Protein secretion systems and adhesins: the molecular armory of Gram-negative pathogens

Protein secretion is a basic cellular function found in organisms of all kingdoms of life. Gram-negative bacteria have evolved a remarkable number of pathways for the transport of proteins across the cell envelope. The secretion systems fulfill general cellular functions but are also essential for pathogenic bacteria during the interaction with eukaryotic host cells. Secretion systems range from relatively simple structures such as type I secretion systems composed of three subunits that only secrete one substrate protein to complex machines such as type III and IV secretion systems composed of more than 20 subunits that can translocate large sets of effector proteins into eukaryotic target cells. In this review, the main structural and functional features of secretion systems are described. One subgroup of substrate proteins of secretion systems are protein adhesins. Despite the conserved function in binding to host cell ligands or to abiotic surfaces, the assembly of the various bacterial adhesins is highly divergent. Here we give an overview on the recent understanding of the assembly of fimbrial and non-fimbrial adhesins and the role of type I, III and V secretion systems and specialized branches of the general secretion pathway in their biogenesis.

E-cadherin is a receptor for the common protein pneumococcal surface adhesin A (PsaA) of Streptococcus pneumoniae

Streptococcus pneumoniae (Pnc) binds to nasopharyngeal (NP) epithelial cells in the first steps of nasopharyngeal carriage and colonization through bacterial adhesins. The pneumococcal surface adhesin A (PsaA) has previously been reported to play a significant role in pneumococcal adherence and colonization. Identification of a receptor for PsaA on human epithelium will aid in understanding the pathogenesis of this bacterium. Using recombinant PsaA covalently bound to fluorescent spheres (fluospheres), we show PsaA binds to NP cells through interaction with the human cellular receptor, E-cadherin. SDS-PAGE silver stain analysis demonstrates binding of PsaA to E-cadherin. Recombinant human E-cadherin binds to and blocks PsaA-coated fluospheres and whole transparent bacteria from adhering to NP cells, but does not block a Pnc PsaA(-) mutant. Recombinant E-selectin and human alpha(5)beta(1) integrin did not bind to or block PsaA-coated fluosphere adherence to NP cells. Likewise, if NP cells were preincubated with anti-E-cadherin antibody, there was a significant decrease (46%, P=0.05) in PsaA-coated fluosphere adherence to the cells. Additionally, when using E-cadherin transfected cells, we observed PsaA-coated fluospheres bind more efficiently to cells which express E-cadherin. This work identifies E-cadherin as a receptor on human epithelial cells for the pneumococcal surface adhesin, PsaA.

Erosion from Staphylococcus aureus biofilms grown under physiologically relevant fluid shear forces yields bacterial cells with reduced avidity to collagen

An estimated 65% of infective diseases are associated with the presence of bacterial biofilms. Biofilm-issued planktonic cells promote blood-borne, secondary sites of infection by the inoculation of the infected sites with bacteria from the intravascular space. To investigate the potential role of early detachment events in initiating secondary infections, we studied the phenotypic attributes of Staphylococcus aureus planktonic cells eroding from biofilms with respect to expression of the collagen adhesin, CNA. The collagen-binding abilities of S. aureus have been correlated to the development of osteomyelitis and septic arthritis. In this study, we focused on the impact of CNA expression on S. aureus adhesion to immobilized collagen in vitro under physiologically relevant shear forces. In contrast to the growth phase-dependent adhesion properties characteristic of S. aureus cells grown in suspension, eroding planktonic cells expressed invariant and lower effective adhesion rates regardless of the age of the biofilm from which they originated. These results correlated directly with the surface expression level of CNA. However, subsequent analysis revealed no qualitative differences between biofilms initiated with suspension cells and secondary biofilms initiated with biofilm-shed planktonic cells. Taken together, our findings suggest that, despite their low levels of CNA expression, S. aureus planktonic cells shed from biofilms retain the capacity for metastatic spread and the initiation of secondary infection. These findings demonstrate the need for a better understanding of the phenotypic properties of eroding planktonic cells, which could lead to new therapeutic strategies to target secondary infections.

