Plasmid carriage and the serum sensitivity of enterobacteria

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.

TraT lipoprotein, a plasmid-specified mediator of interactions between gram-negative bacteria and their environment

The TraT protein is a cell-surface-exposed, outer membrane lipoprotein specified by large, usually conjugative, F-like plasmids. Two biological activities have been associated with the protein: (i) prevention of self-mating of cells carrying identical or closely related conjugative plasmids, by blocking the formation of stable mating aggregates; and (ii) resistance to the bactericidal activities of serum, possibly by inhibiting the correct assembly or efficient functioning of the terminal membrane attack complex of complement. The protein therefore interacts not only with components of the outer membrane but also with specific external agents. In conjugative plasmids the traT gene lies within the region necessary for the conjugal transfer of DNA (tra), although its expression is not necessarily dependent on the expression of other tra genes. Recently, however, the gene has been discovered in isolation from other tra genes in nonconjugative virulence-associated plasmids, providing further evidence that the TraT protein may have a role in pathogenesis. The nucleotide sequences of several traT genes have been determined, and comparison of the corresponding amino acid sequences suggests that a central region of five amino acid residues flanked by hydrophobic domains determines the specificity of the protein in surface exclusion. Additionally, studies of mutants with different amino acid alterations within the hydrophobic domains have shown that insertion of charged residues disrupts normal outer membrane integrity. This review considers our current knowledge of the distribution, structure, and biological role(s) of the protein. Recent applications of the protein in studies of the unusual permeability properties of the outer membrane and for the transport of foreign antigenic determinants to the bacterial cell surface are also discussed.

The pYV plasmid of Yersinia encodes a lipoprotein, YlpA, related to TraT

A series of lipoproteins was detected in the membrane fraction of Yersinia enterocolitica W227, a typical strain from serotype O:9. At least two of them, YlpA and YlpB, are encoded by the pYV plasmid. The sequence of ylpA reveals the presence of a typical lipoprotein signal peptide. The mature YlpA protein would be 223 residues long with a calculated molecular weight of 23798 for the proteic moiety of the molecule. YlpA shares 88% identical residues with the TraT protein encoded by plasmid pED208, 80% identity with TraT proteins encoded by plasmids R100 and F, and 77% identity with the TraT protein encoded by the virulence plasmid of Salmonella typhimurium. The ylpA gene hybridized with the pYV plasmid of Yersinia pseudotuberculosis, suggesting that this gene is conserved among Yersinia spp. The production of YlpA is controlled by virF and only occurs at 37 degrees C in the absence of Ca2+ ions. This co-regulation with the yop genes suggests that ylpA is a virulence determinant. However, mutations in ylpA clearly affect neither the resistance to human serum nor the virulence for intravenously inoculated mice.

Studies on serum resistance in Escherichia coli

Serum-sensitive mutants and their serum-resistant smooth parental E. coli strains (Wf8, Wf26, and WF 52) have been investigated in respect to their binding of different complement components. These pairs consisting of a wild-type and its mutants represent a better model for the investigation of the mechanism of serum resistance than the comparison of unrelated strains. Both strains of a pair bind equivalent amounts of C3. In binding assays using radiolabeled terminal components C6, C7, C8, and C9, the serum-sensitive strains do bind more late acting components than their resistant parental strains. An active membrane attack complex stably bound to the cell surface was found on the mutants, whereas with wild-type bacteria a complex could be isolated from the supernatant which is composed of the late acting complement components and S-protein. This complex is released from the surface of the wild-type bacteria without participation of C9.

Comparison of virulence factors and R plasmids of Salmonella spp. isolated from healthy and ill swine

