Quantitative relationship between capsular content and killing of K1-encapsulated Escherichia coli

More than a Pore: Nonlytic Antimicrobial Functions of Complement and Bacterial Strategies for Evasion

The complement system is an evolutionarily ancient defense mechanism against foreign substances. Consisting of three proteolytic activation pathways, complement converges on a common effector cascade terminating in the formation of a lytic pore on the target surface. The classical and lectin pathways are initiated by pattern recognition molecules binding to specific ligands, while the alternative pathway is constitutively active at low levels in circulation. Complement-mediated killing is essential for defense against many Gram-negative bacterial pathogens, and genetic deficiencies in complement can render individuals highly susceptible to infection, for example, invasive meningococcal disease. In contrast, Gram-positive bacteria are inherently resistant to the direct bactericidal activity of complement due to their thick layer of cell wall peptidoglycan. However, complement also serves diverse roles in immune defense against all bacteria by flagging them for opsonization and killing by professional phagocytes, synergizing with neutrophils, modulating inflammatory responses, regulating T cell development, and cross talk with coagulation cascades. In this review, we discuss newly appreciated roles for complement beyond direct membrane lysis, incorporate nonlytic roles of complement into immunological paradigms of host-pathogen interactions, and identify bacterial strategies for complement evasion.

How Escherichia coli Circumvent Complement-Mediated Killing

Complement is a crucial arm of the innate immune response against invading bacterial pathogens, and one of its main functions is to recognize and destroy target cells. Similar to other pathogens, Escherichia coli has evolved mechanisms to overcome complement activation. It is well known that capsular polysaccharide may confer resistance to complement-mediated killing and phagocytosis, being one of the strategies adopted by this bacterium to survive in serum. In addition, proteases produced by E. coli have been shown to downregulate the complement system. Pic, an autotransporter secreted by different pathogens in the Enterobacteriaceae family, is able to cleave C2, C3/C3b, and C4/C4b and works synergistically with human Factor I and Factor H (FH), thereby promoting inactivation of C3b. Extracellular serine protease P, a serine protease of enterohemorrhagic E. coli (EHEC), downregulates complement activation by cleaving C3/C3b and C5. StcE, a metalloprotease secreted by EHEC, inhibits the classical complement-mediated cell lysis by potentiating the action of C1 inhibitor, and the periplasmic protease Prc contributes to E. coli complement evasion by interfering with the classical pathway activation and by preventing membrane attack complex deposition. Finally, it has been described that E. coli proteins interact with negative complement regulators to modulate complement activation. The functional consequences resulting from the interaction of outer membrane protein A, new lipoprotein I, outer membrane protein W, and Stx2 with proteins of the FH family and C4b-binding protein (C4BP) are discussed in detail. In brief, in this review, we focused on the different mechanisms used by pathogenic E. coli to circumvent complement attack, allowing these bacteria to promote a successful infection.

Analysis of the SDS-PAGE patterns of outer membrane proteins from Escherichia coli strains that have lost the ability to form K1 antigen and varied in the susceptibility to normal human serum

We used SDS-polyacrylamide gel electrophoresis to investigate the outer membrane proteins (OMPs) band composition of 19 Escherichia coli K1 strains that have spontaneously lost the ability to form K1 polysaccharide capsule (E. coli K1-) and demonstrated different degrees of susceptibility to the bactericidal action of normal human serum. Presented results showed that there were differences between E. coli K1- strains in OMPs expressing capacity. The analysis performed on OMPs has not revealed a direct association between the different OMPs band composition and the susceptibility of these strains to the serum.

Prc contributes to Escherichia coli evasion of classical complement-mediated serum killing

Escherichia coli is a common Gram-negative organism that causes bacteremia. Prc, a bacterial periplasmic protease, and its homologues are known to be involved in the pathogenesis of Gram-negative bacterial infections. The present study examined the role of Prc in E. coli bacteremia and characterized the ability of the prc mutant of the pathogenic E. coli strain RS218 to cause bacteremia and survive in human serum. The prc mutant of RS218 exhibited a decreased ability to cause a high level of bacteremia and was more sensitive to serum killing than strain RS218. This sensitivity was due to the mutant's decreased ability to avoid the activation of the antibody-dependent and -independent classical complement cascades as well as its decreased resistance to killing mediated by the membrane attack complex, the end product of complement system activation. The demonstration of Prc in the evasion of classical complement-mediated serum killing of pathogenic E. coli makes this factor a potential target for the development of therapeutic and preventive measures against E. coli bacteremia.

