Relative contribution of ColV plasmid and K1 antigen to the pathogenicity of Escherichia coli

Extraintestinal Pathogenic Escherichia coli: Virulence Factors and Antibiotic Resistance

The One Health approach emphasizes the importance of antimicrobial resistance (AMR) as a major concern both in public health and in food animal production systems. As a general classification, E. coli can be distinguished based on the ability to cause infection of the gastrointestinal system (IPEC) or outside of it (ExPEC). Among the different pathogens, E. coli are becoming of great importance, and it has been suggested that ExPEC may harbor resistance genes that may be transferred to pathogenic or opportunistic bacteria. ExPEC strains are versatile bacteria that can cause urinary tract, bloodstream, prostate, and other infections at non-intestinal sites. In this context of rapidly increasing multidrug-resistance worldwide and a diminishingly effective antimicrobial arsenal to tackle resistant strains. ExPEC infections are now a serious public health threat worldwide. However, the clinical and economic impact of these infections and their optimal management are challenging, and consequently, there is an increasing awareness of the importance of ExPECs amongst healthcare professionals and the general public alike. This review aims to describe pathotype characteristics of ExPEC to increase our knowledge of these bacteria and, consequently, to increase our chances to control them and reduce the risk for AMR, following a One Health approach.

The plasmid of Escherichia coli strain S88 (O45:K1:H7) that causes neonatal meningitis is closely related to avian pathogenic E. coli plasmids and is associated with high-level bacteremia in a neonatal rat meningitis model

A new Escherichia coli virulent clonal group, O45:K1, belonging to the highly virulent subgroup B2(1) was recently identified in France, where it accounts for one-third of E. coli neonatal meningitis cases. Here we describe the sequence, epidemiology and function of the large plasmid harbored by strain S88, which is representative of the O45:K1 clonal group. Plasmid pS88 is 133,853 bp long and contains 144 protein-coding genes. It harbors three different iron uptake systems (aerobactin, salmochelin, and the sitABCD genes) and other putative virulence genes (iss, etsABC, ompT(P), and hlyF). The pS88 sequence is composed of several gene blocks homologous to avian pathogenic E. coli plasmids pAPEC-O2-ColV and pAPEC-O1-ColBM. PCR amplification of 11 open reading frames scattered throughout the plasmid was used to investigate the distribution of pS88 and showed that a pS88-like plasmid is present in other meningitis clonal groups such as O18:K1, O1:K1, and O83:K1. A pS88-like plasmid was also found in avian pathogenic strains and human urosepsis strains belonging to subgroup B2(1). A variant of S88 cured of its plasmid displayed a marked loss of virulence relative to the wild-type strain in a neonatal rat model, with bacteremia more than 2 log CFU/ml lower. The salmochelin siderophore, a known meningovirulence factor, could not alone explain the plasmid's contribution to virulence, as a salmochelin mutant displayed only a minor fall in bacteremia (0.9 log CFU/ml). Thus, pS88 is a major virulence determinant related to avian pathogenic plasmids that has spread not only through meningitis clonal groups but also human urosepsis and avian pathogenic strains.

Resistance to serum complement, iss, and virulence of avian Escherichia coli

Control of avian colibacillosis is hampered by lack of easily identifiable markers for virulent Escherichia coli. Resistance to serum complement appears to be a widespread trait of virulent avian E. coil, suggesting that bacterial factors promoting survival in serum may be useful in discriminating between virulent and avirulent isolates. Such distinguishing factors may prove useful in diagnostic protocols or as targets in future colibacillosis control protocols. Interestingly, the factors responsible for resistance to complement differ in the E. coli isolated from mammalian and avian hosts, which may reflect differences in the nature of avian and mammalian colibacillosis. In some cases, genetic determinants for serum complement resistance in avian E. coli are found on aerobactin- or Colicin V-encoding plasmids. One such gene, iss, first described for its role in the serum resistance associated with a ColV plasmid from a human E. coli isolate, occurs much more frequently in isolates from birds with colibacillosis than in faecal isolates from healthy birds. Efforts to identify the genomic location of iss in a single, virulent avian E. coli isolate have revealed that it occurs in association with several purported virulence genes, all linked to a large conjugative R plasmid. At this time, it is not known whether iss merely marks the presence of a larger pathogenicity unit or is itself a contributor to virulence. Nevertheless, the presence of the complement-resistance determinant, iss, may be a marker of virulent avian E. coli exploitable in controlling avian colibacillosis.

