Search Results (66)

Background/Objectives: Multidrug-resistant (MDR) strains of Streptococcus suis are increasingly prevalent and present significant challenges in clinical management. Given that the development of new antibiotics is a resource-intensive process and time-consuming, there is an urgent need for alternative therapeutic strategies to address resistance in the short term. One promising approach is the use of combination therapy, which involves pairing potent antibiotics with agents that may be less effective on their own, to enhance therapeutic efficacy and potentially overcome resistance mechanisms. This study aimed to investigate the in vitro antibacterial activity of combining two classes of antibiotics with distinct mechanisms of action—cell wall synthesis inhibitors and protein synthesis inhibitors—against MDR S. suis strains isolated from diseased pigs. Methods: A total of 36 MDR S. suis strains were tested using a microbroth dilution checkerboard assay to determine the minimum inhibitory concentration (MIC) of four cell wall synthesis inhibitors —amoxicillin/clavulanic acid (AMC), ampicillin (AMP), penicillin G (PEN), and vancomycin (VAN)— in combination with four protein synthesis inhibitors —gentamicin (GEN), neomycin (NEO), tilmicosin (TMS), and tylosin (TYL). Time–kill curve assays were conducted to evaluate the in vitro bactericidal activity of synergistic antibiotic combinations (PEN–GEN and AMP–NEO) against Beta-lactam-resistant and Beta-lactam-susceptible MDR S. suis strains. Results: Checkerboard analysis revealed that penicillin-gentamicin combination exhibited the most effective synergistic activity against the MDR S. suis strains (10/19, 52.6%), with ∑FIC values of 0.25–1.06 and MIC reductions from resistant to susceptible levels. Time-kill assays further confirmed the synergistic bactericidal effect of the combination, demonstrating complete bacterial clearance within 6–9 h, markedly rapid bacterial killing compared to monotherapy. Conclusions: This study demonstrates that antibiotic combinations, particularly Beta-lactams combined with aminoglycosides, show synergistic activity against pig-isolated S. suis MDR strains. The PEN-GEN combination exhibited strong synergistic and bactericidal effects, supporting combination therapy as a potential strategy to address antimicrobial resistance. Further evaluation in diverse strain backgrounds and prudent antibiotic use are essential to confirm efficacy and limit the emergence of antibiotic resistance. Full article

Traditional macrolide antimicrobials are inhibitors of cytochrome P4503A (CYP3A) in cattle liver. Monensin (MON), an ionophore with a narrow safety margin, undergoes CYP3A-dependent O-demethylation, and its incompatibility with macrolides is well recognized in livestock animals. This study evaluated the effects of newer macrolides—tilmicosin (TIL), tulathromycin (TUL), and gamithromycin (GAM)—on CYP3A-dependent metabolism in bovine liver microsomes and examined how these drugs influence MON hepatic metabolism. Molecular docking studies were also performed to predict their interactions with CYP3A enzymes. The CYP3A-dependent enzyme activity, testosterone 6β-hydroxylase, was inhibited in the presence of triacetyl-oleandomycin (used as a reference macrolide), as well as with MON. None of the other macrolides tested affected this enzymatic activity. All macrolides inhibited MON metabolism, but the extent of inhibition observed with triacetyl-oleandomycin was higher than that produced by TIL, TUL, and GAM. Molecular docking analyses indicated that triacetyl-oleandomycin and MON exhibited the highest binding affinities for the active site of CYP3A isozymes, compared with TIL, TUL, and GAM. The agreement between enzymatic data and in silico predictions indicates that TIL, TUL, and GAM are weaker inhibitors of CYP3A-mediated MON metabolism. The modest reduction in MON hepatic metabolism caused by these macrolides—commonly used in cattle feedlots—suggests a low likelihood of clinically relevant drug–drug interactions under typical dosing conditions. Full article