Urothelial cultures support intracellular bacterial community formation by uropathogenic Escherichia coli

Uropathogenic Escherichia coli (UPEC) causes most community-acquired and nosocomial urinary tract infections (UTI). In a mouse model of UTI, UPEC invades superficial bladder cells and proliferates rapidly, forming biofilm-like structures called intracellular bacterial communities (IBCs). Using a gentamicin protection assay and fluorescence microscopy, we developed an in vitro model for studying UPEC proliferation within immortalized human urothelial cells. By pharmacologic manipulation of urothelial cells with the cholesterol-sequestering drug filipin, numbers of intracellular UPEC CFU increased 8 h and 24 h postinfection relative to untreated cultures. Enhanced UPEC intracellular proliferation required that the urothelial cells, but not the bacteria, be filipin treated prior to infection. However, neither UPEC frequency of invasion nor early intracellular trafficking events to a Lamp1-positive compartment were modulated by filipin. Upon inspection by fluorescence microscopy, cultures with enhanced UPEC intracellular proliferation exhibited large, dense bacterial aggregates within cells that resembled IBCs but were contained with Lamp1-positive vacuoles. While an isogenic fimH mutant was capable of forming these IBC-like structures, the mutant formed significantly fewer than wild-type UPEC. Similar to IBCs, expression of E. coli iron acquisition systems was upregulated by intracellular UPEC. Expression of other putative virulence factors, including hlyA, cnf1, fliC, kpsD, and the biofilm adhesin yfaL also increased, while expression of fimA decreased and that of flu did not change. These results indicate that UPEC differentially regulates virulence factors in the intracellular environment. Thus, immortalized urothelial cultures that recapitulate IBC formation in vitro represent a novel system for the molecular and biochemical characterization of the UPEC intracellular life cycle.

Logarithmic phase Escherichia coli K1 efficiently avoids serum killing by promoting C4bp-mediated C3b and C4b degradation

Meningitis caused by Escherichia coli K1 is a serious illness in neonates with neurological sequelae in up to 50% of survivors. A high degree of bacteremia is required for E. coli K1 to cross the blood-brain barrier, which suggests that the bacterium must evade the host defence mechanisms and survive in the bloodstream. We previously showed that outer membrane protein A (OmpA) of E. coli binds C4b-binding protein (C4bp), an inhibitor of complement activation via the classical pathway. Nevertheless, the exact mechanism by which E. coli K1 survives in serum remains elusive. Here, we demonstrate that log phase (LP) OmpA+ E. coli K1 avoids serum bactericidal activity more effectively than postexponential phase bacteria. OmpA- E. coli cannot survive in serum grown to either phase. The increased serum resistance of LP OmpA+ E. coli is the result of increased binding of C4bp, with a concomitant decrease in the deposition of C3b and the downstream complement proteins responsible for the formation of the membrane attack complex. C4bp bound to E. coli K1 acts as a cofactor to factor I in the cleavage of both C3b and C4b, which shuts down the ensuing complement cascade. Accordingly, a peptide corresponding to the complement control protein domain 3 of C4bp sequence, was able to compete with C4bp binding to OmpA and cause increased deposition of C3b. Thus, binding of C4bp appears to be responsible for survival of E. coli K1 in human serum.

Experimental models of gram-negative sepsis

BACKGROUND

The mortality rate from sepsis has improved little over the past two decades. One reason for this has been the use of flawed or inappropriate experimental models in preclinical sepsis studies.

METHODS

A literature review of animal models of sepsis was performed following a Medline search based on the following medical subject headings: disease models, endotoxin, inflammation, peritonitis and sepsis. Additional references were identified from the papers identified in the search.