Salmonella typhi iron uptake mutants are attenuated in mice

Iron starvation interferes drastically with the multiplication and virulence of Salmonella typhi mutants defective in enterochelin synthesis or enterochelin transport. Growth of these mutants is inhibited in the presence of human sera and unsaturated transferrin and is restored by fully saturated transferrin. The mutants exhibit decreased ability to grow in HeLa cell monolayers and are attenuated in mice. These findings are consistent with the S. typhi enterochelin system playing a role in the pathogenesis of typhoid fever.

Expression of capsular polysaccharide determines serum resistance in Escherichia coli K92

The amount of capsular polysaccharide expression has been shown to be the major determinant of serum resistance in Escherichia coli K1. E. coli K92, like K1, is a polymer of sialic acid molecules. It differs from K1 by containing both alpha (2.8) and alpha (2.9) linkages. Four strains of E. coli K92 were tested for serum resistance. Three strains were serum-resistant (50% normal human serum), one strain was moderately serum-sensitive. The serum-resistant strains expressed significantly more capsular polysaccharide than did the serum-sensitive strain. For each of the serum-resistant strains, six mutants were isolated by selection for resistance against infection with a K92-specific bacteriophage. All of the mutants expressed less capsular polysaccharide than the respective wild-type strains. All mutants were more sensitive to serum killing than the wild-type strains. In all groups, the mutants with lowest expression of capsular polysaccharide were highly serum-sensitive. Changes of outer membrane proteins or lipopolysaccharide patterns that were present in some mutants did not correlate with serum resistance properties of the mutants. Furthermore, it was investigated whether the presence of active serum had an influence on capsule expression. In the serum-sensitive strain, the presence of serum induced a significant and concentration-dependent increase of capsule expression. Serum had no effect on capsule expression by the serum-resistant strains. We conclude from the data that the expression of K92 capsular polysaccharide determines serum resistance in the strains examined.

TnphoA-mediated disruption of K54 capsular polysaccharide genes in Escherichia coli confers serum sensitivity

To assess whether non-K1, group 2 capsular serotypes are important in conferring serum resistance to extraintestinal isolates of Escherichia coli, a K54 blood isolate (CP9) was evaluated as a model pathogen. Transposon mutagenesis (TnphoA) was used to generate isogenic capsule-negative mutants. CP9 was resistant to the bactericidal effects of serum, growing in 80% serum. In contrast, all of the capsule-negative mutants had an increased sensitivity to 80% normal human serum, undergoing a 2- to 3-log kill over 3 h when starting inocula of 10(4) to 10(7) CFU/ml were used. The killing of the capsule-negative strains was mediated through the alternative complement pathway and not by lysozyme or beta-lysins. The protective effect of the K54 capsule against the bactericidal activity of serum was not through inhibition of the complement cascade, nor did it appear to be through a difference in the binding of C3.

Lipopolysaccharide-independent radioimmunoprecipitation and identification of structural and in vivo induced immunogenic surface proteins of Salmonella typhi in typhoid fever

The humoral response to Salmonella typhi is important for protective immunity against typhoid fever, as indicated by the protection obtained with killed cell vaccines and component vaccines (outer membrane proteins, Vi antigen) in animals and human beings. Nonetheless, analysis and interpretation of host humoral immune response to S. typhi surface antigens have been difficult because of the complex structure of the S. typhi envelope and the lack of purified reagents for detection of immune response to individual surface components. Normal and convalescent human sera from typhoid fever patients were absorbed with S. typhi lipopolysaccharide. These sera were used in radioimmunoprecipitation assays of whole S. typhi cells and S. typhi membranes labelled with either 125I or 35S-methionine. This strategy has permitted the unequivocal identification of a humoral immune response to structural and in vivo induced outer membrane proteins of S. typhi. In this manner, we have identified the porins, lipoprotein, the iron-starvation-induced proteins, and three proteins of 30, 18.5 and 15 kDa as surface-exposed immunogens of S. typhi in patients with typhoid fever. These studies suggest that further experimental work is needed to characterize the relevance of both anti-S. typhi outer membrane protein and antilipopolysaccharide antibodies in recovery from S. typhi infections and protective immunity.