Pradofloxacin is a third-generation dual enzyme targeting bactericidal veterinary fluoroquinolone, recently approved for use in cattle for bovine respiratory disease, which is active against Gram-positive/negative, atypical and anaerobic bacteria. We compared in vitro killing by pradofloxacin to that by ceftiofur, danofloxacin, enrofloxacin, florfenicol, marbofloxacin, tildipirosin, tilmicosin and tulathromycin against bovine isolates of Mannheimia haemolytica and Pasteurella multocida over a range of bacterial densities (10⁶–10⁹ cfu/mL). Drug concentrations used in the kill assays included the minimum inhibitory and mutant prevention drug concentrations and maximum serum and maximum tissue drug concentrations. Regardless of bacteria density tested and drug concentration used, pradofloxacin consistently killed as many or more (but not fewer) bacterial cells than any other drug tested against M. haemolytica strains. At the 10⁸–10⁹ cfu/mL densities, pradofloxacin killed 99–99.9%, 100% and 100% of bacterial cells at the MPC, maximum serum and maximum tissue drug concentrations, respectively, following 24 h of drug exposure. Indeed, pradofloxacin killed 99.9–99.99% of cells following 30–60 min of exposure to the maximum serum concentration. Similar trends were seen with killing of P. multocida strains by pradofloxacin. Against high-density bacterial populations, pradofloxacin was rapidly bactericidal and consistently killed more cells than the other agents tested. This manuscript represents the most comprehensive comparative in vitro kill study completed to date. Full article

Macrolides are crucial for the management and treatment of bovine respiratory disease (BRD). However, antimicrobial resistance (AMR) threatens the efficacy of these and other antimicrobials. We developed real-time recombinase polymerase amplification (RPA) assays targeting three clinically relevant macrolide antimicrobial resistance genes (ARGs)—msrE-mphE and erm42—in ≤30 min using extracted DNA. A set of 199 deep nasopharyngeal swabs (DNPS) collected from feedlot calves near the time of arrival were selected based on bacterial culture (BC) results for Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni and antimicrobial susceptibility testing (AST) for tulathromycin, tilmicosin, tildipirosin, or gamithromycin. Samples were also tested for the same targets using RPA and polymerase chain reaction (PCR). In samples that were culture-positive for one or more macrolide-resistant BRD-associated bacteria (n = 101), msrE-mphE and/or erm42 were detected in 95% of cases using RPA. The remaining 98 samples were either culture-negative, or the recovered bacteria were macrolide-susceptible: 43% of these were RPA-positive for at least one macrolide ARG. Together with BC-AST and PCR, Bayesian latent class modelling estimated the clinical sensitivity of RPA for macrolide ARGs to be 95% and specificity to be 58%, with moderate agreement between RPA and BC-AST (κ = 0.52) or PCR (κ = 0.55). The estimated sensitivity of the RPA multiplex assay for the targeted macrolide ARGs was very good, although estimated specificity was limited. However, Sanger sequencing confirmed RPA detection of msrE-mphE in BC-AST/PCR-negative samples (n = 23), reflecting the presence of this locus in non-target bacteria, as well as potential ARG variants among BRD bacteria. These findings support the potential of RPA for rapid ARG detection from extracted DNA. Continued assay optimization and evaluation for detection of respiratory bacteria and ARGs will further enhance its diagnostic utility. Full article