RESULTS AND CONCLUSION

Many animal models of sepsis have been described but none has proved to be superior. Extrapolation of results from endotoxicosis or bacterial infusion models should be regarded with caution. Peritonitis models should be accepted as the 'gold standard' but the use of appropriate virulent bacterial species needs to be ensured. A standardized panel of animal models for the preclinical assessment of immunomodulatory agents should be established, including at least one immuno- suppressed model to simulate the immunocompromised patient with sepsis. A uniform and valid definition of sepsis applicable to both small and large animal species is required.

Loss of resistance to ingestion and phagocytic killing by O(-) and K(-) mutants of a uropathogenic Escherichia coli O75:K5 strain

To determine the importance of the O75 O antigen and the K5 capsular antigen in resistance to phagocytosis and phagocytic killing, we used previously described O75(-) and K5(-) mutants from an O75(+) K5(+) wild-type uropathogenic Escherichia coli strain in phagocytosis assays with polymorphonuclear leukocytes (PMNs) and monocytes. At a 10-to-1 ratio of bacteria to phagocytes and in the presence of 10% serum, the parental strain GR-12 was resistant to both PMNs and monocytes over a 2-h incubation period. The O75(-) and K5(-) mutants were similar in sensitivity to killing by both PMNs and monocytes, decreasing in viability by 80% in the first hour. Yet, a significant difference in killing between the O75(-) and K5(-) mutants was observed in the first 15 min of incubation. The K5(-) mutant decreased in numbers by almost 60%, while the O75(-) mutant increased in numbers similarly to GR-12 in the first 15 min. The difference in killing was found not to be due to the rate of opsonization. To further determine the mechanism of resistance, a fluorescence assay was used to differentiate attached and internalized bacteria. The K5 capsule hindered the association of both the wild-type strain and the O75(-) mutant in the initial incubation time with PMNs. In conclusion, both the K5 capsule and O75 O antigen play crucial roles in resistance to phagocytosis over time.

Differential expression of Vibrio vulnificus capsular polysaccharide

Vibrio vulnificus is a human pathogen whose virulence has been associated with the expression of capsular polysaccharide (CPS). Multiple CPS types have been described; however, virulence does not appear to correlate with a particular CPS composition. Reversible-phase variation for opaque and translucent colony morphologies is characterized by changes in CPS expression, as suggested by electron microscopy of cells stained nonspecifically with ruthenium red. Isolates with opaque colony morphologies are virulent and appear to be more thickly encapsulated than naturally occurring translucent-phase variants, which have reduced, patchy, or absent CPS. Previously, we have shown that the virulence of translucent-phase variants was intermediate between opaque-phase variants and acapsular transposon mutants, suggesting a correlation between virulence and the amount of CPS expressed. In the present study, CPS expression of phase variants and genetically defined mutants of V. vulnificus M06-24/O was examined by using a CPS-specific monoclonal antibody with an enzyme-linked immunosorbent assay, flow cytometry, and immunoelectron microscopy. Semiquantitative analyses of CPS expression correlated well among these assays, confirming that the translucent-phase variant was intermediate in CPS expression and retained type I CPS-specific epitopes. Cell surface expression of CPS varied with the growth phase, increasing during logarithmic growth and declining in stationary culture. Significantly greater CPS expression (P = 0.026) was observed for cells grown at 30 degrees C than for those at 37 degrees C. These studies confirm that phase variation and virulence in V. vulnificus correlate with the amount of CPS expressed and demonstrate the fluidity of bacterial polysaccharide expression in response to environmental conditions.

Relationship between resistance to complement, virulence and outer membrane protein patterns in pathogenic Escherichia coli O2 isolates

To establish a possible relationship between resistance to complement, virulence and outer membrane protein banding patterns, ten E. coli O2 strains isolated from chickens with colibacillosis were studied for: (1) resistance to the bactericidal effect of complement by a quantitative microtiter method, (2) virulence, as determined by chicken lethality test, and (3) outer membrane protein banding patterns yielded by SDS-polyacrylamide gel electrophoresis. The ten isolates were classified into three groups: (1) Group 1, consisting of four isolates showed: (a) high resistance to complement, (b) high virulence, and (c) different pattern between 35 and 40 kDa with a weak peptide band at 35 kDa. (2) Group 2, consisting of one isolate showed: (a) high resistance to complement, (b) low virulence, and (c) a weak peptide band at 35 kDa. (3) Group 3, consisting of five isolates showed: (a) low resistance to complement, (b) low virulence, and (c) identical OMP pattern between 35 and 40 kDa exhibiting a strong peptide band at 35 kDa. The results suggest that high resistance to complement may be necessary but no sufficient for virulence and that OMP banding patterns may be a marker for virulence.

