Presence Of Virulence Determinants Research Articles

Antimicrobial Resistance Profiles of Enterococcus Faecalis and Enterococcus Faecium Isolated from Clinical Specimens at Kitwe Teaching Hospital in Zambia

This research examined the occurrence of Enterococcus faecalis and Enterococcus faecium in clinical samples processed at Kitwe Teaching Hospital Laboratory, along with their resistance to antimicrobials and presence of virulence determinants. A total of 230 clinical specimens were analyzed to identify suspected Enterococcus strains. Standard laboratory techniques were employed to isolate, characterize, and confirm the species of Enterococcus, followed by testing their susceptibility to antimicrobials and detection of resistance and virulence genes using polymerase chain reaction. Among the 230 cultured specimens, 89 were stool samples, 3 were swabs, and 138 were urine specimens. Out of these, 71 isolates were confirmed Enterococcus by PCR using genus specific primers. The resulting prevalence rate of 30.9% was thus obtained. The most prevalent species was Enterococcus faecalis, accounting for 56.3% of the 71 Enterococcus isolates. The prevalence of E. faecium was 14.1%. All tested E. faecalis isolates displayed resistance to ciprofloxacin and erythromycin. Furthermore, high rates of resistance were observed among E. faecalis isolates for chloramphenicol and tetracycline (97.5%), vancomycin (90.0%), nitrofurantoin (75.0%), penicillin (75.0%) and ampicillin (67.5%). Only two E. faecium isolates exhibited susceptibility to tetracycline and vancomycin, while the remaining isolates were 100% resistant to all other tested antimicrobials. Multidrug resistance was detected in all E. faecium isolates. In addition, various antibiotic resistance genes (aac(6′)-Ie-aph(2″)-LA, ermA, ermB, tetK, tetL, tetM, tetX, vanA) were identified in both E. faecalis and E. faecium isolates. A positive association between phenotype and genotype was found for tetracycline and erythromycin.

Biofilm Formation Capacity and Presence of Virulence Determinants among Enterococcus Species from Milk and Raw Milk Cheeses

Bacterial biofilm is one of the major hazards facing the food industry. Biofilm-forming ability is one of the most important virulence properties of enterococci. The genus Enterococcus includes pathogenic, spoilage, and pro-technological bacteria. The presence of enterococci in milk and dairy products is usually associated with inadequate hygiene practices. The study examined the isolates' capacity for biofilm formation and identification of the genetic determinants of its formation among 85 Enterococcus strains isolated from raw milk (n = 49) and soft-ripened cheeses made from unpasteurized milk (n = 36). E. faecalis and E. faecium were the dominant species. The obtained results showed that 41.4% isolates from milk and 50.0% isolates from cheeses were able to form biofilm. All of the isolates analyzed had at least one of the studied genes. As regards the isolates from raw milk, the most prevalent gene was the gelE (85.6%), followed by the asa1 (66.7%). None of the isolates from cheeses showed the presence of cylA and sprE. The most prevalent gene among the strains from this source was the epbC (94.4%), followed by the gelE (88.9%). In isolates from both sources, the presence of proteins from the Fsr group was noted the least frequently. Nevertheless, results showed that were no significant differences between the biofilm-producing Enterococcus spp. and non-biofilm-producing isolates in term of occurrences of tested virulence genes. The ability to produce a biofilm by enterococci isolated from raw milk or ready-to-eat products emphasizes the need for continuous monitoring of the mechanisms of microbial adhesion.

Virulence Profiling, Multidrug Resistance and Molecular Mechanisms of Campylobacter Strains from Chicken Carcasses in Tunisia.