Survey of Escherichia coli septicemia over a six-year period

Escherichia coli was the most frequent species isolated from blood cultures in the Hospital Covadonga of Oviedo (Spain) over a six-year period (474 episodes, 15.3% of the total septicemias and 2.7 episodes per 1,000 patients). Escherichia coli strains were susceptible in greater than 95% of episodes to cefoxitin, cefotaxime, gentamicin, tobramycin and amikacin. In a series of 72 episodes, microbiological features and host factors were studied. No endemic stains were found. Type 1 fimbria was detected in 73.6% of strains and P-fimbriae in 12.5%, without correlation between P-fimbria and urinary infection; 84.7% of the strains were resistant to decomplement human serum; 61.1% produced aerobactin and 20.8% were hemolytic. Factors such as age, hospital location, metastatic focus and surgical treatment were significantly correlated with morbidity and mortality. The global mortality rate was 18%, and in 8.3% of cases was directly associated with septicemia.

Virulence factors in Escherichia coli urinary tract infection

Uropathogenic strains of Escherichia coli are characterized by the expression of distinctive bacterial properties, products, or structures referred to as virulence factors because they help the organism overcome host defenses and colonize or invade the urinary tract. Virulence factors of recognized importance in the pathogenesis of urinary tract infection (UTI) include adhesins (P fimbriae, certain other mannose-resistant adhesins, and type 1 fimbriae), the aerobactin system, hemolysin, K capsule, and resistance to serum killing. This review summarizes the virtual explosion of information regarding the epidemiology, biochemistry, mechanisms of action, and genetic basis of these urovirulence factors that has occurred in the past decade and identifies areas in need of further study. Virulence factor expression is more common among certain genetically related groups of E. coli which constitute virulent clones within the larger E. coli population. In general, the more virulence factors a strain expresses, the more severe an infection it is able to cause. Certain virulence factors specifically favor the development of pyelonephritis, others favor cystitis, and others favor asymptomatic bacteriuria. The currently defined virulence factors clearly contribute to the virulence of wild-type strains but are usually insufficient in themselves to transform an avirulent organism into a pathogen, demonstrating that other as-yet-undefined virulence properties await discovery. Virulence factor testing is a useful epidemiological and research tool but as yet has no defined clinical role. Immunological and biochemical anti-virulence factor interventions are effective in animal models of UTI and hold promise for the prevention of UTI in humans.

Virulence-related genes in ColV plasmids of Escherichia coli isolated from human blood and intestines

DNA probes for the colicin V, traT, iss, and iu genes were used in this study of four representative ColV plasmids together with 200 Escherichia coli strains isolated from the stools of patients with diarrhea and 146 E. coli strains isolated from the blood of patients with bacteremia. The study indicated that the ColV plasmids are heterogeneous. Southern and colony hybridization analyses showed that in most of the colicin V-producing intestinal E. coli strains, the colicin V genes are located in the chromosome (14 of 16); in most of the colicin V-producing E. coli strains isolated from the blood, they are located in plasmids (18 of 22). In both intestinal and blood E. coli isolates, the traT, iss, and aerobactin receptor genes were present at similar frequencies, but the frequency of the aerobactin synthesis genes was significantly different. The aerobactin receptor gene was present in 25% of the intestinal E. coli strains that lack the aerobactin synthesis gene. In the blood isolates, the aerobactin synthesis and receptor genes were present at almost equal frequencies. Among the colicin V-producing isolates, the iss, traT, and iu genes were present in 95.5, 86.4, and 90.9% of the blood isolates and in only 68.8, 43.8, and 81.3% of the intestinal isolates, respectively. The ColV plasmids from blood isolates that were tested for the presence of traT, iss, and iu genes were homogeneous and had DNA sequences that hybridized with each of the probes. On the other hand, the two intestinal strains containing ColV genes in a plasmid were heterogeneous in regard to the carriage of these genes. The presence of ColV is not restricted to specific O types.