The antibiotic resistance and virulence profiles of Salmonella spp. isolated from healthy (group 1) and ill (group 2) swine were compared. Parameters studied included colicin and siderophore production; mannose-sensitive hemagglutination of erythrocytes; resistance to the lethal effect of serum complement; resistance to antibiotics; and the transmissibility of these characteristics to recipient organisms. Group 1 (19 isolates) had 14 serotypes, and group 2 (20 isolates) had 2 serotypes. Isolates from group 2 were resistant to more antibiotics and had a greater ability to hemagglutinate erythrocytes and transfer R plasmids to recipient organisms, but a lesser ability to produce siderophore than group 1. All 39 isolates resisted the lethal effects of serum complement. Colicin was produced by 1 of 19 from group 1 and 0 of 20 from group 2. A donor Escherichia coli isolated from a pig with enteritis transferred R plasmids to 62% of group 1 and 0% of group 2 Salmonella spp. when they were used as recipient organisms. A transconjugant from the mating of donor E. coli to a group 1 Salmonella spp. was further able to pass an R plasmid to recipient E. coli and salmonellae. Plasmid isolation from group 1 yielded 1 of 19 strains with a 56-megadalton plasmid, while 20 of 20 strains from group 2 contained three to five plasmids from 2.4 to 60 megadaltons in size.

Influence of cloned Escherichia coli hemolysin genes, S-fimbriae and serum resistance on pathogenicity in different animal models

The virulence of the uropathogenic E. coli strain 536 (O6:K15:H31) which produces the S-fimbrial adhesin (Sfa+), is serum-resistant (Sre+) and hemolytic (Hly+) and its derivatives were assessed in five different animal models. Cloned hemolysin (hly) determinants from the chromosomes of O6, O18 and O75 E. coli strains and from the plasmid pHly152 were introduced into the spontaneous Sfa-, Sre-, Hly- mutant 536-21 and its Sfa+, Sre+, Hly- variant 536-31. As already demonstrated for the 536-21 strains (Infect. Immun. 42: 57-63) the O18-hly determinant but not the plasmid-encoded hly determinant of pHly152 transformed into 536-31 contribute to lethality in a mouse peritonitis model. Similar results were obtained with both Hly- host strains and their Hly+ transformants in a chicken embryo test and in a mouse nephropathogenicity assay in which the renal bacterial counts were measured 15 min to 8 hours after i.v. infection. S-fimbriae and serum resistance had only a marginal influence in these three in vivo systems. In contrast all three factors, S-fimbriae, serum resistance and hemolysin, were necessary for full virulence in a respiratory mouse infection assay. In a subcutaneously-induced sepsis model in the mouse restoration of S-fimbriae and serum resistance and separately chromosomally-encoded hemolysis increased virulence to a level comparable to that of the parental 536 strain.

Large, unstable inserts in the chromosome affect virulence properties of uropathogenic Escherichia coli O6 strain 536

The hemolytic, uropathogenic Escherichia coli 536 (O6:K15:H31) contains two inserts in its chromosome (insert I and insert II), both of which carried hly genes, were rather unstable, and were deleted spontaneously with a frequency of 10(-3) to 10(-4). These inserts were not found in the chromosome of two nonhemolytic E. coli strains, whereas the chromosomal sequences adjacent to these inserts appeared to be again homologous in the uropathogenic and two other E. coli strains. Insert I was 75 kilobases in size and was flanked at both ends by 16 base pairs (bp) (TTCGACTCCTGTGATC) which were arranged in direct orientation. For insert I it was demonstrated that deletion occurred by recombination between the two 16-bp flanking sequences, since mutants lacking this insert still carried a single copy of the 16-bp sequence in the chromosome. Both inserts contained a functional hemolysin determinant. However, the loss of the inserts not only affected the hemolytic phenotype but led to a considerable reduction in serum resistance and the loss of mannose-resistant hemagglutination, caused by the presence of S-type fimbriae (sfa). It is shown that the Sfa-negative phenotype is due to a block in transcription of the sfa genes. Mutants of strain 536 which lacked both inserts were entirely avirulent when tested in several animal model systems.

Bactericidal and bacteriolytic activity of serum against gram-negative bacteria

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K antigen and serum sensitivity of rough Escherichia coli

We prepared bacterial hybrids which express both K1 and K27 antigens and examined the relative contributions of these capsules to serum resistance. Escherichia coli Hfr strain F639 (rough, K27+, serum sensitive) was conjugated with E. coli recipient strain E412 (rough, K1+, His- Trp- Strr, serum resistant). Transconjugants which inherited both the his and trp linked genes for K27 antigen synthesis were analyzed. These hybrids retained and expressed the K1 antigen since the K1 locus is nonallelic with K27 gene loci. Hybrid strains which express both K1 and K27 antigens exhibit serum resistance, but not at the level of the K1+ parental strain. An isogenic K1 derivative of a hybrid which expressed only K27 antigen was serum sensitive (greater than 99% kill, 60 min). These findings indicate that the presence of the K1 capsular antigen can protect some rough strains of E. coli from serum bactericidal activity, whereas K27 and perhaps other K antigens fail to provide such a protective effect.