Antibiotic resistance in foodborne pathogens is an emergent global health concern. The objectives of this study were to assess antimicrobial resistance (AMR) in Campylobacter isolates from chicken carcasses and to investigate the AMR molecular mechanisms as well as the presence of virulence determinants. The study was performed on 257 samples collected from abattoirs and retail shops in northeastern Tunisia. Forty-eight Campylobacter isolates were recovered and identified as C. jejuni (n = 33) and C. coli (n = 15). Antibiotic resistance was tested against eight antibiotics and high resistance rates were observed against tetracycline (100%), erythromycin (97.9%), ciprofloxacin (73%), nalidixic acid (85.4%), ampicillin (83.3%), amoxicillin/clavulanic acid (22.9%), chloramphenicol (75%), and gentamicin (27.1%). All isolates were multidrug-resistant, and 22 resistance patterns were found. All isolates were screened for AMR genes (tet(O), tet(A), tet(B), tet(L), cmeB, ermB, blaOXA-61, and aphA-3), and for point mutations in gyrA (C257T substitution) and 23SrRNA (A2075G/A2074C) genes. All screened AMR genes, as well as the C257T and the A2075G mutations, were detected. The virulence genotypes were also determined, and all isolates carried the motility (flaA) and invasion (cadF) genes. Most of them also harbored the cdtA, cdtB, and cdtC genes, encoding the Campylobacter toxin. The screening of the cgtB and the wlaN genes, involved in Guillain-Barré Syndrome expression, revealed the presence of the cgtB in 21.2% of C. jejuni strains, whereas none of them carried the wlaN gene. Our findings highlight the emergence of Campylobacter strains simultaneously harboring several virulence and AMR determinants, which emphasizes the risk of transmission of MDR strains to humans via the food chain. Hence, controlling the dissemination of foodborne pathogens “from the farm to the fork” as well as restricting the use of antimicrobials in husbandry are mandatory to prevent the risk for consumers and to mitigate the dissemination of MDR pathogens.

Antimicrobial resistance, virulence genes, and biofilm formation in Staphylococcus aureus strains isolated from meat and meat products

ABSTRACTStaphylococcus aureus is an important food pathogen that has the ability to form biofilms. This pathogen has been implicated in foodborne outbreaks associated with the consumption of meat and meat products. Since the role of meat products as reservoirs of antimicrobial resistant S. aureus and biofilm formers has not been studied, this research was conducted to assess the incidence, antimicrobial resistance, and presence of biofilm‐forming genes in S. aureus bacteria from meat and meat products. For this purpose, 440 samples of meat and meat products were randomly collected from supermarkets of Shahrekord city, Iran. S. aureus isolates were highlighted and identified through biochemical tests. All S. aureus isolates were profiled for susceptibility to 14 antibiotics and the corresponding resistance genes were searched. The quantification of biofilm formation followed by a screening of biofilm formation encoding genes was done. Of the 440 samples collected, 61 were found to contain S. aureus. The incidence of S. aureus among the beef meat (16/61) was the highest followed by its incidence in sausage (10/61) and lunch meat (9/61). At least one isolate of S. aureus carries one of the following resistance gene blaZ, mecA, tet K, linA, tetM, erm A, ermB, and aac A‐D. High distribution of icaA, icaB, icaC, icaD, clfB, and clfA, biofilm formation genes and Enterotoxin encoding genes sea, seb, sec, and sed. The ability of biofilm production and the presence of virulence determinants in the genome of S. aureus can contribute to their pathogenicity. This research demonstrated the roles of meat and meat products as reservoirs of biofilm‐forming and antimicrobial‐resistant strains of S. aureus. This poses a food safety and public health problem. However, further work needs to be done to better understand the implication of icaABCD biofilm forming genes present in S. aureus from meat and meat products in their resistance.

CRISPR-Cas Loci of Yersinia pseudotuberculosis Strains with Different Genetic Determinants

Relevance. Yersinia pseudotuberculosis is a causative agent of pseudotuberculosis, a disease with polymorphism of clinical manifestation that is determined by the presence of specific virulence determinants: plasmid pVM82, pathogenicity islands HPI and YAPI, and superantigen YPM. Occurrence of new determinants depends on horizontal transfer of mobile genetic elements, hence, systems regulating horizontal transfer participate in evolution of pathogenic species. CRISPR-Cas is and adaptive protection system of prokaryotes against mobile genetic elements. Aim. The study analyzed an interaction between CRISPR-loci of Y. pseudotuberculosis and virulence determinants. Results. 86% of strains includes three CRISPR-loci: YP1, YP2, and YP3. Length of locus YP3 mostly depends on presence of virulence determinants in strains of Y. pseudotuberculosis serotype O:1b. Strains with virulence genes are able to cause a severe form of pseudotuberculosis and have longer locus than strains without determinants. Conclusion. Therefore, CRIPSRCas system of Y. pseudotuberculosis may participate in formation of a certain strain genotype that defines clinical manifestation of pseudotuberculosis.

