1. Introduction
Colon diseases are a common complex of diseases in humans ranging from inflammatory bowel disease (IBD) to colorectal cancer (CRC). Cancer is the abnormal growth of cells in determinate tissue, and is the result of multiple alterations in the cell cycle [
1]. CRC is the third most common malignancy, and fourth most common cause of cancer mortality worldwide; in fact, it is estimated that there are more than 1 million new cases of CRC each year [
2], and CRC is responsible for 896,000 deaths per year worldwide [
3]. The process of CRC starts with chronic inflammation in the gut, and then the patient progresses to colitis. After that, DNA damage and mutations in oncogenes and tumor suppressor genes occur in cells in the intestine. These phenomena lead to the production of polypi or diverticula, and the constant damage to DNA will evolve into CRC [
4,
5].
There are many factors associated with CRC, such as diet, lifestyle, genetic predisposition, and, recently, the microbiota [
6]. Evidence shows that the composition of the human intestinal microbiota influences the host health status. In fact, microbial dysbiosis has been observed in CRC patients, and there is evidence that infection from bacteria, including
Streptococcus bovis,
Enterococcus spp.,
Helicobacter pylori,
Bacteroides fragilis,
Escherichia coli, and
Klebsiella spp., can result in CRC [
7]. Some studies have demonstrated a high presence of
E. coli in colon diseases in comparison with controls. Studies have revealed that from 70% of CRC biopsies,
E. coli has been isolated in comparison with healthy subjects [
8]. Interestingly, most
E. coli isolated from colon diseases belong to phylogroup B2; of all the
E. coli phylogroups, B2 is related to strong adherent-invasive ability and high virulence skills that are needed to cause damage to the host [
9]. Other bacteria that form part of the normal microbiota and have been isolated from CRC biopsies are
K. pneumoniae; this bacterium has been isolated from 13.5% of CRC patients in comparison with healthy subjects [
10].
E. coli and
Klebsiella are Gram-negative bacteria that colonize in humans, starting from the first hours of life. These bacteria are a component of normal colon-rectal flora; however, cyclomodulins have been identified in colon-rectal flora isolated from colon diseases. Cyclomodulins are genotoxins that modulate the cellular cycle, differentiation, apoptosis, and proliferation [
7]. Different cyclomodulins have been identified, some of which are (i) colibactin, which is in the
pks genomic island, and causes DNA double-strand breaks and chromosomal instability in human eukaryotic cells [
11]; (ii) cytotoxic necrotizing factor (CNF), which activates Rho GTPases, leading to cytoskeletal alterations, and affecting the cell cycle [
12]; (iii) cycle-inhibiting factor (CIF), which is related to proliferation inhibition by mitosis inhibition, blocking the cell cycle G2/M transition [
13]; and (iv) cytolethal distending toxin (CDT), which apparently induces DNA damage by DNAse activity [
14]. The presence of cyclomodulin-positive bacteria in biopsies of colon diseases suggests that these bacteria play an important role in the genesis of these diseases.
Despite the importance of these bacterial cyclomodulins in colon diseases, there are no studies about the prevalence of these bacteria in Mexican subjects with colon diseases. Therefore, in this comprehensive study, we investigated the associations and prevalence of cyclomodulins-positive (pks, cdt, cnf, and cif) diarrheagenic E. coli (DEC), non-DEC, and Klebsiella spp. strains isolated from Mexican subjects with colon diseases (IBD, polypus, diverticulosis, and CRC).
3. Discussion
In this study, we demonstrated that E. coli (non-DEC, EPEC, and DAEC) from phylogroup B2 was the most prevalent bacterium isolated from the colon disease biopsies (CRC, polypus, diverticulosis, and IBD), followed by Klebsiella and Enterococcus. A total of 80.4% of the isolated strains were CM+, of which non-DEC was statically associated with cdt, EPEC with cif and its combinations, whereas Klebsiella was associated with pks and cdt. Lastly, 34.3% of the isolated strains presented the MDR phenotype, and 48.6% presented XDR; surprisingly, more than 95% of the CM+ strains were resistant to two or more antibiotics.