The widespread use of antimicrobial drugs (AMDs) in livestock production contributes to antimicrobial resistance (AMR), a global One Health concern affecting humans, animals, and the environment. This study analyzed AMD residues and the AMR profiles in Escherichia coli and Enterococcus spp./Streptococcus spp. (ES) isolated from lagoon water samples collected from nine California dairies. Antimicrobial susceptibility testing was performed using the microbroth dilution method, and enzyme-linked immunosorbent assay (ELISA) kits were used to detect AMD residues in lagoon water. Overall, residues of florfenicol and tilmicosin were detected in more than 90% of the samples, while tetracycline was detected in 74.2 ± 4.6% of the samples. In contrast, penicillin and sulfamethazone residues were low, observed in only 3.4 ± 1.9% and 32.3 ± 5.0% of samples, respectively. The very low prevalence of penicillin was likely due to limited use in dairy cattle, given its prolonged withdrawal period. Prevalence estimates for AMR in the lagoon samples showed 100% resistance of E. coli to tiamulin, tilmicosin or tylosin and high prevalence against florfenicol (96.0% ± 2.0) or gamithromycin (92.0% ± 1.9). However, low AMR estimates (less than 10%) were observed against other AMDs tested. Similarly, the prevalence estimates for AMR of ES isolates in the studied lagoon were high against florfenicol (95.1% ± 2.0), tildipirosin (97.6% ± 1.7), or tilmicosin (98.8% ± 1.2), but low against ampicillin (4.9% ± 1.9) and penicillin (8.5% ± 2.4). Despite numerical differences in AMR prevalence by season, region, and sampling point, these variations were not statistically significant. Logistic regression models were applied to explore associations between AMD residues and AMR phenotypes where appropriate. Tilmicosin residues were significantly associated with reduced resistance to danofloxacin, enrofloxacin, and tildipirosin in E. coli isolates, while sulfamethoxazole residues were linked to increased tetracycline resistance in Enterococcus spp. The presence of florfenicol residues, potentially originating from treated calves and heifers, helps explain the high prevalence of resistance to this drug in both bacterial species. However, not all AMD residues were associated with AMR, underscoring the complex ecological and genetic factors involved in the development and maintenance of resistance in dairy environments. These findings underscore the importance of integrating AMR surveillance and prudent AMD use practices across all segments of dairy production systems. Full article

The impact of varying antibiotic residue levels on antibiotic resistance gene (ARG) removal during composting is still unclear. This study investigated the impact of different residue levels of tilmicosin (TIM), a common veterinary macrolide antibiotic, on ARG removal during pig manure composting. Three groups were used: the CK group (no TIM), the L group (246.49 ± 22.83 mg/kg TIM), and the H group (529.99 ± 16.15 mg/kg TIM). Composting removed most targeted macrolide resistance genes (MRGs) like ereA, ermC, and ermF (>90% removal), and reduced ermB, ermX, ermQ, acrA, acrB, and mefA (30–70% removal). However, ermA increased in abundance. TIM altered compost community structure, driving succession through a deterministic process. At low doses, TIM reduced MRG–bacteria co-occurrence, with horizontal gene transfer via intI1 being the main cause of ermA rebound. In conclusion, composting reduces many MRG levels in pig manure, but the persistence and rebound of genes like ermA reveal the complex interactions between composting conditions and microbial gene transfer. Full article

Pasteurella multocida (PM), Glaesserella parasuis (GPS), and Actinobacillus pleuropneumoniae (APP) are among the species with the top five isolation rates on Chinese pig farms annually. To understand the antimicrobial susceptibility and genotypes of these three pathogens that are currently prevalent on pig farms, we investigated 151 bacterial strains (64 PM, 48 GPS, and 39 APP) isolated from 4190 samples from farms in 12 Chinese provinces between 2021 and 2023. The prevalent serotypes were PM type D (50.0%), GPS type 5/12 (47.92%), and APP type 7 (35.90%). A relatively high proportion of PM and APP were resistant to ampicillin (PM, 93.75%; APP, 71.79%), tilmicosin (PM, 64.06%; APP, 58.97%), tetracycline (PM, 43.75%; APP, 61.54%), and enrofloxacin (PM, 34.38%; APP, 10.26%). Ampicillin, tetracycline, and enrofloxacin exhibited low MIC₉₀ values against GPS (8 µg/mL), while sulfamethoxazole-trimethoprim had a high MIC₉₀ value (512 µg/mL). A total of 18 genes conferring resistance to various antimicrobial classes were identified, and tet(L), tet(M), tet(A), bla_TEM, sul2, aph(3′)-Ia, dfrA12, qnrS1, strA, sul3, and mef(B) exhibited a high frequency of identification (≥70%). The analysis of regular virulence factor genes showed that several genes, including fimB, fimA, fimD, fimF, and fepG, were found in all PM, GPS, and APP strains. However, certain genes exhibited species-specific preferences, even if they belonged to the same category. Full article