CRISPR-Cas Loci of Yersinia pseudotuberculosis Strains with Different Genetic Determinants

Relevance. Yersinia pseudotuberculosis is a causative agent of pseudotuberculosis, a disease with polymorphism of clinical manifestation that is determined by the presence of specific virulence determinants: plasmid pVM82, pathogenicity islands HPI and YAPI, and superantigen YPM. Occurrence of new determinants depends on horizontal transfer of mobile genetic elements, hence, systems regulating horizontal transfer participate in evolution of pathogenic species. CRISPR-Cas is and adaptive protection system of prokaryotes against mobile genetic elements. Aim. The study analyzed an interaction between CRISPR-loci of Y. pseudotuberculosis and virulence determinants. Results. 86% of strains includes three CRISPR-loci: YP1, YP2, and YP3. Length of locus YP3 mostly depends on presence of virulence determinants in strains of Y. pseudotuberculosis serotype O:1b. Strains with virulence genes are able to cause a severe form of pseudotuberculosis and have longer locus than strains without determinants. Conclusion. Therefore, CRIPSRCas system of Y. pseudotuberculosis may participate in formation of a certain strain genotype that defines clinical manifestation of pseudotuberculosis.

Genetic & virulence profiling of ESBL-positive E. coli from nosocomial & veterinary sources

CTX-M genes are the most prevalent ESBL globally, infiltrating nosocomial, community and environmental settings. Wild and domesticated animals may act as effective vectors for the dissemination of CTX-producing Enterobacteriaceae. This study aimed to contextualise blaCTX-M-14-positive, cephalosporin-resistant Enterobacteriaceae human infections and compared resistance and pathogenicity markers with veterinary isolates.Epidemiologically related human (n=18) and veterinary (n=4) blaCTX-M-14-positive E. coli were fully characterised. All were typed by XbaI pulsed field gel electrophoresis and ST. Chromosomal/plasmidic locations of blaCTX-M-14 were deduced by S1-nuclease digestion, and association with ISEcp1 was investigated by sequencing. Conjugation experiments assessed transmissibility of plasmids carrying blaCTX-M-14. Presence of virulence determinants was screened by PCR assay and pathogenicity potential was determined by in vitro Galleria mellonella infection models.84% of clinical E. coli originated from community patients. blaCTX-M-14 was found ubiquitously downstream of ISEcp1 upon conjugative plasmids (25–150kb). blaCTX-M-14 was also found upon the chromosome of eight E. coli isolates. CTX-M-14-producing E. coli were found at multiple hospital sites. Clonal commonality between patient, hospitals and livestock microbial populations was found. In vivo model survival rates from clinical isolates (30%) and veterinary isolates (0%) were significantly different (p<0.05). Co-transfer of blaCTX-M-14 and virulence determinants was demonstrated.There is evidence of clonal spread of blaCTX-M-14-positive E. coli involving community patients and farm livestock. blaCTX-M-14 positive human clinical isolates carry a lower intrinsic pathogenic potential than veterinary E. coli highlighting the need for greater veterinary practices in preventing dissemination of MDR E. coli among livestock.

Frequent occurrence of oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) strains in two African countries.

Oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) isolates have been increasingly reported worldwide, but data regarding the African continent have not been available. Between 2010 and 2014, 1462 inpatients and healthcare workers were screened for MRSA nasal carriage in São Tomé and Príncipe (STP) and Angola, two Portuguese-speaking African countries (PALOP countries). We determined the presence of the mecA gene and the antimicrobial susceptibility profiles of the isolates. OS-MRSA clonal lineages were identified as well as the presence of virulence determinants, including Panton-Valentine leucocidin (PVL). Out of 164 S. aureus hospital isolates tested, 29 (17.7%) were mecA positive, but susceptible to oxacillin, showing oxacillin MICs ≤3 mg/L. All OS-MRSA isolates were resistant to cefoxitin and most of them were also resistant to at least two antimicrobials other than β-lactams. The 29 OS-MRSA were distributed into two major clonal lineages: (i) PFGE type B-ST88-SCCmec IVa, associated with spa types t186/t325/t786/t1814/t1951, detected in Angola (n = 5) and STP (n = 10); and (ii) PFGE type C-t451/t648-ST8-SCCmec V, exclusively found in STP (n = 9). OS-MRSA showed at least two virulence determinants. PVL was detected in an isolate recovered in STP. We describe a high prevalence of OS-MRSA among S. aureus strains recovered in two African countries. OS-MRSA in PALOP countries were mainly associated with ST88 and ST8, two prevalent MRSA clonal types in these countries. If direct testing for mecA is not available, cefoxitin susceptibility testing is highly recommended to avoid the misidentification of OS-MRSA.