Klebsiella spp. as nosocomial pathogens: epidemiology, taxonomy, typing methods, and pathogenicity factors

Bacteria belonging to the genus Klebsiella frequently cause human nosocomial infections. In particular, the medically most important Klebsiella species, Klebsiella pneumoniae, accounts for a significant proportion of hospital-acquired urinary tract infections, pneumonia, septicemias, and soft tissue infections. The principal pathogenic reservoirs for transmission of Klebsiella are the gastrointestinal tract and the hands of hospital personnel. Because of their ability to spread rapidly in the hospital environment, these bacteria tend to cause nosocomial outbreaks. Hospital outbreaks of multidrug-resistant Klebsiella spp., especially those in neonatal wards, are often caused by new types of strains, the so-called extended-spectrum-beta-lactamase (ESBL) producers. The incidence of ESBL-producing strains among clinical Klebsiella isolates has been steadily increasing over the past years. The resulting limitations on the therapeutic options demand new measures for the management of Klebsiella hospital infections. While the different typing methods are useful epidemiological tools for infection control, recent findings about Klebsiella virulence factors have provided new insights into the pathogenic strategies of these bacteria. Klebsiella pathogenicity factors such as capsules or lipopolysaccharides are presently considered to be promising candidates for vaccination efforts that may serve as immunological infection control measures.

Comparison of loss of serum resistance by defined lipopolysaccharide mutants and an acapsular mutant of uropathogenic Escherichia coli O75:K5

In order to determine the importance of the O75 O antigen versus the K5 capsular antigen and the bimodal distribution of lipopolysaccharides (LPSs) in protection from complement-mediated lysis, mutants were made by insertion of a cat or an aphA gene in or in place of genes necessary for the synthesis of LPS and/or the K antigen of an O75(+) K5(+) uropathogenic Escherichia coli strain, GR-12. Mutations were made in the following genes: the rfbD gene (required for the synthesis of TDP-rhamnose), the rfbKM genes (necessary for the synthesis of GDP-mannose), the rol gene (regulating O-antigen length), the kfiC gene (encoding a putative glycosyltransferase), and the kfiC-rfbD genes. The resulting phenotypes were rough (O75(-)), core plus one partial O-antigen subunit, random distribution of O-antigen chain lengths, acapsular (K5(-)), and O75(-) K5(-), respectively. All five mutants and GR-12 were analyzed for survival in 80% serum. The GR-12 parent was resistant, exhibiting a 500% increase in numbers. The rol, rfbKM, rfbD, and kfiC-rfbD mutants were sensitive, experiencing 99%, 99.9%, 99.9%, and at least 99.999% killing, respectively, in the first hour. The kfiC mutant, however, increased in numbers in the first hour but experienced delayed sensitivity, decreasing in viability by 80% in the third hour. Single mutants were complemented with the wild-type gene in trans, showing restoration of the wild-type phenotype and serum resistance. Therefore, the O75 antigen is more important for survival in serum than the K5 antigen, and regulation of the O75 O-antigen chain length is crucial for protection of the bacteria from complement-mediated lysis.

Analysis of the G93E mutant allele of KpsM, the membrane component of an ABC transporter involved in polysialic acid translocation in Escherichia coli K1

KpsM is an integral membrane protein involved in the translocation of the polysialic acid capsule of Escherichia coli K1. The kpsMG93E allele is a point mutation in the first cytoplasmic loop (Cl) of KpsM which partially disrupts translocation of the capsule. While producing polymer of wild-type length, strains harboring the G93E allele exhibit a decreased production of capsular polymer and a reduced rate of polymer translocation to the cell surface.