Commercial organic fertilizers (COFs), as an alternative to chemical fertilizers, have been widely promoted and applied to agricultural soils to improve soil fertility and develop green agriculture. However, the residues of veterinary antimicrobials in COFs could be transferred to agricultural soils and pose ecological risk that should not be ignored. This study quantified the occurrence of fifty-seven veterinary antimicrobials, covering five classes (i.e., twenty-three sulfonamides, nineteen quinolones, seven macrolides, six tetracyclines, and two lincosamides) in ninety-three COFs collected from five provinces in China. Twenty-two veterinary antimicrobials, including eleven quinolones, six sulfonamides, four macrolides, and one lincosamide, were detected in the COFs with total contents up to 3870 ng/g. The contents of individual antimicrobials ranged from 0.66 to 3310 ng/g, and their detection frequencies were between 2% and 49%. The composition and contents of antimicrobials in the COFs varied significantly, depending on their raw materials, production processes, and source regions. Seven antimicrobials, including ciprofloxacin, enrofloxacin, and tilmicosin, could pose low to medium potential ecological risk to soil organisms in the amended soils. The wide occurrence of antimicrobials in COFs and their potential ecological risk indicate the urgent need to establish regulatory limits of antimicrobial residues in COFs to control and prevent antimicrobial pollution in agricultural soils brought by their amendment. Full article

Bovine respiratory disease causes significant economic losses in cattle farming due to mortality, treatment costs, and reduced productivity. It involves viral and bacterial infections, with Pasteurella multocida and Mannheimia haemolytica key bacterial pathogens. These bacteria contribute to severe pneumonia and are often found together. Poland has one of the highest levels of antimicrobial use in food-producing animals among European Union countries. A total of 70 bacterial strains were analyzed, 48 P. multocida and 22 M. haemolytica, collected from affected calves’ respiratory tracts. The bacterial species were confirmed molecularly using PCR, which was also employed to detect antimicrobial resistance and virulence-associated genes. Antimicrobial susceptibility was determined using the broth microdilution method. Antimicrobial resistance varied between the two bacterial species studied. The highest resistance in P. multocida was to chlortetracycline 79.2% (38/48) and oxytetracycline 81.3% (39/48), while M. haemolytica showed 63.6% (14/22) resistance to penicillin and tilmicosin. The highest susceptibility was found for fluoroquinolones: P. multocida demonstrated 91.7% (44/48) susceptibility to enrofloxacin and 87.5% (42/48) to danofloxacin, while 77.3% (17/22) of M. haemolytica were susceptible to both tested fluoroquinolones. The tetH and tetR genes were observed only in P. multocida, at frequencies of 20.8% (10/48) and 16.7% (8/48), respectively. Both species carried the mphE and msrE genes, though at lower frequencies. All M. haemolytica contained the lkt, gs60, and gcp genes. All P. multocida carried the sodA gene, while the hgbB and ompH genes were present in 37.5% (18/48) and 20.8% (10/48) of strains, respectively. The highest resistance was observed against the most commonly used antibiotics in the European Union, although the resistance differed between the studied bacterial species and each strain exhibited the presence of at least one virulence gene. Full article

Bacterial killing is important for recovering from infection. Pasteurella multocida is a key bacterial pathogen causing swine respiratory disease and is associated with substantial mortality. Antimicrobial therapy remains an important therapeutic intervention for treating infected animals. Pradofloxacin (fluoroquinolone) is the most recently approved antimicrobial agent for treating pigs with swine respiratory disease. We compared in vitro killing of swine P. multocida strains by pradofloxacin in comparison to ceftiofur, enrofloxacin, florfenicol, marbofloxacin, tildipirosin, tilmicosin, and tulathromycin over a range of bacterial densities and four clinically relevant drug concentrations. Pradofloxacin killed 92–96.9% of cells across 10⁶–10⁸ cfu/mL densities at the mutant prevention drug concentration following 2–24 h of drug exposure, 96.9–98.9% of cells across 10⁶–10⁹ cfu/mL at the maximum serum drug concentration following 30 min of drug exposure, increasing to 99.9–100% kill following 12–24 h of drug exposure. At the maximum tissue drug concentration and against bacterial densities of 10⁶–10⁹ cfu/mL, pradofloxacin killed 91.3–99.8% of cells following 2 h of drug exposure, which increased to 99.9–100% kill following 12–24 h of drug exposure. Pradofloxacin was rapidly bactericidal across a range of bacterial densities and at clinically relevant drug concentrations. Pradofloxacin will be an important antibiotic for treating pigs with swine respiratory disease and where clinically indicated. Full article