Agr dysfunction affects staphylococcal cassette chromosome mec type-dependent clinical outcomes in methicillin-resistant Staphylococcus aureus bacteremia.

Staphylococcal cassette chromosome mec element (SCCmec) type-dependent clinical outcomes may vary due to geographical variation in the presence of virulence determinants. We compared the microbiological factors and mortality attributed to methicillin-resistant Staphylococcus aureus (MRSA) bacteremia between SCCmec types II/III and type IV. All episodes of MRSA bacteremia in a tertiary-care hospital (South Korea) over a 4.5-year period were reviewed. We studied the microbiological factors associated with all blood MRSA isolates, including spa type, agr type, agr dysfunction, and the genes for Panton-Valentine leukocidin (PVL) and phenol-soluble modulin (PSM)-mec, in addition to SCCmec type. Of 195 cases, 137 involved SCCmec types II/III, and 58 involved type IV. The mortality attributed to MRSA bacteremia was less frequent among the SCCmec type IV (5/58) than that among types II/III (39/137, P = 0.002). This difference remained significant when adjusted for clinical factors (adjusted odds ratio [aOR], 0.14; 95% confidence interval [CI], 0.04 to 0.49; P = 0.002). Of the microbiological factors tested, agr dysfunction was the only significant factor that showed different positivity between the SCCmec types, and it was independently associated with MRSA bacteremia-attributed mortality (aOR, 4.71; 95% CI, 1.72 to 12.92; P = 0.003). SCCmec type IV is associated with lower MRSA bacteremia-attributed mortality than are types II/III, which might be explained by the high rate of agr dysfunction in SCCmec types II/III in South Korea.

Composition of Escherichia coli population in the neonatal gut: phylogroups and virulence determinants

The composition of Escherichia coli in the neonatal gut has rarely been studied in developing countries. To gain insight into the composition of E. coli in the neonatal gut and to assess factors that could influence colonization by E. coli, analysis of the phylogenetic groups and virulence determinants of E. coli isolated from the guts of neonates in a tertiary care hospital was carried out. The distribution of the phylogroups of 124 E. coli isolates recovered showed that phylogroups A (23 %) and B1 (49 %) accounted for 72 % of the isolates. Isolates of the phylogenetic group B2 were rare (8 %). Virulence factors were also rare with the exception of aerobactin (iucC), which was detected in 45 % of the isolates and was significantly associated with phylogroup B1. Multinomial logistic regression established that colonization with phylogroup B1 was associated with a stay in the neonatal intensive care unit; phylogroup A was associated with a stay on the ward; and phylogroups B2 and D were associated with neonates delivered vaginally. Evaluation of the effect of different E. coli phylogroups, with and without identified virulence determinants, on the gut of neonatal mice showed histopathological changes in the mucosa. The severity of the changes could be correlated with the presence of virulence determinants, irrespective of the phylogroup.

Genetic characterisation of extended-spectrum β-lactamases in Escherichia coli isolated from retail chicken products including CTX-M-9 containing isolates: a food safety risk factor

1. Bacterial resistance to β-lactam antibiotics has risen dramatically in Escherichia coli from food animals. In a previous study, 29 randomly selected chicken products, collected in Portugal, were analysed for the presence of extended-spectrum β-lactamases (ESBLs)-producing E. coli; and during this study the genetic characterisation of ESBLs genes was investigated. 2. The presence of genes encoding TEM, OXA, SHV, and CTX-M type beta-lactamases was studied by PCR followed by sequencing. Additionally, other mechanisms of antimicrobial resistance, phylogenetic groups and the presence of virulence determinants were evaluated among the isolates. 3. β-lactamases genes were identified as follows: bla CTX-M-14 (n = 4), bla CTX-M-1 (n = 2), bla CTX-M-9 (n = 4) and bla TEM-52 (n = 13). Mutations at positions −42, −18, −1, and +58 of ampC promoter region were identified in 4 non-ESBL-producing isolates. The tet(A) or tet(B) genes were identified in all tetracycline-resistant isolates; the aadA gene detected in 8 of 10 streptomycin-resistant isolates; the aac(3)-II gene in all gentamicin-resistant isolates; the cmlA gene in the chloramphenicol-resistant isolate; and sul1 and/or sul2 and/or sul3 genes were found in all trimethoprim-sulfamethoxazole-resistant isolates. The intI1 gene was detected in 8 trimethoprim-sulfamethoxazole-resistant isolates and the intI2 gene in 4 isolates; one gene cassette arrangements were identified among class 1 integrons (dfrA1 + aadA1) and among the class 2 integrons (dfrA1 + sat2 + aadA1). Among cefotaxime-resistant isolates, 16 belonged to A or B1 phylogenetic groups, while 11 isolates were classified into the D or B2 phylogroups. At least one virulence-associated gene (aer, fimA, or papC) was detected in 74·1% of the cefotaxime-resistant isolates. 4. Because ESBLs-producing bacteria are resistant to a broad range of β-lactams, infections caused by these organisms complicate therapy and limit treatment options.