Cell growth rate regulates expression of group B Streptococcus type III capsular polysaccharide

The capsular polysaccharide (CPS) of group B streptococci (GBS) is an important virulence factor that also serves to protect cells from nonspecific host defense mechanisms. Expression of CPS by GBS, as with other encapsulated bacterial pathogens, is not constitutive but varies during growth in vitro and in primary cultures isolated from different sites of infection. Despite this understanding, little is known about regulation of this surface-expressed carbohydrate antigen in GBS. Here we report that expression of type III CPS by GBS strain M781 grown in continuous culture with a modified chemically defined medium is regulated by growth rate. Cells in steady state at mass doubling times (tds) of 0.8, 1.4, and 1.6 h expressed an average of sixfold more cell-associated CPS than did cells held at tds of 2.3 and 11 h. Strain M781 grown at a td of 1.4 h repeatedly produced more type III CPS than those held at a td of 11.0 h, even when limited for glucose, pyridoxamine, or thiamine. In our studies, > or = 93% of the total CPS expressed by strain M781 was cell associated. Strain M781 grown at a td of 11.0 h (i.e., lowered CPS expression) was susceptible to in vitro complement-mediated opsonophagocytosis and killing by human peripheral blood leukocytes, whereas cells grown at a td of 1.4 h (i.e., higher CPS expression) were not killed unless type III CPS-specific antibody was present. Factors that allow GBS to asymptomatically colonize women yet cause invasive infection to both mother and infant are poorly understood. Our results shed new light on parameters that regulate the pathogenic potential of GBS and may also serve as a way to discern more fully the genetics and biochemistry of GBS capsule synthesis.

Enhanced protection by use of a combination of anticapsule and antilipopolysaccharide monoclonal antibodies against lethal Escherichia coli O18K5 infection of mice

To study antibody-mediated protection against Escherichia coli peritonitis in BALB/c mice, monoclonal antibodies (MAbs) were generated against the capsule (K5) and the lipopolysaccharide (O18) of E. coli. Flow cytometric analysis with two selected immunoglobulin M MAbs revealed that bacteria were antigenically heterogeneous. Arbitrarily, three subpopulations in E. coli O18K5 cultures could be distinguished by double immunofluorescence. A subpopulation bound only the anti-K5 MAb, and another subpopulation bound only the anti-O18 MAb. An intermediate subpopulation, however, bound both MAbs. In agreement with this result, combinations of both MAbs enhanced phagocytosis of fluorescein isothiocyanate-labeled bacteria by human polymorphonuclear leukocytes and mouse macrophage J774 cells as well. In protection experiments, combinations of both MAbs, preincubated with 3 50% lethal doses of E. coli O18K5, protected all mice upon intraperitoneal challenge. Relatively high doses of either MAb alone proved to be not fully protective in this infection model. Protection of mice by the combination of MAbs was associated with significantly lower (P < 0.02) tumor necrosis factor levels in serum 90 min after challenge compared with any other treatment group. Similarly, prophylactic administration of MAbs yielded significantly lower (P < 0.01) tumor necrosis factor levels in mice that received the combination of MAbs than in any other treatment group.

The importance of a lipopolysaccharide-initiated, cytokine-mediated host defense mechanism in mice against extraintestinally invasive Escherichia coli

Extraintestinally invasive Escherichia coli (EC) that possess both a complete LPS and K1 capsule evade both complement-mediated bacteriolysis and neutrophil-mediated killing. Since C3H/HeJ mice that are hyporesponsive to LPS were uniquely susceptible to lethal infection with EC of this phenotype, we speculated there was an LPS-initiated host defense mechanism against this pathogenic phenotype. The LPS-normoresponsive C3H/HeN as well as the C3H/HeJ mice cleared these EC from the circulation within 4 h of intravenous administration. Whereas electron micrographs of the liver demonstrated these EC undergoing degeneration within the phagolysosomes of of both macrophages and Kupffer cells of C3H/HeN mice, these EC replicated within these cells of the C3H/HeJ mice. Restoration of anti-EC activity of C3H/HeJ mice occurred with activation of Kupffer cells and peritoneal macrophages in vivo with BCG and in vitro with IFN-gamma, but not with LPS. Pretreatment of C3H/HeJ mice with a combination of recombinant murine IL-1 and TNF-alpha also restored the killing of K1(+)-EC but did not enhance the killing of a K1(-)-EC mutant. These data are consistent with the hypothesis that (a) there is no intrinsic inability of C3H/HeJ phagocytes to kill EC, but (b) an LPS-initiated, cytokine-mediated host defense mechanism is required for such killing. These studies emphasize the importance of bacterial surface characteristics in the interaction with specific host defenses.