E. coli and
Klebsiella are classified as important commensal bacteria in the human gut, but recently, these bacteria have been isolated from lesions in the gut related to colon diseases. In a previous work, a prevalence of 39.5% to 12.9% of
E. coli in colon disease biopsies, such as CRC and diverticulosis [
8], was found, whereas
K. pneumoniae was found in 4–27% of patients with CRC [
15,
16], data that are similar to those found in this study. Other bacteria isolated from colon diseases in this study were
Enterococcus, which was associated with CRC patients. The presence of this bacterium in CRC has also been reported by other researchers, but its mechanism of producing damage is still unclear [
17].
Different
E. coli pathotypes (EPEC and DAEC) were found in patients with colonic diseases in this study, with EPEC being the most prevalent. The presence of EPEC has previously been associated with CRC. It has reported an EPEC prevalence of 50.7% in CRC patients in Egypt compared with 20.1% in healthy patients, and this prevalence was higher than that found in the Mexican population [
18]. Prorok-Hamon et al. (2014) found a high prevalence of DAEC
afaC+ and
lpfA+ in CRC and IBD patients in comparison with that in controls [
19]. Both pathotypes were considered in this study due to their strong ability to adhere to enterocytes, which is the first step in producing damage in cells. In the case of EPEC, this pathotype harbors
eae (
E. coli attachment effacement), which helps to form attaching-and-effacing (A/E) lesions on intestinal cells, and contains a type 3 secretion system [
20,
21], whereas DAEC encodes important adhesins, such as afimbrial (
Afa) or fimbrial (
Dr) adhesins [
22].
Regarding the
E. coli phylogroups, most of the
E. coli strains isolated in this study belonged to B2, and these data match those of other studies. In a work, it was reported that 47.5% and 32.6% of
E. coli isolated from CRC and diverticulosis patients, respectively, belonged to phylogroup B2 [
9]. Also, 66.4%
E. coli B2 prevalence was found in biopsies of patients with colon diseases (CRC and diverticulosis) in French subjects [
8]. This
E. coli is frequently found in colon diseases because it is well documented that B2
E. coli have a greater ability to colonize the human intestine, as this bacterium encodes fitness factors, such as pili and adhesins [
23]. Despite the methodology applied to clean the biopsies and remove the mucus surface,
E. coli remained in the samples, which demonstrated the high capacity of
E. coli B2 to attach to the surface; nevertheless, the probability of bacteria internalized into enterocytes cannot be discarded. Moreover,
E. coli belonging to phylogroup B2 has been related to the presence of cyclomodulins, which are responsible for causing damage to the DNA of host cells, and altering the cellular cycle [
24].
Cyclomodulins have been identified in different bacteria, such as
E. coli or
Klebsiella, that were isolated from colon diseases. The prevalence of bacterial cyclomodulin positivity in colon diseases differs depending on the population. In this study, approximately 80% of the bacteria were cyclomodulin-positive, which was higher than that in other studies, such as that in French subjects, in which 52.3% of
E. coli strains from patients with colon diseases were positive for this genotoxin [
24]. Other work reported an
E. coli cyclomodulin positivity of 66.7% in CRC patients, and 40% in patients with IBD [
25]. Also, the presence of 25.6% cyclomodulin-positive
K. pneumoniae was observed [
15]. The high prevalence of bacteria positive for cyclomodulins in colon diseases, specifically in lesion zones, suggests that these bacteria have a role in the development of colon diseases. In the case of
pks-encoded colibactin, this genotoxin causes DNA double-strand breaks and chromosomal instability in human eukaryotic cells, and its relation to CRC is well established [
11]. Previous reports indicate that
pks is most frequently found in
E. coli isolated from patients with CRC [
8]. By contrast, in our study, we found that this cyclomodulin was associated with
Klebsiella spp., and was found in similar proportions in all colon diseases from IBD to CRC, suggesting that
pks could cause damage to DNA during the early steps of colon diseases. Additionally, based on the
K. pneumonia pks+ prevalence shown in this study, it is important to increase the research focus on this bacterium as an etiologic agent of colon diseases. Moreover, the high presence of bacterial
pks+ in this study, as in other studies, demonstrated the importance of this genotoxin in the development of colon diseases.