Antibiotic resistance and virulence traits in clinical and environmental Enterococcus faecalis and Enterococcus faecium isolates

This study compared virulence and antibiotic resistance traits in clinical and environmental Enterococcus faecalis and Enterococcus faecium isolates. E. faecalis isolates harboured a broader spectrum of virulence determinants compared to E. faecium isolates. The virulence traits Cyl-A, Cyl-B, Cyl-M, gel-E, esp and acm were tested and environmental isolates predominantly harboured gel-E (80% of E. faecalis and 31.9% of E. faecium) whereas esp was more prevalent in clinical isolates (67.8% of E. faecalis and 70.4% of E. faecium). E. faecalis and E. faecium isolated from water had different antibiotic resistance patterns compared to those isolated from clinical samples. Linezolid resistance was not observed in any isolates tested and vancomycin resistance was observed only in clinical isolates. Resistance to other antibiotics (tetracycline, gentamicin, ciprofloxacin and ampicillin) was detected in both clinical and water isolates. Clinical isolates were more resistant to all the antibiotics tested compared to water isolates. Multi-drug resistance was more prevalent in clinical isolates (71.2% of E. faecalis and 70.3% of E. faecium) compared to water isolates (only 5.7% E. faecium). tet L and tet M genes were predominantly identified in tetracycline-resistant isolates. All water and clinical isolates resistant to ciprofloxacin and ampicillin contained mutations in the gyrA, parC and pbp5 genes. A significant correlation was found between the presence of virulence determinants and antibiotic resistance in all the isolates tested in this study (p<0.05). The presence of antibiotic resistant enterococci, together with associated virulence traits, in surface recreational water could be a public health risk.

Molecular Epidemiologic Analysis ofEnterococcus faecalisIsolates in Cuba by Multilocus Sequence Typing

We carried out the first study of Enterococcus faecalis clinical isolates in Cuba by multilocus sequence typing linking the molecular typing data with the presence of virulence determinants and the antibiotic resistance genes. A total of 23 E. faecalis isolates recovered from several clinic sources and geographic areas of Cuba during a period between 2000 and 2005 were typed by multilocus sequence typing. Thirteen sequence types (STs) including five novel STs were identified, and the ST 64 (clonal complex [CC] 8), ST 6 (CC2), ST 21(CC21), and ST 16 (CC58) were found in more than one strain. Sixty-seven percent of STs corresponded to STs reported previously in Spain, Poland, and The Netherlands, and other STs (ST115, ST64, ST6, and ST40) were genetically close to those detected in the United States. Prevalence of both antimicrobial resistance genes [aac(6')-aph(2''), aph(3'), ant(6), ant(3'')(9), aph(2'')-Id, aph(2'')-Ic, erm(B), erm(A), erm(C), mef(A), tet(M), and tet(L)] and virulence genes (agg, gelE, cylA, esp, ccf, and efaAfs) were examined by polymerase chain reaction. Aminoglycoside resistance genes aac(6')-Ie-aph(2'')-Ia, aph(3'), ant(6), ant(3'')(9) were more frequently detected in ST6, ST16, ST23, ST64, and ST115. The multidrug resistance was distributed to all STs detected, except for ST117 and singleton ST225. The presence of cyl gene was specifically linked to the ST64 and ST16. Presence of the esp, gel, and agg genes was not specific to any particular ST. This research provided the first insight into the population structure of E. faecalis in Cuba, that is, most Cuban strains were related to European strains, whereas others to U.S. strains. The CC2, CC21, and CC8, three of the biggest CCs in the world, were evidently circulating in Cuba, associated with multidrug resistance and virulence traits.