Nucleotide sequence analysis of the complement resistance gene from plasmid R100

The multiple antibiotic resistance plasmid R100 renders Escherichia coli resistant to the bactericidal action of serum complement. We constructed a plasmid (pOW3) consisting of a 1,900-base-pair-long restriction fragment from R100 joined to a 2,900-base-pair-long fragment of pBR322 carrying ampicillin resistance. E. coli strains carrying pOW3 or R100 were up to 10,000-fold less sensitive to killing by serum complement than were plasmid-free bacteria or bacteria carrying pBR322. Nucleotide sequencing revealed that 875 of the 1,900 bases from R100 correspond exactly to part of the bacterial insertion sequence IS2. The remaining 1,075 bases contained only one sizeable open reading frame; it covered 729 base pairs (243 amino acids) and was preceded by nucleotide sequences characteristic of bacterial promoters and ribosome binding sites. The first 20 amino acids of the predicted protein showed features characteristic of a signal sequence. The remainder of the predicted protein showed an amino acid composition almost identical with that determined for the traT protein from the E. coli F factor. Southern blot analysis showed that the resistance gene from R100 does not hybridize to the serum resistance gene from ColV,I-K94 isolated by Binns et al.; we concluded that these genes are distinct.

Influence of Escherichia coli on Streptococcus faecalis in mixed cultures and experimental animal infections

Strains of Escherichia coli (four strains) and Streptococcus faecalis (three strains) randomly isolated from faecal samples were cultured together in vitro and in experimental animal infections in order to evaluate the interaction of these two species. Streptococcus faecalis was eliminated from mixed cultures and infections at population ratios of Escherichia coli to Streptococcus faecalis of 10-100:1. The mechanism involved was probably an antibiotic substance produced by Escherichia coli which was antagonistic to Streptococcus faecalis. The substance appeared to be diffusible, water-soluble, of low molecular weight and insensitive to the protease trypsin. It was produced and remained active in vitro and in vivo and can probably be classified as a microcin.

Serum resistance among Escherichia coli strains causing urinary tract infection in relation to O type and the carriage of hemolysin, colicin, and antibiotic resistance determinants

The sensitivity to normal human serum of 91 smooth strains of Escherichia coli isolated from urinary tract infections was determined. Production of hemolysin, which was common and associated primarily with the types O4, O6, O18, and O75, was significantly correlated with high levels of serum resistance, both within the total population and within individual O types. In contrast, serum resistance was not significantly associated with antibiotic resistance (whether transmissible or not), with colicinogeny in general, or with colicin V production in particular. This indicates that the carriage of R and ColV plasmids, shown previously to be capable of conferring increased levels of serum resistance on individual strains of E. coli isolated from other sources, does not play an important part in determining the serum sensitivity of the E. coli population involved in urinary tract infection.

The role of macrophages in primary and secondary infection of mice with Salmonella typhimurium

Elimination of macrophages with high-molecular dextran sulphate (DS) markedly impairs resistance of mice to primary infection with smooth, virulent strains of Salmonella typhimurium, whereas stimulation of this system by killed Bordetella pertussis organisms increases resistance. In infection with rough, avirulent strains of S. typhimurium the elimination of macrophages was not followed by an essential loss of resistance, and it appears that other non-specific defence mechanisms, for example the complement system, may have compensated for the lack of macrophages. Macrophages, therefore, play an important role in defence during primary infection with virulent strains. In immunity to challenge infection with S. typhimurium, macrophages play an even more significant role. Treatment with DS completely removes immunity, and both humoral and cell-mediated immune mechanisms seem to require the participation of macrophages.

Genetics and pathogenic role of Escherichia coli haemolysin

While clear evidence exists for the direct involvement of cytolysins in the pathogenesis of Gram-positive bacteria, the significance of Gram-negative haemolysins remains unclear. This paper presents briefly data indicating a role for haemolysin production in infections caused by Escherichia coli and also experiments which have allowed an analysis of the molecular basis of the haemolysis among pathogenic and non-pathogenic strains of this species.