cnf is a cyclomodulin that promotes cellular proliferation by stimulating the assembly of actin stress fibers and focal adhesions by deamination and DNA synthesis, and
cnf has only been found in
E. coli and
Y. pseudotuberculosis. Although
cnf has been associated with uropathogenic
E. coli and bladder cancer, in this study,
cnf was found in DEC and non-DEC, and primarily in CRC patients [
26]. A previous report detected
cnf-1 in stool samples for the first time, but only one sample was positive for this genotoxin from healthy people in Puerto Rico. In contrast, in our study,
cnf+ E. coli was isolated from 10 individuals. The high prevalence of
cnf in our study in comparison with that in the Gómez-Moreno study could have occurred because our patients presented colon diseases, whereas the Gómez-Moreno study also included healthy participants [
27].
Another important cyclomodulin found in this study was
cdt, which has phosphodiesterase activity that triggers a DNA damage response resulting in inhibited proliferation by G2–M cell cycle arrest [
12]. There are different types of CDT toxins, and these genes are within the chromosomal DNA, such as
cdtA or
cdtB, and the other is in plasmid DNA, such as
cdt-II. CDTs have also been described in other important bacterial pathogens, such as in
E. coli strains, but to date, still not yet in
Klebsiella spp. or isolated from patients with colon diseases. This is the first study to report the presence of
cdt in
Klebsiella spp.
The active interchange of genetic information (competence or recombination process) in the gut among bacteria could be the factor that helps
Klebsiella spp. acquire this genotoxin, and this fact is very important because bacteria not only disseminate among the genes that confer antibiotic resistance, but also virulence factors, which can be watersheds in the virulence of bacteria [
28,
29]. However, future research is needed to characterize the structure and function of this
cdt in
Klebsiella spp. and compare it with the
cdt of other bacterial species, and also identify the
Klebsiella species by biochemical and molecular methods in this way to know what
Klebsiella species is positive to
pks or
cdt. Notably,
cdt was the most prevalent cyclomodulin in this study; despite the relationship between
cdt and DNA damage, this toxin has also been related to serious cases of diarrhea in Mexican subjects due to
E. coli [
30,
31]. However, we found an association of
cdt with non-DECs, but recent work indicates that non-DECs with supplementary virulence genes have high levels of virulence, and are capable of producing diarrhea [
32]. The high prevalence of diarrheas from
E. coli in Mexico could explain the amount of
cdt in our study population [
33].
Regarding
cif, this genotoxin is related to proliferation inhibition by mitosis inhibition, which blocks the cell cycle G2/M transition [
13]. In this study, the cycle-inhibiting factor was found only in
E. coli, specifically in EPEC, because
cif must secrete type III into the system to translocate to the cytoplasm of the host cells, where it induces a progressive cytopathic effect [
34]. This cyclomodulin was less prevalent in Mexican subjects, which is consistent with other studies [
8]. This is the first study to identify
E. coli with a combination of three cyclomodulins. In a previous study,
E. coli with a combination of two cyclomodulins (
cnf/cdtdf and
cnf/pks) was isolated [
8].
The high antibiotic resistance of strains isolated from patients with colon diseases in this study is alarming.
E. coli and
Klebsiella spp. were resistant to multiple antibiotics, as observed in other similar studies in Mexico [
32,
35,
36]. A study conducted in Egypt also reported a high resistance to antibiotics in
E. coli and in
K. pneumoniae isolated from patients with CRC, where more than 71% of those strains were resistant to 10 antibiotics. [
37]. However, other studies have reported low or moderate resistance of
E. coli and
Klebsiella positive for cyclomodulins to antibiotics [
38,
39]. More evidence is needed to establish a possible relationship between antibiotic resistance and cyclomodulins; nevertheless, we cannot rule out the resistance of the present microbiome in subjects from other countries. In the case of Mexico, bacteria isolated from humans, water, or foods have a high level of resistance to antimicrobials [
32,
35,
36].