Relationship between O-antigen subtypes, bacterial surface structures and O-antigen gene clusters in Escherichia coli O123 strains carrying genes for Shiga toxins and intimin

Escherichia coli O123 strains express a broad spectrum of phenotypes, H serotypes and virulence markers and are able to colonize and to cause disease in different hosts including humans. In this study, two subtypes of E. coli O123 antigen (group I and group II) have been identified based on their cross-reactions with other E. coli O antigens. Investigation of the relationship between O123 group I and group II strains by O serotyping and Fourier transform infrared spectroscopy of whole bacteria revealed surface structural differences between these two groups of E. coli O123 strains. Nucleotide sequence analysis of the O-antigen gene clusters of two E. coli O123 strains representing O123 group I and group II revealed no change at the amino acid level. These findings indicate that the differences in the surface structures of group I and group II strains are not related to genetic heterogeneity in their O-antigen gene clusters. A PCR assay based on O123 antigen-specific wzx and wzy genes was developed and found to be suitable for reliable detection of all subtypes of E. coli O123 strains, which bears an advantage over traditional serological detection.

Reduced phase switch capacity and functional adhesin expression of type 1-fimbriated Escherichia coli from immunoglobulin A-deficient individuals

The mannose-specific adhesin of type 1 fimbriae is the most common adhesin in Escherichia coli. One receptor for this adhesin is the carbohydrate chains of secretory immunoglobulin A (S-IgA), and intestinal E. coli from IgA-deficient individuals has a reduced capacity to adhere to mannose-containing receptors. Here, we investigated the expression of the mannose-specific adhesin and its capacity to switch to the fimbriated phenotype in colonic resident and transient E. coli strains isolated from control (n = 16) and IgA-deficient (n = 17) persons. Resident E. coli strains from IgA-deficient individuals displayed weaker mannose-specific adherence to colonic cells than resident strains from control individuals (21 versus 44 bacteria/cell, P = 0.0009) due to three mechanisms: a lower carriage rate of the fimH gene (90% versus 97%, not significant), more frequent failure to switch on the fim genes (30% versus 6%, P = 0.02), and the reduced adhesive potential of fimH(+) isolates capable of phase switch (26 versus 46 bacteria/cell, P = 0.02). On the other hand, resident strains from IgA-deficient individuals displayed stronger mannose-resistant adherence than resident strains from control individuals (P = 0.04) and transient strains from IgA-deficient individuals (P = 0.01). The presence of S-IgA appears to favor the establishment of E. coli clones which readily express mannose-specific adhesins in the bowel microbiota.

Mycobacterium smegmatis produces an HBHA homologue which is not involved in epithelial adherence

Mycobacterium tuberculosis produces heparin-binding hemagglutinin (TB-HBHA), an adhesin involved in binding to non-professional phagocytes and in extrapulmonary dissemination. TB-HBHA binds sulphated glycoconjugates through its C-terminal lysine-rich domain and can be purified by heparin-Sepharose chromatography. Homologues of HBHA are found in other pathogenic mycobacteria, but previous investigations failed to demonstrate them in non-pathogenic Mycobacterium smegmatis. We identified a gene encoding a HBHA-like protein, named MS-HBHA, from the complete M. smegmatis genome. The deduced MS-HBHA amino acid sequence revealed 68% identity with that of TB-HBHA and contains lysine-rich repeats in its C-terminal domain. However, in contrast to TB-HBHA, the lysine-rich domain of MS-HBHA is preceded by a stretch of acidic residues. This difference likely explains the low affinity for heparin displayed by MS-HBHA compared to TB-HBHA. Isolation by heparin-Sepharose chromatography procedure and mass spectrometry analysis indicated that MS-HBHA, similar to TB-HBHA contains several methylated lysine residues in its C-terminal domain. Although MS-HBHA is associated with M. smegmatis cell wall fractions, it does not seem to play a role in epithelial adherence and its function remains unknown. We therefore conclude that TB-HBHA may have evolved as an adhesin in pathogenic mycobacteria from a homolog that serves a different function in a saprophytic mycobacterium.