Spread of R-plasmids among Escherichia coli causing urinary tract infections

The incidence of multiple-antibiotic resistance among Escherichia coli isolated from urinary tract infections at Charing Cross Hospital, London, increased over the last 10 years, and its distribution was related to O-type. Among strains of the eight O-types most frequently causing such infections at this hospital, O4, O9, and O18 had a high incidence of multiple resistance (35, 22, and 19%, respectively); O2 and O6 had a intermediate incidence (14 and 11%, respectively); and O7, O1, and O75 had a low incidence (8, 6, and less than 3%, respectively). This nonrandom distribution appears to be a consequence of unequal plasmid recipient ability. After overnight mating with antibiotic-resistant donors, R-plasmid infection frequencies among antibiotic-sensitive urinary tract isolates differed by up to 10(5)-fold, and such differences were correlated with the variation in the incidence of antibiotic resistance among the O-types. The inherent differences in the ability to achieve significant R-plasmid spread, which appear to be determined by the host, not the plasmid, may be compounded in some cases by the inhibition of potential mating partners by colicin production.

Determinants that increase the serum resistance of Escherichia coli

The rfb locus, determining biosynthesis of O8-specific lipopolysaccharide side chains, was transferred to a rough mutant of Escherichia coli; recombinants producing a complete lipopolysaccharide were more resistant to the complement-mediated bactericidal action of human serum than the rough recipient. Inheritance of the his-linked genes for K27 antigen production did not alter the response to serum. The serum resistance of strains carrying O8 side chains, but not of strains with incomplete lipopolysaccharides, was further increased by inheritance of plasmids R1 and NR1.20

Modification of bacterial serum susceptibility by rifampin

A subinhibitory concentration of rifampin converted a strain of Escherichia coli from serum resistant to serum susceptible. When continually cultured in nutrient broth containing 1.5 microgram of rifampin per ml, this strain of E. coli became susceptible to killing by both normal human serum and normal rabbit serum. Copared to the original strain, the rifampin-treated E. coli displayed no detectable change in adherence capability, but appeared less virulent in the rabbit model of endocarditis. A rifampin-resistant mutant of the E. coli strain was not found to undergo conversion to serum susceptibility upon culture in rifampin.

Bactericidal activity of human serum against Escherichia coli chi1776

Escherichia coli chi1776 is a host strain providing the EK2 level of biological containment. Laboratories equipped to use this strain may be located near medical care facilities where patients prone to infection may be found. We have therefore documented the marked susceptibility of chi1776 to killing by human serum from normal as well as ill individuals.

Plasmid-determined resistance to serum bactericidal activity: a major outer membrane protein, the traT gene product, is responsible for plasmid-specified serum resistance in Escherichia coli

Resistance to the bactericidal activity of serum appears to be an important virulence property of invasive bacteria. The conjugative multiple-antibiotic-resistance plasmid R6-5 was found to confer upon Escherichia coli host bacteria increased resistance against rabbit serum. Gene-cloning techniques were used to localize the serum resistance determinant of R6-5 to a segment of the plasmid that encodes conjugal transfer functions, and a pACYC184 hybrid plasmid, designated pKT107, that contains this segment was constructed. The generation and analysis of deletion and insertion mutant derivatives of the pKT107 plasmid that no longer specify serum resistance permitted precise localization of the serum-resistance cistron on the R6-5 map and demonstrated that this locus is coincident with that of traT, one of the two surface exclusion genes of R6-5. Examination of the proteins synthesized in E. coli minicells of pKT107 and its serum-sensitive mutant derivative plasmids confirmed that the serum-resistance gene product of R6-5 is the traT protein and showed that this protein is a major structural component (about 21,000 copies per cell) of the bacterial outer membrane.

Cloned fragments of the plasmid ColV,I-K94 specifying virulence and serum resistance

A cloned BamH1-generated fragment of ColV,I-K94 increased the virulence of Escherichia coli, causing an approximately 100-fold reduction in LD50 for chicks. A genetic determinant for resistance to the bactericidal effects of serum was mapped to a 5,300 base-pair sequence within the fragment. Neither colicin V nor immunity to colicin V affected the pathogenicity of E. coli for chicks.