Glaesserella parasuis is the etiological agent of Glässer’s disease, which causes high morbidity and mortality in pigs worldwide. Macrolide resistance poses an urgent threat to their treatment, as macrolides are widely used for preventing and treating G. parasuis infections. Here, we determined the susceptibilities to five macrolides and characterized the genetic markers of macrolide resistance. The antimicrobial susceptibility of 117 G. parasuis isolates to erythromycin, tulathromycin, gamithromycin, tylosin, and tilmicosin was evaluated using broth microdilution method. Erythromycin-resistant isolates were sequenced using whole-genome sequencing. Further analysis of these sequences revealed the genetic basis of macrolide resistance in G. parasuis. Our results show that most G. parasuis isolates remained susceptible to the macrolide drugs. For commonly used agents (e.g., tylosin and tilmicosin), elevated minimum inhibitory concentrations (MICs) were observed, whereas for the newer macrolides (e.g., tulathromycin and gamithromycin), the MICs remained almost unchanged. The macrolide resistance gene erm(T) and the A2059G mutation in 23S rRNA were detected in the current study. To the best of our knowledge, integrative and conjugative element (ICE)-borne erm(T) in G. parasuis is reported for the first time in this study. Taken together, these results provide insights into the susceptibility of G. parasuis to macrolides. The presence of erm(T) on ICEs may facilitate its transfer, reducing the effectiveness of macrolide treatment. Full article

The impact of antibiotic therapy on the spread of antibiotic resistance genes (ARGs) and its relationship to gut microbiota remains unclear. This study investigated changes in ARGs, mobile genetic elements (MGEs), and gut microbial composition following tilmicosin administration in pigs. Thirty pigs were randomly divided into control (CK), low-concentration (0.2 g/kg; L), and high-concentration (0.4 g/kg; H) groups. Tilmicosin concentration in manure peaked on day 16 of dosing and dropped below detectable levels by day 13 of the withdrawal period. While tilmicosin did not significantly affect the total abundance of macrolide resistance genes (MRGs) (p > 0.05), it significantly increased the abundance of the multidrug resistance gene tolC in the H group compared with the L and CK groups during the withdrawal period (p < 0.05). This increase was associated with a coincidental rise in the abundance of MGEs (e.g., int1 and int2) and the growth of potential tolC-hosting bacteria such as Paenalcaligenes and Proteiniclasticum. Redundancy analysis showed gut microbial composition as the primary driver of MRG abundance, with MGEs, tilmicosin concentration, and manure physicochemical properties playing secondary roles. These findings suggest that high-dose tilmicosin may alter the gut microbiota and promote ARG spread via MGE-mediated transfer. Full article