Adenylate cyclase influences filamentous haemagglutinin-mediated attachment of Bordetella pertussis to epithelial alveolar cells

Attachment to epithelial cells in the respiratory tract is a key event in Bordetella pertussis colonization. Filamentous haemagglutinin (FHA) is an important virulence factor mediating adhesion to host cells. In this study, the relevance of the interaction between FHA and adenylate cyclase toxin (ACT) during bacterial attachment was investigated. Mutants lacking either FHA or ACT showed significantly decreased adherence to epithelial respiratory cells. The use of several ACT-specific monoclonal antibodies and antiserum showed that the decrease in attachment of strains lacking ACT expression could not be explained by the adhesin-like activity of ACT, or a change of any of the biological activities of ACT. Immunoblot analysis showed that the lack of ACT expression did not interfere with FHA localization. An heparin-inhibitable carbohydrate-binding site is crucial in the process of FHA-mediated bacterial binding to epithelial cells. In the presence of heparin attachment of wild-type B. pertussis, but not of the isogenic ACT defective mutant, to epithelial cells was significantly decreased. These results suggest that ACT enhances the adhesive functions of FHA, and modifies the performance of the FHA heparin-inhibitable carbohydrate binding site. We propose that the presence of ACT in the outer membrane of B. pertussis to play a role in the functionality of FHA.

Expression of the short fimbriae of Porphyromonas gingivalis is regulated in oral bacterial consortia

The Mfa1 protein of Porphyromonas gingivalis is the structural subunit of the short fimbriae and mediates coadhesion between P. gingivalis and Streptococcus gordonii. We utilized a promoter-lacZ reporter construct to examine the regulation of mfa1 expression in consortia with common oral plaque bacteria. Promoter activity of mfa1 was inhibited by S. gordonii, Streptococcus sanguinis and Streptococcus mitis. In contrast, Streptococcus mutans, Streptococcus cristatus, Actinomyces naeslundii, Actinobacillus actinomycetemcomitans and Fusobacterium nucleatum did not affect mfa1 expression. Expression of SspA/B, the streptococcal receptor for Mfa1, was not required for regulation of mfa1 promoter activity. Proteinaceous molecule(s) in oral streptococci may be responsible for regulation of Mfa1 expression. Porphyromonas gingivalis is capable of detecting heterologous organisms, and responds to selected organisms by specific gene regulation.

Bacterial-bacterial cell interactions in biofilms: detection of polysaccharide intercellular adhesins by blotting and confocal microscopy

Adhesive interactions between bacterial cells coupled with adherence to a solid surface can lead to the formation of a biofilm. The important role of biofilm formation in the pathogenesis of certain types of infection, especially those involving indwelling medical devices, is becoming increasingly apparent. Critical to the development of a biofilm is the elaboration of exo-polysaccharide that contributes to substrate and intercellular adhesion. The synthesis and secretion of large exo-polysaccharides is a metabolically expensive process and is therefore often suppressed under conditions that favor the planktonic mode of growth. One way to identify the environmental cues that cause a given bacterial species to switch to the biofilm mode of growth is to monitor exo-polysaccharide elaboration in vitro. The exo-polysaccharide involved in biofilm formation in a number of bacterial species is a polymer of N-acetyl-glucosamine. In this chapter, we outline two methods that use wheat germ agglutinin, a lectin that binds to N-acetyl-glucosamine, to evaluate extracellular polysaccharide production by a variety of bacterial species.

Analysis of Bartonella adhesin A expression reveals differences between various B. henselae strains

Bartonella henselae causes cat scratch disease and the vasculoproliferative disorders bacillary angiomatosis and peliosis hepatis in humans. One of the best known pathogenicity factors of B. henselae is Bartonella adhesin A (BadA), which is modularly constructed, consisting of head, neck/stalk, and membrane anchor domains. BadA is important for the adhesion of B. henselae to extracellular-matrix proteins and endothelial cells (ECs). In this study, we analyzed different B. henselae strains for BadA expression, autoagglutination, fibronectin (Fn) binding, and adhesion to ECs. We found that the B. henselae strains Marseille, ATCC 49882, Freiburg 96BK3 (FR96BK3), FR96BK38, and G-5436 express BadA. Remarkably, BadA expression was lacking in a B. henselae ATCC 49882 variant, in strains ATCC 49793 and Berlin-1, and in the majority of bacteria of strain Berlin-2. Adherence of B. henselae to ECs and Fn reliably correlated with BadA expression. badA was present in all tested strains, although the length of the gene varied significantly due to length variations of the stalk region. Sequencing of the promoter, head, and membrane anchor regions revealed only minor differences that did not correlate with BadA expression, apart from strain Berlin-1, in which a 1-bp deletion led to a frameshift in the head region of BadA. Our data suggest that, apart from the identified genetic modifications (frameshift deletion and recombination), other so-far-unknown regulatory mechanisms influence BadA expression. Because of variations between and within different B. henselae isolates, BadA expression should be analyzed before performing infection experiments with B. henselae.

