This recommendation applied regardless of in vitro susceptibility profiles, due to concerns over potential PBP saturation and testing inaccuracies. A significant shift occurred in 2024, when sulbactam–durlobactam, administered with a carbapenem, became the first-line treatment [275]. Ampicillin–sulbactam was subsequently reclassified as an alternative therapy, limited to the higher sulbactam dose of 9 g per day. Polymyxin B, minocycline, and cefiderocol are recommended as combination partners in such regimens. Differences between ESCMID and Infectious Diseases Society of America (IDSA) guidelines likely reflect the regional timing of sulbactam–durlobactam’s approval in 2023 across the United States and Europe [275]. Xacduro, a fixed-dose sulbactam–durlobactam therapy approved in the U.S., treats HAP and VAP caused by the A. baumannii–calcoaceticus complex. Sulbactam acts as the antibacterial agent, while durlobactam, a DBO beta-lactamase (classes A, C, and D) inhibitor, shields it from enzymatic breakdown. Though durlobactam lacks direct killing power, it boosts sulbactam’s efficacy. In vitro, the combo rivals colistin and outperforms several other antibiotics against CRAB. Clinically, it improves outcomes with fewer deaths and less kidney toxicity than colistin. Since its effect without carbapenems remains uncertain, the IDSA advises using it alongside imipenem–cilastatin or meropenem [115,276,277,278,279,280].

5.3. Tigecycline

Tigecycline (PubChem CID: 54686904, Figure 2c), a glycylcycline and derivative of minocycline, reaches high concentrations in the lungs, skin, soft tissues, and bones [281]. It demonstrates strong activity against CRAB strains; however, exposure to suboptimal tigecycline levels may promote resistance development [282].

Compared to colistin in treating CRAB pneumonia, tigecycline monotherapy has been associated with higher mortality and lower clinical response [283,284,285]. Conversely, one study reported better survival rates with tigecycline than colistin [286], though overall evidence remains inconclusive [249].

When assessed against ampicillin–sulbactam, tigecycline-based therapies showed lower clinical and microbiological failure rates [287]. In contrast, another study found significantly lower 28-day mortality with cefoperazone–sulbactam compared to tigecycline [288]. However, the evidence for the superiority of sulbactam-based regimens over tigecycline is still of low certainty [249].

5.4. Fosfomycin

Fosfomycin (PubChem CID: 446987, Figure 2d) is an older antibiotic that has been reintroduced, primarily for treating uncomplicated urinary tract infections (UTIs) caused by MDR uropathogens [87,96,289,290,291]. In the context of CRAB infections, it has been used in combination therapies for VAP and bacteremia. However, its clinical effectiveness in these settings requires further confirmation through larger, well-designed studies [292,293].

5.5. Plazomicin

Plazomicin (PubChem CID: 42613186, Figure 2e) is a synthetic aminoglycoside approved in 2018 for the treatment of complicated urinary tract infections (cUTIs) and pyelonephritis [248]. Early studies suggest it has some in vitro activity against CRAB [294]. Despite its enhanced potency compared to other aminoglycosides, plazomicin is not regarded as a first-line treatment for CRAB infections [249,295].

5.6. Eravacycline

Eravacycline (PubChem CID: 54726192, Figure 2f) is a fluorocycline that is two to eight times more potent than tigecycline against CRAB. While some clinical evidence supports its effectiveness, eravacycline is currently considered a last-resort treatment, and further research is needed to validate its clinical utility [249,296].

5.7. Cefiderocol

Cefiderocol (PubChem CID: 77843966, Figure 2g) is a novel siderophore cephalosporin approved by the FDA for the treatment of cUTIs in 2019 and VAP in 2020 [297]. It also received marketing authorization from the European Medicines Agency (EMA) for treating infections caused by aerobic MDR Gram-negative bacteria in adults with limited treatment options [298]. Cefiderocol utilizes siderophore-mediated transport to penetrate the bacterial outer membrane and accumulate in the periplasmic space, enabling it to inhibit a broad range of MDR bacteria regardless of the resistance mechanism [299].

Against CRAB, cefiderocol shows a minimum inhibitory concentration (MIC) of ≤2 μg/mL and has demonstrated a 70% survival rate in critically ill patients with bacteremia or VAP in one study [300]. However, the CREDIBLE-CR randomized controlled trial comparing cefiderocol with best available therapy for carbapenem-resistant Gram-negative infections reported a higher 28-day mortality rate among CRAB patients treated with cefiderocol (49%) compared to those receiving other treatments (18%) [301]. While cefiderocol has been used in managing severe CRAB outbreaks [210], emerging resistance poses a growing concern, with some studies linking cefiderocol resistance to clinical failure. As a result, cefiderocol monotherapy for CRAB is generally not recommended [302]. Overall, data on its efficacy against CRAB remain limited, and further research is required [249].

5.8. Temocillin

Temocillin (PubChem CID: 171758, Figure 2h) is a 6-α-methoxy derivative of ticarcillin introduced in the UK during the 1980s. While it exhibits activity against various Enterobacterales, it has no efficacy against A. baumannii or CRAB [248,303].

5.9. Ceftolozane–Tazobactam

Ceftolozane–tazobactam (PubChem CID: 172973390, Figure 2i) received FDA approval in 2014 for the treatment of cUTIs and intra-abdominal infections (IAIs), with its use later extended to VAP in 2019 [248]. In 2022, approval was further expanded to include pediatric patients (from birth to under 18 years) for cIAIs and cUTIs [304]. However, it shows limited in vivo activity against CRAB [305,306].