Polysaccharide capsule-mediated resistance to opsonophagocytosis in Klebsiella pneumoniae

The polysaccharide capsule of Klebsiella pneumoniae is an important virulence factor that confers resistance to phagocytosis. The treatment of encapsulated bacteria with salicylate to inhibit capsule expression was found to enhance the phagocytosis of encapsulated bacteria by human neutrophils only in the presence of cell surface-specific antibodies. Both type-specific rabbit antisera and anticapsular human hyperimmune globulin were employed as opsonins. Salicylate significantly enhanced phagocytosis with homologous, but not heterologous, whole-cell antisera. Antisera, diluted 1:40, no longer opsonized fully encapsulated bacteria but promoted the uptake of multiple salicylate-treated bacteria in > 90% of neutrophils. Salicylate (0.25 to 1.0 mM) also enhanced opsonization with globulin against homologous bacteria. Higher salicylate levels (1 to 2.5 mM) enhanced the opsonization of heterologous serotypes with human globulin. The nature of antibody attachment to encapsulated bacteria was determined by immunofluorescence. Even after the addition of purified capsular polysaccharide to prevent phagocytosis, K-specific antibodies attached in large amounts to bacteria. K-specific antibodies reacted with antigens throughout the capsule and showed a predilection for a denser inner layer of the capsule, indicating that many of the K-specific antibodies may be masked underneath the capsule surface. K-specific antibodies can also be rendered nonfunctional by soluble, cell-free capsular antigen. In culture, large quantities of soluble capsular polysaccharide extrude from bacteria after overnight growth. The reduction in capsule expression caused by salicylate largely affected the soluble, cell-free fraction. Purified capsular polysaccharide was shown to retard the opsonophagocytosis of salicylate-treated bacteria in a concentration-dependent manner. However, extensive washing of encapsulated bacteria to remove loosely attached capsular material did not significantly enhance opsonophagocytosis. In conclusion, cell-free capsule and cell-associated capsule are antiphagocytic; both act to neutralize K-specific antibodies by binding or concealment. Salicylate-mediated inhibition of capsule expression, particularly of the cell-free fraction, improved K-specific opsonization dramatically.

Current status of meningococcal group B vaccine candidates: capsular or noncapsular?

Meningococcal meningitis is a severe, life-threatening infection for which no adequate vaccine exists. Current vaccines, based on the group-specific capsular polysaccharides, provide short-term protection in adults against serogroups A and C but are ineffective in infants and do not induce protection against group B strains, the predominant cause of infection in western countries, because the purified serogroup B polysaccharide fails to elicit human bactericidal antibodies. Because of the poor immunogenicity of group B capsular polysaccharide, different noncapsular antigens have been considered for inclusion in a vaccine against this serogroup: outer membrane proteins, lipooligosaccharides, iron-regulated proteins, Lip, pili, CtrA, and the immunoglobulin A proteases. Alternatively, attempts to increase the immunogenicity of the capsular polysaccharide have been made by using noncovalent complexes with outer membrane proteins, chemical modifications, and structural analogs. Here, we review the strategies employed for the development of a vaccine for Neisseria meningitidis serogroup B; the difficulties associated with the different approaches are discussed.

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.

Influence of some bacterial and host factors on colonization and invasiveness of Escherichia coli K1 in neonatal rats

Of 209 healthy infants examined, 44 (21.1%) carried Escherichia coli K1 in their feces. Of these 44 isolates, 36 (81.8%) were attributed to 10 different known clonal groups of E. coli K1 and 4 isolates represented unknown types. The influence of mannose-resistant (MR) adhesins, aerobactin production, and resistance to serum on colonization and invasiveness of E. coli K1 in orally infected inbred LEW baby rats was investigated. Strains expressing MR adhesins had significantly higher colonization and invasion rates than non-MR strains did. Mixed-infection experiments of LEW rats revealed interactions between different types of E. coli K1 strains affecting colonization and invasion rats. P-fimbriated strains appeared to have a selective advantage for colonization. The bacteremic potentials of different E. coli K1 strains could not be associated with their resistance to sera from LEW rats free of members of the family Enterobacteriaceae. No differences in virulence between fecal E. coli K1 isolates and clinical isolates from diseased humans were found. An influence of the major histocompatibility complex on host susceptibility to invasive E. coli K1 was indicated by comparing the parental LEW rat strain with different congenic LEW strains (RT1).