Adhesion characteristics of Listeria adhesion protein (LAP)-expressing Escherichia coli to Caco-2 cells and of recombinant LAP to eukaryotic receptor Hsp60 as examined in a surface plasmon resonance sensor

Listeria adhesion protein (LAP) is an important adhesion factor in Listeria monocytogenes and interacts with its cognate receptor, mammalian heat shock protein 60 (Hsp60). The genetic identity of LAP was determined to be alcohol acetaldehyde dehydrogenase (Aad). A recombinant Escherichia coli strain expressing aad confirmed the involvement of Aad in adhesion to Caco-2 cells. Binding kinetics (ka) of recombinant LAP (rLAP) to Hsp60 was examined in a surface plasmon resonance sensor and was determined to be 5.35 x 10(8) M(-1) s(-1) and it was equivalent to the binding of anti-Hsp60 antibody (ka = 2.15 x 10(9) M(-1) s(-1)) to Hsp60. In contrast, Internalin B, an adhesion/invasion protein from L. monocytogenes, used as a control, had binding kinetics (ka) of only 2.9 x 10(6) M(-1) s(-1). The KD value of rLAP was 1.68 x 10(-8) M, which was significantly lower than Internalin B (KD = 6.5 x 10(-4) M). These results suggest that Hsp60 has significantly higher avidity for anti-Hsp60 antibody and LAP than Internalin B. In summary, LAP is identified as an alcohol acetaldehyde dehydrogenase and binding of recombinant E. coli to Caco-2 cells or rLAP to Hsp60 protein was found to be highly specific.

Genotypic profile of the outer membrane proteins BabA and BabB in clinical isolates of Helicobacter pylori

Helicobacter pylori BabA is the ABO blood group antigen binding adhesin, which has a closely related paralogue (BabB) whose function is unknown. PCR and DNA sequence analysis showed extensive genotypic diversity in babA and babB across different strains, as well as within a strain colonizing an individual patient. We hypothesize that diverse profiles of babA and babB reflect selective pressures for adhesion, which may differ across different hosts and within an individual over time.

Yersinia pseudotuberculosis adhesins regulate tissue-specific colonization and immune cell localization in a mouse model of systemic infection

Yersinia pseudotuberculosis mutants deficient for the adhesins invasin and/or YadA were injected intravenously into BALB/c mice. Invasin expression inhibited colonization of the liver and spleen. YadA decreased liver colonization but promoted growth within the lung. The persistence of leukocytes within liver microabscesses correlated with enhanced colonization and lack of adhesin expression.

Demonstration of regulatory cross-talk between P fimbriae and type 1 fimbriae in uropathogenic Escherichia coli

The majority of Escherichia coli strains isolated from urinary tract infections have the potential to express multiple fimbriae. Two of the most common fimbrial adhesins are type 1 fimbriae and pyelonephritis-associated pili (Pap). Previous research has shown that induced, plasmid-based expression of a Pap regulator, papB, and its close homologues can prevent inversion of the fim switch controlling the expression of type 1 fimbriae. The aim of the present study was to determine if this cross-regulation occurs when PapB is expressed from its native promoter in the chromosome of E. coli K-12 and clinical isolates. The regulation was examined in three ways: (1) mutated alleles of the pap regulatory region, including papB and papI, that maintain the pap promoter in either the off or the on phase were exchanged into the chromosome of both E. coli K-12 and the clinical isolate E. coli CFT073, and the effect on type 1 fimbrial expression was measured; (2) type 1 fimbrial expression was determined using a novel fimS : : gfp(+) reporter system in mutants of the clinical isolate E. coli 536 in which combinations of complete fimbrial clusters had been deleted; (3) type 1 fimbrial expression was determined in a range of clinical isolates and compared with both the number of P clusters and their expression. All three approaches demonstrated that P expression represses type 1 fimbrial expression. Using a number of novel genetic approaches, this work extends the initial finding that PapB inhibits FimB recombination to the impact of this regulation in clinical isolates.