To the best of our knowledge, this is the first paper to report the presence of cyclomodulins in Mexican subjects. We also highlighted that this is the first time that cdt has been identified and associated with Klebsiella spp. Lastly, we demonstrated that cyclomodulin-positive bacteria have high resistance to antibiotics, and that one bacterium can harbor up to three cyclomodulin genes.
In conclusion, this study provides evidence regarding the high prevalence of cyclomodulin-positive bacteria in patients with colon disease in Mexican subjects. E. coli belonging to phylogroup B2 was the most frequently isolated from patients. Some cyclomodulins were associated with different bacteria, and no difference was found among cyclomodulins and colon diseases. Moreover, high antibiotic resistance was present in CM-positive bacteria. The findings presented in this study support that the presence of bacteria positive for these genotoxins may have a role in the development of colon diseases, and special attention should be given to these bacteria as etiologic agents of colon diseases. In this context, new strategies must be established to detect and eradicate bacteria harboring the CM gene, and prevent the development of colon diseases.
4. Material and Methods
4.1. Ethical Considerations
This research project was approved by the Committee of Ethics in Research and the Research Committee of Regional Hospital of High Specialty of Oaxaca (HRAEO) under registration number HRAEO-CIC-CEI-015-16. The patients were informed, and we asked for their participation in the protocol. They were informed about the protocol, procedures, and risks of sampling. We voluntarily obtained written consent from the patients, and they were informed that they could withdraw their consent at any time.
4.2. Patients
In this study, 43 adult patients of both sexes were recruited from February 2015 to May 2017 from the HRAEO. Sixteen patients had CRC, nine had polypus, thirteen had inflammatory bowel disease, and five had diverticulosis. The age range of patients with cancer was 35 to 84 years (median 57), the age of diverticula patients ranged from 32 to 72 years (median 58), the age of polypus patients ranged from 20 to 77 years (median 54), and the age of inflammatory bowel disease patients ranged from 39 to 84 years (median 50.5).
The sex proportion (men/women) was 1.6 in CRC patients, 3.5 in diverticula patients, 0.22 in polypus patients, and 5 in patients with inflammation. Biopsies were taken by the HRAEO medical team from injured or inflamed tissue. For the colonoscopy, patients fasted for 8 h, and they took polyethylene glycol or sodium picosulfate to prepare their intestines for the procedure.
4.3. Bacteria Strains
The reference strains used in this study belong to our laboratory collection. To identify DEC, we used DAEC (daaE+), ETEC (lt+ and/or st+), EPEC E2348/69 (eae+ and bfp+), and enteroaggregative E. coli (EAEC O42; pCVD432+ and aafII+). Klebsiella pneumoniae subsp. pneumoniae (Schroeter) Trevisan ATCC 13883, ATCC 13182 was used to identify Klebsiella. Enterococcus faecalis ATCC 19433 to Enterococcus. E. coli (Migula) Castellani and Chalmers ATCC, ATCC BAA-2452, ATCC 25922, and ATCC 35218 were used as control in antimicrobial susceptibility tests. E. coli (Migula) Castellani and Chalmers ATCC 11775 and E. coli K12 (HB101) were used in quality control of culture media. The bacteria were cultured in Luria-Bertani (LB) broth at 37 °C, and frozen in 15% glycerol at −70 °C for future experiments.
4.4. Bacterial Isolation and Identification from Biopsies
Biopsies from colon mucosa were collocated in tubes containing sterile phosphate saline solution (PBS), and the biopsies were washed three times to clean the fecal waste. To obtain bacteria from the biopsies, Triton X-100 (0.1X) was added to the samples, and processed using a Tissue Ruptor II (Qiagen, Valencia, CA, USA). To identify bacteria in biopsies, we performed biochemical and molecular analyses. First, crushed biopsies were inoculated into selective agar culture media, namely, McConkey (MCD LAB, Oaxaca, Mexico), CHROMagar Orientation Medium (BD BBL, Franklin Lakes, NJ, USA), and Enterococcus Agar (BD BBL, Franklin Lakes, NJ, USA), to identify bacteria, such as E. coli, Klebsiella spp., and Enterococcus spp. These samples were incubated at 37 °C for 24 h. The colonies that grew in selective media were collected, and every colony was characteristic for each bacterium. To complete the biochemical analysis, the colony identities were confirmed using an API 20 E kit test (bioMérieux, Marcy l’Etoile, France) according to the manufacturer’s instructions. Twelve colonies were selected per sample, which were cultivated in LB broth, and frozen in 15% glycerol at −70 °C for future experiments.