Structures of the O1B and O1C lipopolysaccharide antigens of Escherichia coli

The O-specific moieties of the O1B antigen (lipopolysaccharide) from Escherichia coli O1B:K1 and the O1C antigen from E. coli O1C:K- both consist of L-rhamnose, D-galactose, N-acetyl-D-glucosamine, and N-acetyl-D-mannosamine in a molar ratio of 2:1:1:1. By using fragmentation procedures, methylation analysis, and one- and two-dimensional nuclear magnetic resonance spectroscopy, the structures of these polysaccharides were found to be [formula: see text] In the O1B polysaccharide X is 2, and in the O1C polysaccharide X is 3. With the recently published structure of the O1A polysaccharides (B. Jann, A. S. Shashkov, D. S. Gupta, S. M. Panasenko, and K. Jann, Carbohydr. Polym. 18:51-57 1992), three related O1 antigens are now known. Their common (O1-specific) epitope is suggested to be the side-chain N-acetyl-D-mannosamine residue.

The K1 capsule is the critical determinant in the development of Escherichia coli meningitis in the rat

Although Escherichia coli strains possessing the K1 capsule are predominant among isolates from neonatal E. coli meningitis and most of these K1 isolates are associated with a limited number of 0 lipopolysaccharide (LPS) types, the basis of this association of K1 and certain 0 antigens with neonatal E. coli meningitis is not clear. The present study examined in experimental E. coli bacteremia and meningitis in newborn and adult rats whether or not the K1 capsule and/or O-LPS antigen are critical determinants in the development of meningitis. Rats received subcutaneously at K1 E. coli strain (018+K1+) or mutants lacking either the K1 capsule (018+K1-) or 0 side-chain (018-K1+). 12-24 h later, blood and cerebrospinal fluid (CSF) specimens were obtained for quantitative cultures. The isolation of E. coli from CSF was observed in both newborn and adult rats infected with K1+ strains regardless of LPS phenotype (018+ or 18-) who also developed a high degree of bacteremia (e.g., greater than 10(4) CFU/ml of blood). In contrast, none of the newborn and adult rats infected with 018+K1- and developing bacteremia of greater than 10(4) were found to have positive CSF cultures. These findings indicate that the presence of the K1 capsule and a high degree of bacteremia are key determinants in the development of E. coli meningitis, suggesting that there may be specific binding sites present in the brain which have an affinity for the K1 capsule and thus may be responsible for the entry of K1-encapsulated E. coli into the meninges.

Salicylate or bismuth salts enhance opsonophagocytosis of Klebsiella pneumoniae

After treatment of encapsulated Klebsiella pneumoniae with salicylate or bismuth compounds, phagocytic uptake by human peripheral white blood cells or rat alveolar macrophages was assessed. Without salicylate pretreatment of bacteria, a 30-60% net increase in viable bacteria resulted in phagocytic assays after a 1 hour incubation. With salicylate pretreatment, dose-related decreases in bacterial counts were seen, achieving a maximal reduction of 60% with 240 microM salicylate pretreatment. Bacterial variants producing less capsule were more serum sensitive and more readily phagocytosed. Micrographs of Giemsa-stained cells revealed phagocytic uptake of multiple bacteria after salicylate pretreatment, but virtually no uptake of untreated bacteria. Opsonization with polyclonal antiserum decreased bacterial cell counts by 20% without and by 90% with salicylate pretreatment of bacteria. Pretreatment of bacteria with bismuth salts also enhanced opsonophagocytosis of encapsulated bacteria. Thus, agents known to reduce capsule expression in K. pneumoniae also enhance phagocytic uptake of bacteria.

The mechanism by which microorganisms avoid complement attack

The complement system provides a critical level of defense against bacterial invasion. Various microorganisms have evolved a variety of mechanisms to allow them to avoid complement lytic and opsonic activity. These range from the formation of factors that destroy activity of complement proteins to the evolution of surface structures that fail to bind, facilitate degradation of, or shed, complement proteins. The range of factors associated with bacterial complement resistance is reviewed here.