Production of protective antibodies againstPorphyromonas gingivalisstrains by immunization with recombinant gingipain domains

We studied the effect of antibodies against Porphyromonas gingivalis gingipain domains, preparing them against three recombinant fragments of RgpA (catalytic domain, r-Rgp CAT; hemagglutinin domains, r-Rgp 44 and r-Rgps 15-27) and one fragment of Kgp (catalytic domain, r-Kgp CAT). Enhancement of opsonization and killing by human polymorphonuclear leukocytes were measured in the noninvasive FDC 381 and invasive W50 strains of P. gingivalis. Anti-r-Rgp 44 was the most effective in both strains of P. gingivalis. The present findings lead us to recommend RgpA 44 as a candidate immunogen for vaccines against P. gingivalis.

Synthesis, stability, and function of cytadhesin P1 and accessory protein B/C complex of Mycoplasma pneumoniae

The genes MPN141 and MPN142 encode the major adhesin P1 and the cytadherence-related B/C proteins (P90/P40), respectively, in Mycoplasma pneumoniae. Using reverse transcriptase PCR we found open reading frames MPN140 to MPN142 constitute a polycistronic transcriptional unit. Cytadherence mutant IV-22 has a frameshift mutation in MPN141 and lacks the P1, B, or C proteins. Recombinant MPN141 and/or MPN142 were introduced into mutant IV-22 by transposon delivery in several configurations, and the levels of the P1, B, and C proteins were assessed by immunoblotting. MPN142 in mutant IV-22 has a wild-type nucleotide sequence, yet the introduction of recombinant MPN141 alone to mutant IV-22, although it restored P1 levels, failed to restore levels of B or C. In contrast, recombinant MPN141 and MPN142 delivered in cis or in trans were sufficient to restore all three proteins. Taken together, our data indicated that some but not all synthesis of B or C is dependent on coupling to the translation of P1 immediately upstream of MPN142 and demonstrated that proteins B and C are not stable in the absence of P1. The linkage of MPN141 and MPN142 at the levels of transcription, translation, and protein stability, in addition to their previously demonstrated colocalization and the requirement of B and/or C for P1 function, reinforces the conclusion that these proteins constitute a multiprotein complex that functions in receptor binding.

Induction of systemic and mucosal immune response and decrease in Streptococcus pneumoniae colonization by nasal inoculation of mice with recombinant lactic acid bacteria expressing pneumococcal surface antigen A

Mucosal epithelia constitute the first barriers to be overcome by pathogens during infection. The induction of protective IgA in this location is important for the prevention of infection and can be achieved through different mucosal immunization strategies. Lactic acid bacteria have been tested in the last few years as live vectors for the delivery of antigens at mucosal sites, with promising results. In this work, Streptococcus pneumoniae PsaA antigen was expressed in different species of lactic acid bacteria, such as Lactococcus lactis, Lactobacillus casei, Lactobacillus plantarum, and Lactobacillus helveticus. After nasal inoculation of C57Bl/6 mice, their ability to induce both systemic (IgG in serum) and mucosal (IgA in saliva, nasal and bronchial washes) anti-PsaA antibodies was determined. Immunization with L. lactis MG1363 induced very low levels of IgA and IgG, possibly by the low amount of PsaA expressed in this strain and its short persistence in the nasal mucosa. All three lactobacilli persisted in the nasal mucosa for 3 days and produced a similar amount of PsaA protein (150–250 ng per 10⁹ CFU). However, L. plantarum NCDO1193 and L. helveticus ATCC15009 elicited the highest antibody response (IgA and IgG). Vaccination with recombinant lactobacilli but not with recombinant L. lactis led to a decrease in S. pneumoniae recovery from nasal mucosa upon a colonization challenge. Our results confirm that certain Lactobacillus strains have intrinsic properties that make them suitable candidates for mucosal vaccination experiments.

Surgical injury and metabolic stress enhance the virulence of the human opportunistic pathogen Pseudomonas aeruginosa

BACKGROUND

We have shown previously that the PA-I lectin of Pseudomonas aeruginosa plays a key role in gut-derived sepsis during surgical stress. The aims of this study were to determine if the intestinal tract lumen of a stressed host contained soluble factors that could induce the expression of PA-I.