Mycoplasma hyopneumoniae, an important cause of enzootic pneumonia in pigs in many countries, has recently been shown to exhibit reduced susceptibility to several antimicrobial classes. In the present study, a total of 185 pig lung tissue samples were collected from abattoirs in Australia, from which 21 isolates of M. hyopneumoniae were obtained. The antimicrobial resistance profile of the isolates was determined for 12 antimicrobials using minimum inhibitory concentration (MIC) testing, and a subset (n = 14) underwent whole-genome sequence analysis. MIC testing revealed uniformly low values for enrofloxacin (≤1 μg/mL), florfenicol (≤8 μg/mL), lincomycin (≤4 μg/mL), spectinomycin (≤4 μg/mL), tetracycline (≤0.5 μg/mL), tiamulin (≤2 μg/mL), tildipirosin (≤4 μg/mL), tilmicosin (≤16 μg/mL) tulathromycin (≤2 μg/mL), and tylosin (≤2 μg/mL). Higher MICs were observed for erythromycin (MIC range: 16–32 μg/mL), gamithromycin, and tilmicosin (MIC range of both: 32–64 μg/mL). Whole-genome sequencing of the isolates and additional screening using mismatch amplification mutation assay PCR did not identify any known genetic resistance markers within 23S rRNA (macrolides), DNA gyrase A, and topoisomerase IV genes (fluoroquinolones). The WGS data also indicated that the Australian M. hyopneumoniae isolates exhibited limited genetic diversity and formed a distinct monophylectic clade when compared to isolates from other countries. These findings indicate that Australian M. hyopneumoniae likely remains susceptible to the major antimicrobials used to treat enzootic pneumonia in pigs and have evolved in isolation from strains identified in other pig-producing countries. Full article

Tilmicosin, an antibiotic widely used in animal husbandry to prevent and treat bacterial infections, raises concerns due to its residual accumulation, which impacts both animal health and food safety. In this study, we conducted a comprehensive analysis of tilmicosin clearance patterns in different tissues, assessed physiological impacts through blood biochemistry, and investigated changes in gut microbial composition with 16S rRNA sequencing of the tilmicosin-treated Silkie chickens. Initially, we observed rapid peaks in tilmicosin residues in all tissues within 1 day after treatment, but complete metabolism took longer, extending beyond 9 days. Moreover, tilmicosin treatment significantly decreased serum levels of total bile acid, blood urea nitrogen, and uric acid, while increasing the levels of direct bilirubin, total bilirubin, and glutathione peroxidase at day 3, followed by a decrease from day 5 onwards. The effects of tilmicosin use on microbial composition and diversity lasted for an extended period, with the relative abundance of Proteobacteria remaining significantly different between the control and tilmicosin-treated groups at 120 days. Additionally, correlation analysis revealed a strong positive correlation between Mucispirillum_schaedleri and tilmicosin residue in all tissues, while Parabbacteroide_distasonis, Faecalibacterium_prausnitzii, and others exhibited negative correlations with tilmicosin residue. Overall, our study indicates a significant correlation between intestinal microbes and antibiotic residues, providing a theoretical basis for guiding the withdrawal period after antibiotic use. Full article

Pradofloxacin is a dual targeting, bactericidal fluoroquinolone recently approved for treating bacteria causing swine respiratory disease. Currently, an abundance of in vitro data does not exist for pradofloxacin. We determined the minimum inhibitory concentration (MIC) and mutant prevention concentrations (MPC) of pradofloxacin compared to ceftiofur, enrofloxacin, florfenicol, marbofloxacin, tildipirosin, tilmicosin and tulathromycin against swine isolates of Actinobacillus pleuropneumoniae and Pasteurella multocida. Overall, pradofloxacin had the lowest MIC and MPC values as compared to the other agents tested. For example, pradofloxacin MIC values for 50%, 90% and 100% of A. pleuropneumoniae strains were ≤0.016 µg/mL, ≤0.016 µg/mL and ≤0.016 µg/mL and for P. multocida were ≤0.016 µg/mL, ≤0.016 µg/mL and 0.031 µg/mL, respectively. The MPC values for 50%, 90% and 100% of A. pleuropneumoniae strains were 0.031 µg/mL, 0.063 µg/mL and 0.125 µg/mL and for P. multocida were ≤0.016 µg/mL, 0.031 µg/mL and 0.0.063 µg/mL, respectively. By MPC testing, all strains were at or below the susceptibility breakpoint. Based on MPC testing, pradofloxacin appears to have a low likelihood for resistance selection. This study represents the most comprehensive in vitro comparison of the above noted drugs and the first report for pradofloxacin and tildipirosin. Full article