Enhanced enzyme-linked immunosorbant assay on membranes for the identification of mutants and pathogens

With variations in the concentrations of antibodies and blocking agents and reduction in incubation times, qualitative enzyme-linked immunosorbant assay can be performed in a matter of two hours, for more rapid identification of mutants and antigens.

Bacterial resistance to antibiotics: the role of biofilms

Bacteria adhere to natural and synthetic, medically important surfaces within an extracellular polymer generically termed the glycocalyx. This quasi-structure is a biofilm. The enhanced antibiotic resistance of biofilm bacteria, relative to floating (planktonic) bacteria, encourages the establishment of chronic bacterial infections. Resistance mechanisms include the hinderance of antibiotic diffusion by the glycocalyx, the physiology of the bacteria and the environment conditions of the niche in which the biofilm resides.

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.

Further electron microscopic studies on the expression of Escherichia coli group II capsules

The de novo expression of Escherichia coli K1, K5, and K12 capsules was analyzed with immunoelectron microscopy in temperature upshift experiments, with upshift from 18 degrees C (capsule restrictive) to 37 degrees C (capsule permissive). Newly produced capsular polysaccharides appeared at the cell surface atop membrane adhesion sites (Bayer's junctions). After plasmolysis of the bacteria at an early expression stage, the capsular polysaccharides were labeled at discrete sites in the periplasm by the immunogold technique. After temperature upshift in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) or chloramphenicol, the polysaccharides were labeled in the cytoplasm.

Influence of beta-lactam antibiotics on serum resistance of K1-positive blood culture isolates of Escherichia coli

The K1-positive strains of Escherichia coli are a group with considerable clinical importance, serum resistance being a common virulence factor of these strains. In the present paper, the influences of cephaloridine, imipenem, and ceftazidime on the serum resistance of eight serum-resistant K1-positive E. coli blood culture isolates with smooth-type lipopolysaccharide were studied. All strains were rendered more serum sensitive by treatment with subinhibitory concentrations of antibiotics. The amount of the reduction of serum resistance was dependent on the concentration of the antibiotic. Amounts of K1 produced under the influence of the antibiotics were measured and were found to be reduced for almost all strains tested. To further test the hypothesis that antibiotic-induced reduction of serum resistance is mediated by inhibition of K1 expression, isogenic mutants of one strain were produced by selection for resistance against infection with K1-specific bacteriophages. These mutants were found to be highly serum sensitive. We conclude from this study that beta-lactam antibiotics can render K1-positive serum-resistant strains of E. coli highly serum sensitive and that this effect is mediated by inhibition of K1 expression.

The capsular polysaccharide is a major determinant of serum resistance in K-1-positive blood culture isolates of Escherichia coli

Serum resistance is a major virulence factor of gram-negative bacteria, and K-1 polysaccharide has been shown to contribute to serum resistance in selected strains. To obtain further information about the role of K-1 in serum resistance and to find out whether loss of the ability to produce K-1 can induce loss of serum resistance, we studied the serum resistance of mutants derived from completely serum-resistant, K-1-positive blood culture isolates of Escherichia coli by selection for resistance to infection with K-1 specific bacteriophages. The amounts of K-1 polysaccharide produced by wild-type strains and mutants were measured, and outer membrane protein and lipopolysaccharide (LPS) patterns were analyzed. In each group of mutants, several highly serum-sensitive strains were found. All mutant strains expressed less K-1 than did the corresponding wild-type strains. Mutants that became highly serum sensitive always had less K-1 than did mutants with less-pronounced changes of serum resistance. A few mutants derived from different wild-type strains showed increased expression of outer membrane proteins with molecular weights of about 46,000 and 67,000. All of the wild-type strains examined had smooth-type LPS, and only two mutants had altered LPS structures; alterations of mutants in outer membrane proteins and LPS could not be correlated with alterations of serum resistance. The results indicate that for K-1-positive blood culture strains of E. coli, K-1 expression is a prerequisite for serum resistance, and loss of ability to synthesize K-1 leads to loss of serum resistance.