METHODS

Mice were subjected to either 30% surgical hepatectomy or sham-laparotomy, and P. aeruginosa was introduced into the cecum. Twenty-four hours later, feces were recovered, and PA-I and exotoxin A were determined by real-time polymerase chain reaction (PCR). In reiterative experiments, fecal filtrates from both hepatectomy and sham-operated mice were tested for their ability to induce PA-I expression in cultures of P. aeruginosa. Finally, the media from cultured human intestinal epithelial (Caco-2) cells stressed with excess glutamine was tested for its ability to induce the expression of PA-I in cultures of P. aeruginosa.

RESULTS

Both PA-I and exotoxin A mRNA were increased in vivo in the intestinal tract of mice subjected to 30% hepatectomy. Soluble fecal filtrates from hepatectomy mice induced PA-I in vitro. Media from epithelial cells exposed to excess glutamine alone induced PA-I expression.

CONCLUSIONS

The intestinal environment of a stressed host contains soluble factors capable of inducing lethal virulence traits in human opportunistic pathogen P. aeruginosa.

Characterization of FimH adhesins expressed by Salmonella enterica serovar Gallinarum biovars Gallinarum and Pullorum: reconstitution of mannose-binding properties by single amino acid substitution

Recombinant FimH adhesins of type 1 fimbriae from Salmonella enterica serovar Gallinarum biovars Gallinarum and Pullorum, in contrast to those of Salmonella enterica serovar Typhimurium, did not bind to high-mannose oligosaccharides or to human colon carcinoma HT-29 cells. However, mutated FimH proteins from biovar Gallinarum and biovar Pullorum, in which the isoleucine at position 78 was replaced by the threonine found in S. enterica serovar Typhimurium, bound well to glycoproteins carrying high-mannose oligosaccharides and colon carcinoma cells. The loss of sugar-binding properties by biovar Gallinarum and biovar Pullorum FimH adhesins, which are a part of the type 1 fimbriae, is most probably the result of a single T78I mutation, as was proven by site-directed mutagenesis of FimH proteins.

[Preparation of Pseudomonas aeruginosa recombinant outer-membrane protein F and the study of its antigenic properties]

In Escherichia coli M15, the gene of P. aeruginosa recombinant outer-membrane protein F (OprF) was cloned. OprF, chromatographically purified on Ni-agarose and containing an additional sequence of 6 histidines on the N-end, was obtained. The purified OprF specifically reacted with rabbit serum, hyperimmune to P. aeruginosa, and in the mice injected with this protein specific IgG antibodies were synthesized. The optimum concentrations of P. aeruginosa OprF were selected for further tests of its protective properties from infection induced by P. aeruginosa.

Neisseria meningitidis lactate permease is required for nasopharyngeal colonization

Neisseria meningitidis is a human specific pathogen that is part of the normal nasopharyngeal flora. Little is known about the metabolic constraints on survival of the meningococcus during colonization of the upper airways. Here we show that glucose and lactate, both carbon energy sources for meningococcal growth, are present in millimolar concentrations within nasopharyngeal tissue. We used a mutant defective for the uptake of lactate (C311DeltalctP) to investigate the contribution of this energy source during colonization. Explants of nasopharyngeal tissue were inoculated with the wild-type strain (C311) and C311DeltalctP; the mutant was recovered at significantly lower levels (P = 0.01) than C311 18 h later. This defect was not due to changes in the expression of adhesins or initial adhesion in C311DeltalctP to epithelial cells. Instead, lactate appears to be important energy source for the bacterium during colonization and is necessary for growth of the bacterium in nasopharyngeal tissue. Studies with other strains defective for the uptake of specific nutrients should provide valuable information about the environment in which N. meningitidis persists during carriage.

Identification of putative adhesin genes in shigatoxigenic Escherichia coli isolated from different sources