ColV increases the virulence of Escherichia coli K1 strains in animal models of neonatal meningitis and urinary infection

Isogenic Escherichia coli strains, differing in their expression of K1 antigen and ColV plasmid, were studied for their ability to produce disease. Newborn rats were used to test the ability of these strains to colonize the intestine and to produce bacteremia and meningitis; adult rats were used to test their ability to produce urinary tract infection. Colonization of intestine and bladder by K1+ ColV+ E. coli was associated with rapid induction of bacteremia and higher mortalities compared with colonization with K1+ ColV- strains. These findings suggest that the ColV plasmid could play a role in the pathogenesis of human infections.

Immune response to the iron-deprivation-induced proteins of Salmonella typhi in typhoid fever

Iron starvation conditions limited the growth of Salmonella typhi, as evidenced by an increase in the lag phase of a culture and a decrease in the number of bacteria reached in the stationary phase. The analysis of the outer membrane of bacteria grown under these conditions identified new protein components with apparent molecular weights of 83,000, 78,000, and 69,000. The extent of induction of these proteins was regulated by increased iron deprivation. Immunoblot analysis showed that the serum of patients with typhoid fever exhibited an immunoglobulin G response to these iron-deprivation-induced proteins. The results of bioassays and DNA-DNA hybridization experiments indicated that pathogenic strains of S. typhi produced enterochelin but not aerobactin. Immunodetection with an anti-FepA antiserum confirmed that one of the induced proteins is the S. typhi analog of the Escherichia coli fepA gene product. These studies suggest a role for iron uptake in the pathogenesis of typhoid fever and confirm the immunogenicity of some of the outer membrane proteins of this pathogen.

Cloning and DNA sequence of plasmid determinant iss, coding for increased serum survival and surface exclusion, which has homology with lambda DNA

Escherichia coli K12 cells carrying a cloned 1.4 kb HindIII fragment from plasmid ColV2-K94, showed increased survival in guinea pig serum. The recombinant plasmid also conferred group II surface exclusion, i.e. the cells were reduced in recipient ability towards the incoming plasmid R538drd in conjugation experiments. Southern blotting suggested homology with bacteriophage lambda DNA and to the insertion element IS2. Determination of the DNA sequence of the fragment demonstrated the presence of a truncated IS2 (165 bp), separated by 250 bp from a 900 bp stretch of homology with lambda DNA, beginning within the Rz gene and continuing in the rightward direction on the lambda map. A 97 amino acid open reading frame (ORF) adjacent to Rz and on the opposite strand, remained intact in iss, with several amino acid changes. The ORF in iss is preceded by sequences resembling prokaryotic ribosome binding sites and promoters.

Passive transfer of meningococcal group B polysaccharide antibodies to the offspring of pregnant rabbits and their protective role against infection with Escherichia coli K1

Pregnant rabbits vaccinated with meningococcal group B polysaccharide complexed to outer membrane proteins (serotype 6) responded to produce IgG, IgM and IgA anti-B polysaccharide antibodies, which were passively transferred to the offspring (IgG preferentially) and could be detected in their sera immediately after birth. These antibody levels were sustained in the mothers but diminished in the offspring to background levels at day 22 after birth. In a subsequent experiment, rabbits immunized with the group B vaccine had offspring that proved considerably more resistant to infection with Escherichia coli K1 than the control litters from non-immune mothers. Although not complete, protection was statistically of high significance and correlated well with the anti-B polysaccharide titres obtained in the mothers.

Invasion and lysis of HeLa cell monolayers by Salmonella typhi: the role of lipopolysaccharide

Adhesion to and penetration of HeLa cell monolayers by Salmonella typhi Ty2 requires the presence of a complete lipopolysaccharide as demonstrated by the inability of polysaccharide-defective mutants to invade the monolayer. Lysis of HeLa cell monolayers by Salmonella typhi Ty2 is associated with intracellular bacterial multiplication and no detectable production of extracellular toxins. The ability of Salmonella typhi to invade and lyse monolayers could provide a novel system for the study of its ability to invade the bloodstream from the intestine.