Phenotypic typing, technological properties and safety aspects of Lactococcus garvieae strains from dairy environments

To characterize Lactococcus garvieae strains of dairy origin and to determine their technological properties and safety for their possible use in starter culture preparation. Forty-seven L. garvieae isolates, recovered from two artisanal Italian cheeses were studied, in comparison with 12 fish isolates and the type strain of the species. Phenotypic typing revealed that the strains could be differentiated on the basis of their ecological niche of origin in lactose positive strains (all isolated from dairy sources) and lactose negative strains (all isolated from fish). Furthermore, the strains exhibited a high degree of physiological variability, showing the presence of 26 different biotypes. The strains possessed moderate acidifying and proteolytic activities and did not produce bacteriocins. A safety investigation revealed that all strains were sensitive to vancomycin and moderately resistant to kanamycin; some biotypes were tetracycline resistant. Production of biogenic amines or presence of genes encoding virulence determinants occurred in some isolates. The prevalence of L. garvieae in some artisanal Italian cheeses can be linked to the typicity of the products. Although in a few cases an antimicrobial resistance or a presence of virulence determinants may imply a potential hygienic risk, most of the strains showed positive properties for their possible adjunction in a starter culture preparation, to preserve the natural bacterial population responsible for the typical sensorial characteristics of the traditional raw milk cheeses. L. garvieae strains can be considered an important part of the microbial population associated with the natural fermentation of artisanal Italian cheeses. A deepened characterization of the strains may aid in understanding the functional and ecological significance of their presence in dairy products and in selecting new strains for the dairy industry.

Molecular characterization, technological properties and safety aspects of enterococci from 'Hussuwa', an African fermented sorghum product

To identify enterococci from Hussuwa, a Sudanese fermented sorghum product, and determine their technological properties and safety for possible inclusion in a starter culture preparation. Twenty-two Enterococcus isolates from Hussuwa were identified as Enterococcus faecium by using phenotypic and genotypic tests such as 16S rDNA gene sequencing, RAPD-PCR and restriction fragment length polymorphism of the 16S/23S intergenic spacer region fingerprinting. Genotyping revealed that strains were not clonally related and exhibited a considerable degree of genomic diversity. Some strains possessed useful technological properties such as production of bacteriocins and H2O2 or utilization of raffinose and stachyose. None produced alpha-amylase or tannase. A safety investigation revealed that all strains were susceptible to the antibiotics ampicillin, gentamicin, chloramphenicol, tetracycline and streptomycin, but some were resistant to ciprofloxacin, erythromycin, penicillin and vancomycin. Production of biogenic amines or presence of genes encoding virulence determinants occurred in some strains. Enterococcus faecium strains are associated with fermentation of Sudanese Hussuwa. Some strains exhibited useful technological properties such as production of antimicrobial agents and fermentation of indigestible sugars, which may aid in stabilizing and improving the digestibility of the product respectively. Enterococci were shown to play a role in the fermentation of African foods. While beneficial properties of these bacteria are indicated, their presence in this food may also imply a hygienic risk as a result of antimicrobial resistances or presence of virulence determinants.

The link between phylogeny and virulence in Escherichia coli extraintestinal infection.

Previous studies suggesting a link between Escherichia coli phylogenetic groups and extraintestinal virulence have been hampered by the difficulty in establishing the intrinsic virulence of a bacterial strain. Indeed, unidentified virulence factors do exist, and the susceptibility of the host to infection is highly variable. To overcome these difficulties, we have developed a mouse model of extraintestinal virulence to test the virulence of the strains under normalized conditions. We then assessed the phylogenetic relationships compared to the E. coli reference (ECOR) collection, the presence of several known virulence determinants, and the lethality to mice of 82 human adult E. coli strains isolated from normal feces and during the course of extraintestinal infections. Commensal strains belong mainly to phylogenetic groups A and B1, are devoid of virulence determinants, and do not kill the mice. Strains exhibiting the same characteristics as the commensal strains can be isolated under pathogenic conditions, thus indicating the role of host-dependent factors, such as susceptibility linked to underlying disease, in the development of infection. Some strains of phylogenetic groups A, B1, and D are able to kill the mice, their virulence being most often correlated with the presence of virulence determinants. Lastly, strains of the B2 phylogenetic group represent a divergent lineage of highly virulent strains which kill the mice at high frequency and possess the highest level of virulence determinants. The observed link between virulence and phylogeny could correspond to the necessity of virulence determinants in a genetic background that is adequate for the emergence of a virulent clone, an expression of the interdependency of pathogenicity and metabolic activities in pathogenic bacteria.