Shiga toxin-producing Escherichia coli (STEC) is an important pathogen responsible for severe human intestinal and systemic infections. The bacterial factors required for colonization of the hosts are still not well defined. In this study, the prevalence of seven putative adhesive genes that are not encoded in the locus of enterocyte effacement (LEE) in 74 STEC strains isolated from humans (n=39), food (n=6), cattle (n=11), and pigs (n=18) was investigated by PCR. In addition, Shiga toxin (stx) and intimin (eaeA including alpha, beta, gamma, delta, epsilon, zeta variants) genes were tested. The most prevalent adhesin was that encoded by toxB gene (52 of 74 isolates; 70.3%). This marker was found in all 12 strains of O157:H7 serotype and in 23 of 32 (71.9%) isolates of the O157:NM serogroup. Moreover, this gene was also present in other 17 STEC of the non-O157 serogroup. The second most prevalent adhesin was that encoded by the lpfAO157/OI-154 gene (43 isolates; 58.1%). This marker was detected in LEE-positive strains of the O157 serogroup but also in 9 LEE-negative isolates of porcine origin. Several STEC isolates tested (42 strains; 56.7%) had the efa1 gene of the Efa1 putative adhesive marker. This adhesin was almost exclusively found among eaeA-positive strains recovered from humans, food and cattle. On the other hand, iha marker was detected either in LEE-positive (29 isolates) or LEE-negative (12 strains) STEC. Only two eaeA-negative strains had the saa putative adhesive gene. These results show that STEC strains may be able to express several putative adhesins. However, further studies are needed to evaluate the role of the genes identified in the present study in the pathogenesis of human infections.

Pathoadaptive mutation that mediates adherence of shiga toxin-producing Escherichia coli O111

The adherence of pathogenic Escherichia coli strains to intestinal epithelium is essential for initiation of infection. The cad operon encodes the lysine decarboxylase (LDC) system responsible for metabolizing lysine, and this operon has been proposed as an antivirulence mechanism in enteroinvasive E. coli and Shigella flexneri and as a factor mediating E. coli O157:H7 adherence. We sought to determine whether the LDC activity was present in a phylogenetically characterized collection of diarrheagenic E. coli (DEC) strains and to establish whether its expression was associated with their adherence to tissue culture cells. LDC activity was found in most of the pathogenic E. coli strains tested and was absent from Shiga toxin-producing E. coli (STEC) O111 strains (DEC pathotype 8). Analysis of the cad region in these O111 strains indicates that the operon has been rearranged and some of the genes are either missing or disrupted. A similar rearrangement was found in an E. coli O111:H8 strain recently isolated from an outbreak in Texas. Complementation of the LDC-negative strains with the cad operon in trans restored the LDC activity and resulted in a reduction in adherence to tissue culture cells. Initial analysis of the protein profiles on the surface of the O111 strains indicates that the LDC activity has an effect on the expression of the adhesin intimin. Cadaverine had a slight effect on LDC-negative strain adhesion but none on intimin expression. Our data suggest that this pathoadaptive mutation is an important mechanism to control functions potentially implicated in the pathogenesis of these organisms.

Recognition of host immune activation by Pseudomonas aeruginosa

It is generally reasoned that lethal infections caused by opportunistic pathogens develop permissively by invading a host that is both physiologically stressed and immunologically compromised. However, an alternative hypothesis might be that opportunistic pathogens actively sense alterations in host immune function and respond by enhancing their virulence phenotype. We demonstrate that interferon-gamma binds to an outer membrane protein in Pseudomonas aeruginosa, OprF, resulting in the expression of a quorum-sensing dependent virulence determinant, the PA-I lectin. These observations provide details of the mechanisms by which prokaryotic organisms are directly signaled by immune activation in their eukaryotic host.

The effect of transformation on the virulence of Streptococcus pneumoniae

Although pneumococcus is one of the most frequently encountered opportunistic pathogen in the world, the mechanisms responsible for its infectiveness have not yet been fully understood. In this paper, we have attempted to characterize the effects of pneumococcal transformation on the pathogenesis of the organism. We constructed three transformation-deficient pneumococcal strains, which were designated as Nos. 1d, 2d, and 22d. The construction of these altered strains was achieved via the insertion of the inactivated gene, comE, to strains 1, 2 and 22. We then conducted a comparison between the virulence of the transformation-deficient strains and that of the wild-type strains, via an evaluation of the ability of each strain to adhere to endothelial cells, and also assessed psaA mRNA expression, and the survival of hosts after bacterial challenge. Compared to what was observed with the wild-type strains, our results indicated that the ability of all of the transformation-deficient strains to adhere to the ECV304 cells had been significantly reduced (p < 0.05), the expression of psaA mRNA was reduced significantly (p < 0.05) in strains 2d and 22d, and the median survival time of mice infected with strains 1d and 2d was increased significantly after intraperitoneal bacterial challenge (p < 0.05). The results of our study also clearly indicated that transformation exerts significant effects on the virulence characteristics of S. pneumoniae, although the degree to which this effect is noted appears to depend primarily on the genetic background of the bacteria.