Contribution of capsular polysaccharide and surface properties to virulence of Escherichia coli K1

We examined the surface properties, susceptibility to the bactericidal activity of serum, and susceptibility to phagocytosis of Escherichia coli K1, a laboratory strain of E. coli (LE392), and strain LE392 carrying a plasmid (pKT274) incorporating a 17-kilobase insert of DNA that encodes the ability to produce surface K1 antigen. As determined by electron microscopy, LE392 was nonencapsulated but both E. coli K1 and LE392(pKT274) possessed a thin capsule. By using charged aqueous two-phase polymer systems, both E. coli K1 and LE392(pKT274) were shown to be significantly more negatively charged than LE392. E. coli K1 was resistant to the bactericidal activity of serum, but both LE392 and LE392(pKT274) were completely inhibited by neonatal serum at a concentration of 20% (vol/vol). As measured by counting endocytosed and nonendocytosed bacteria and by chemiluminescent response, E. coli K1 was highly resistant to phagocytic uptake by polymorphonuclear leukocytes, whereas LE392 was rapidly taken up by such cells; LE392(pKT274) was resistant to endocytosis, although less so than E. coli K1. Most intraphagocytic E. coli LK1 remained structurally intact for up to 60 min, whereas both LE392 and LE392(pKT274) were rapidly degraded.

Acquisition of genes from an O18:K1:H7 ColV+ strain of Escherichia coli renders intracranially-inoculated E. coli K12 highly virulent for chickens, ducks and guinea-pigs but not mice

The virulence of intracranially-inoculated mutant forms of an O18ac:K1:H7 ColV+ strain of Escherichia coli (designated MW) that lacked different combinations of its O and K antigens and ColV, and of an E. coli K12 strain to which these characters had been transmitted was studied in mice, chickens, ducks and guinea-pigs. The O18+K1+ColV+ form of MW was highly virulent for chickens and mice but the corresponding form of K12 was only highly virulent for chickens; the O18-K1-ColV- forms of both strains were of low virulence for chickens and mice. K1 was more important than O18 or ColV in determining virulence for both animal species. Ducks and guinea-pigs resembled chickens, not mice, in their response to infection with the O18+K1+ColV+form of K12. Pathogenesis studies revealed that the virulence of the forms of MW and K12 was associated with their ability to proliferate in the central nervous system; only low numbers of organisms were found in the blood and spleen of inoculated animals. The O18+K1+ColV+ form of K12 multiplied in mouse brain and in mouse blood in vitro; its multiplication in chicken blood was partially inhibited. Agglutinins to this and other forms of K12 were found in chicken serum but not in mouse serum. Large doses of mouse serum given to chickens and large doses of chicken serum given to mice did not alter the manner in which these animals responded to K12 O18+K1+ColV+ infection. Vaccination protected chickens and mice against lethal intracranial infection with the O18+K1+ColV+ forms of K12 or MW; it produced a much stronger immunity in mice against intraperitoneal challenge than against intracranial challenge.

Comparison of the antibacterial effect of uroepithelial cells from healthy donors and children with asymptomatic bacteriuria

Bacterial attachment to uroepithelial cells (UEC) and the effect of UEC on bacterial growth was investigated in 15 healthy persons and 12 patients suffering from asymptomatic bacteriuria (ABU) with recurrent urinary tract infections (UTI). Desquamated UEC and mannose-resistant Escherichia coli were co-cultivated for up to 90 min. While no difference in bacterial adherence was observed between healthy controls and patients, 33.4% of the bacteria attached to normal UEC were found to be dead under microscopic evaluation (acridine orange staining), whereas no killing effect could be observed in patients' UEC 5 min after the onset of incubation. This phenomenon was confirmed by investigating the E. coli growth rate in the presence of UEC, measured by counting bacterial colony forming units (CFU) on agar plates. While E. coli showed exponential growth in RPMI medium, the addition of normal UEC suppressed bacterial growth (P less than 0.01). UEC from patients with ABU, however, did not show this effect. It has been concluded that bacterial adhesion may initiate an epithelial defence function, present in healthy controls and lacking in ABU patients.