4.5. Preparation of Template DNA
To obtain genomic DNA, 10 colonies were taken per sample, and inoculated into LB broth media incubated at 37 °C for 18 h. Each bacterial pellet was collected and suspended in 200 µL of molecular biology grade water, and then the boiling method was applied [
32]. The DNA was stored at −20 °C for future experiments.
4.6. Molecular Identification of Bacterial Species, Pathotypes, Phylogenetic Groups, and Cyclomodulins
To complete the identification of the bacteria isolated from the biopsies, PCRs were performed, and the 16 S rRNA gene was amplified to confirm the identities of
E. coli,
Klebsiella, and
Enterococcus faecalis. The primers used in the reaction are shown in
Supplemental Table S1. Using the
E. coli isolate strains, the following pathotypes were searched: enteropathogenic
E. coli (EPEC) and diffusely adherent
E. coli (DAEC), by identifying the
bfp and
eae genes in EPEC and
daaE genes in DAEC (
Supplemental Table S1). Bacteria that were negative for both reactions were classified as non-DEC, and the positive ones were denoted as DEC. Then, phylogenetic groups A, B1, B2, and D were identified as described by Clermont et al. [
40]. Additionally, the identification of cyclomodulins CNF, CDT, CIF, and PKS was performed using the genes
cnf, cif, cdt III and
IV,
pks, and
clbB as targets. The primer sequences and the sizes of the PCR products are shown with these targets in
Supplemental Table S1. For all PCR reactions, we mixed MyTaq (BIOLINE), 0.5 µM forward and reverse primers, and 2 µL of genomic DNA and molecular biology grade water, and the reactions were performed in a Mastercycler flexlid thermocycler (Eppendorf). The PCR products were visualized using ethidium bromide (1 µg/µL), and the results were analyzed in a PhotoDoc-It imaging system with a Benchtop 2 UV transilluminator (UVP).
4.7. Antimicrobial Agent Susceptibility Testing
Antibiotic susceptibility testing of the isolated bacteria was performed using the Kirby-Bauer disk diffusion method on Mueller-Hinton agar plates [
41] in accordance with the guidelines of the Clinical Laboratory Standard Institute (CLSI) [
42]. Suspensions of isolated bacteria were prepared in LB at a turbidity of 0.5 using the McFarland turbidity standard. Then, they were swabbed on already prepared nutrient agar plates. The plates containing the impregnated antibiotic disks (BD BBL, Franklin Lakes, NJ, USA) were placed aseptically on the inoculated agar. The tested antibiotics were ampicillin, piperacillin, amoxicillin, ampicillin/sulbactam, cefazolin, cefotaxime, cefuroxime, ceftazidime, aztreonam, imipenem, gentamicin, tobramycin, amikacin, kanamycin, streptomycin, tetracycline, ciprofloxacin, levofloxacin, nalidixic acid, and chloramphenicol, and the plates were incubated for 18 h at 37 °C.
E. coli (Migula) Castellani and Chalmers (ATCC
® 25922™) was used as the control. After that, the diameters of the inhibition halos were measured by using the Vernier rule. Antibiotic susceptibility was interpreted according to CLSI guidelines; the bacteria were classified as resistant, intermediate, or sensitive. Isolates that showed non-susceptibility to at least one agent in more than three and six antibiotic categories were classified as multidrug-resistant (MDR) and extremely drug-resistant (XDR), respectively [
43]. The antibiotics were selected based on their use in treating human infections caused by Gram-negative bacteria [
44], and represent different classes of antimicrobial agents that are available to treat these infections in Mexico.
4.8. Statistical Analysis
Associations between nominal variables were analyzed with Fisher’s exact test and/or chi-squared test. Statistical significance was determined when p ≤ 0.05, and analyses were performed with the IBM® SPSS® Statistics program version 20 (New York, NY, USA).