5.10. Imipenem/Cilastatin–Relebactam

Imipenem/cilastatin–relebactam (PubChem CID: --) was approved by the FDA in 2019 for the treatment of cUTIs and IAIs, and in 2020, its approval was extended to include VAP. It has also been authorized by the EMA [248]. Relebactam functions by inhibiting class A and C β-lactamases [307]. However, this combination is not considered effective against CRAB, as the addition of relebactam does not enhance imipenem’s in vitro activity against these strains [308,309].

5.11. Meropenem–Vaborbactam

Vaborbactam is a cyclic boronate β-lactamase inhibitor. When combined with meropenem, it is effective against KPC-producing multidrug-resistant Gram-negative bacteria and shows notable activity against OXA-48 producers. This combination, meropenem–vaborbactam (PubChem CID: 86298703, Figure 2j), is approved for the treatment of cUTIs, IAIs, and VAP [310,311]. However, it is not considered a viable option for CRAB, as vaborbactam does not enhance meropenem’s in vitro activity against these strains [308,309].

5.12. Ceftazidime–Avibactam

Avibactam is a non-β-lactam β-lactamase inhibitor with in vitro activity against Ambler class A and C β-lactamases and partial activity against certain OXA-type enzymes classified under Ambler class D. It was patented in 2011 [312]. Several RCTs have evaluated the safety and effectiveness of the ceftazidime–avibactam combination for treating cUTIs and cIAIs [313].

Ceftazidime–avibactam (PubChem CID: 90643431, Figure 2k) was approved by the FDA in 2015 for the treatment of cUTIs and, in combination with metronidazole, for cIAIs [314]. Its dosing was later reassessed for use in critically ill patients [315]. In 2018, the FDA extended its approval to include HAP/VAP, based on results from the pivotal Phase III REPROVE trial [316]. However, data on its efficacy against CRAB are limited, and it is therefore not routinely recommended for these infections [249,317].

5.13. Aztreonam–Avibactam

Aztreonam–avibactam (PubChem CID: --) is a combination antimicrobial agent effective against MDR bacteria producing MBLs [318]. It was approved by the EMA in 2024 for the treatment of infections such as cIAIs, cUTIs, and HAP/VAP [319]. In February 2025, the FDA approved it for treating cIAIs in adults with limited therapeutic options [320]. In the context of CRAB, aztreonam–avibactam is particularly effective against strains producing MBLs such as VIM or NDM [319,321,322].

6. Guidelines for the Treatment of CRAB Infections

According to the latest guidelines from the European Society of Clinical Microbiology and Infectious Diseases (ESCMID), ampicillin–sulbactam is conditionally recommended for patients with HAP/VAP caused by sulbactam-susceptible CRAB. Alternatives include polymyxins or high-dose tigecycline, provided in vitro activity is confirmed; however, the quality of evidence is low. For CRAB strains resistant to sulbactam, polymyxins or high-dose tigecycline may be considered if susceptibility is demonstrated, though no formal recommendation is provided. ESCMID also conditionally advises against the use of cefiderocol for CRAB infections, citing low-quality evidence for this guidance [249,323].

For patients with severe, high-risk CRAB infections, ESCMID conditionally recommends combination therapy using two antimicrobials with confirmed in vitro activity—such as polymyxins, aminoglycosides, tigecycline, or sulbactam-based regimens. However, this recommendation is based on low-quality evidence. Combination treatments involving polymyxin–meropenem or polymyxin–rifampin are strongly discouraged, supported by high-quality evidence. In cases where CRAB isolates have a meropenem MIC ≤ 8 mg/L, a carbapenem-based combination therapy using high-dose, extended-infusion meropenem is considered good clinical practice, although this guidance is based on expert opinion [249,274,275].

The IDSA recommends ampicillin–sulbactam as the first-line treatment for CRAB isolates susceptible to sulbactam. Alternative options include tetracyclines (such as eravacycline or minocycline), tigecycline, polymyxins, and cefiderocol. For sulbactam-resistant CRAB, IDSA provides no formal recommendation, but the use of high-dose ampicillin–sulbactam in combination with a second active agent may be considered [324,325,326].

For severe CRAB infections, IDSA strongly supports combination therapy using at least two active antimicrobials. High-dose ampicillin–sulbactam should serve as the core agent, combined with another drug such as a tetracycline, polymyxin, extended-infusion meropenem, or cefiderocol. Fosfomycin and rifampin are not recommended as part of combination regimens. Similarly, polymyxin–meropenem combinations should be avoided unless a third agent is included [274,327].

7. Conclusions

Pan-genomic analyses have revealed that A. baumannii has an open pan-genome, containing a wide array of virulence factors. Most of its virulence and ARGs are disseminated via MGEs, including Tns, ISs, Ints, and bacteriophages (Φs)—often transferred through plasmids via HGT. As a result, A. baumannii harbors a robust arsenal of resistance and virulence mechanisms, making CRAB infections a serious public health concern [12,21,24].

Understanding the specific mechanisms of antimicrobial resistance is crucial for selecting effective treatment options. Potential therapies include older agents such as polymyxins, ampicillin–sulbactam, high-dose carbapenems, tigecycline, and minocycline, as well as newer drugs like eravacycline, cefiderocol, and aztreonam–avibactam.