Pathology of cloacal bursae of gnotobiotic turkeys orally inoculated with Escherichia coli

Cloacal bursae from three-week-old gnotobiotic turkeys were examined by light and electron microscopy and bacteriologic techniques at 1, 2, 5, 8, 12, and 16 days after oral inoculation of highly virulent (group 1) and weakly virulent (group 2) Escherichia coli. In both groups a significant decrease in follicular volume and increase in interstitial volume were associated with infection. Follicular cortical, follicular medullary and total follicular transectional areas increased with time in inoculated and control turkeys. In group 1, granulocytic inflammation developed in bursae on day 1 and diminished by day 8 after inoculation. Microabscesses were present on days 5 and 8 after inoculation in less than 1% of follicles. Bacteria were seen in few follicular medullae on days 5 and 8 after inoculation; bacteremia was detected on days 1, 2, and 5 after inoculation. In group 2, pyogranulomatous bursitis was first seen at five days after inoculation and became progressively more severe with time. Follicular alterations in group 2 included abscessation, lymphocyte necrosis, reticuloepithelial hyperplasia and perifollicular fibroplasia. Ultrastructurally, follicular pads had degenerate and necrotic epithelial cells, intercellular edema, and cystic spaces that contained acidic mucosubstances and laminar deposits of calcium salts. Bacteria were seen within necrotic centers of follicular abscesses, in phagosomes of macrophages and multinucleate giant cells and within extracellular spaces of follicular pads and follicular medullae from day 5 to day 16 after inoculation; no bacteremia was detected. We conclude that E. coli passes through the bursal follicle-associated epithelium and replicates within follicular medullae, that extensive follicular necrosis is associated with persistence of E. coli in follicular medullae, and that E. coli of low virulence may cause severe pyogranulomatous bursitis in young turkeys without causing the respiratory or systemic diseases which are commonly associated with organisms of high virulence.

A plasmid-encoded outer membrane protein, TraT, enhances resistance of Escherichia coli to phagocytosis

The presence of the outer membrane protein TraT, encoded by plasmid R6-5, reduces the sensitivity of Escherichia coli cells to phagocytosis by macrophages. This effect is independent of the bacterial capsule and is more evident in the presence of adsorbed normal human serum. The property of inhibiting phagocytosis is specifically abolished by anti-TraT protein antiserum and anti-TraT immunoglobulin G but not by Fab fragments. These results indicate that the TraT protein is a passive inhibitor of phagocytosis. Inhibition of phagocytosis is produced because the TraT protein antagonizes opsonization by complement, such that C3 deposition is reduced and altered in distribution.

Phenotypic modification of the reactivity in human serum of Escherichia coli K1 isolates

Three Escherichia coli strains producing the K1 antigen and shown to be resistant to the complement-mediated bactericidal action of human serum when grown in batch culture, were cultivated in the chemostat under conditions of carbon-, nitrogen-, magnesium- and phosphate-limitation. All strains were fully serum resistant when grown under carbon-limiting conditions but became phenotypically serum sensitive when limited by magnesium. One strain, belonging to serogroup O7:K1, also displayed serum sensitivity when nitrogen limited and showed an intermediate serum response when phosphate was used as the limiting nutrient.

Monoclonal antibody detection of IncF group plasmid-encoded TraT protein in clinical isolates of Escherichia coli

The TraT protein specified by IncF group plasmids mediates surface exclusion and bacterial resistance to the lethal activities of serum. In this study, an anti-TraT protein monoclonal antibody was generated which failed to react with TraT+ bacteria but which efficiently detected solubilized TraT protein in Western blots and in an enzyme-linked immunosorbent assay. Use of this antibody to screen clinical and nonclinical isolates of Escherichia coli for the production of TraT protein revealed its presence in a modest proportion (38%) of normal fecal strains, a significantly higher proportion of clinical strains (51 to 73%), and an even higher proportion (78 to 88%) of clinical strains concomitantly producing the K1 capsule, an important virulence factor of E. coli.

Increased frequency of ColV plasmids and mannose-resistant hemagglutinating activity in an Escherichia coli K1 population

The expression of traits linked to pathogenicity was studied in a population of Escherichia coli K1 strains. It was found that E. coli K1 strains isolated from extraintestinal infection harbor the ColV plasmid and express mannose-resistant hemagglutinating activity type VI with a high frequency. The presence of these properties may play a role in the ability of some E. coli K1 serogroups to invade.