Identification of novel staphylococcal virulence genes by in vivo expression technology

Staphylococcus aureus: the search for novel targets

In the UK, 20,000 cases of Staphylococcus aureus bacteraemia are reported each year, half of which are antibiotic resistant and approximately 4% are fatal, exemplifying a worldwide phenomenon of tremendous economic and human impact. Novel treatments and prophylaxis are urgently required to combat such a serious threat. A common goal in the postgenomic era is to identify new targets for drug intervention (using small molecules) and immunologicals. Several promising cellular targets are now being developed in the quest to control such a life-threatening pathogen.

The significance of infection related to orthopedic devices and issues of antibiotic resistance

Over the last 15 years, with the advent of modern standards in the control of sterility within the operating room environment and adequate protocols of peri-operative antibiotic prophylaxis, the incidence of infections associated to orthopedic implants has become very low. Nevertheless, the event of infection still represents one of the most serious and devastating complications which may involve prosthetic devices. It leads to complex revision procedures and, often, to the failure of the implant and the need for its complete removal. In orthopedics, for the enormous number of surgical procedures involving invasive implant materials, even if nowadays rare, infections have a huge impact in terms of morbidity, mortality, and medical costs. The difficult battle to prevent and fight bacterial infections associated to prosthetic materials must be played on different grounds. A winning strategy requires a clear view of the pathogenesis and the epidemiology of implant-related infections, with a special attention on the alarming phenomenon of antibiotic resistance. In this regard staphylococci are the prevalent and most important causative pathogens involved in orthopedic implant-related infections, and, thus, the main enemy to defeat. In this paper, we offer an overview of the complexity of this battleground and of the current and new, in our opinion most promising, strategies in the field of biomaterials to reduce the risks and counteract the establishment of implant infections.

The surface-associated protein of Staphylococcus saprophyticus is a lipase

Staphylococcus saprophyticus surface-associated protein (Ssp) was the first surface protein described for this organism. Ssp-positive strains display a fuzzy layer of surface-associated material in electron micrographs, whereas Ssp-negative strains appear to be smooth. The physiologic function of Ssp, however, has remained elusive. To clone the associated gene, we determined the N-terminal sequence, as well as an internal amino acid sequence, of the purified protein. We derived two degenerate primers from these peptide sequences, which we used to identify the ssp gene from genomic DNA of S. saprophyticus 7108. The gene was cloned by PCR techniques and was found to be homologous to genes encoding staphylococcal lipases. In keeping with this finding, strains 7108 and 9325, which are Ssp positive, showed lipase activity on tributyrylglycerol agar plates, whereas the Ssp-negative strain CCM883 did not. Association of enzyme activity with the cloned DNA was proven by introducing the gene into Staphylococcus carnosus TM300. When wild-type strain 7108 and an isogenic mutant were analyzed by transmission electron microscopy, strain 7108 exhibited the fuzzy surface layer, whereas the mutant appeared to be smooth. Lipase activity and the surface appendages could be restored by reintroduction of the cloned gene into the mutant. Experiments using immobilized collagen type I did not provide evidence for the involvement of Ssp in adherence to this matrix protein. Our experiments thus provided evidence that Ssp is a surface-associated lipase of S. saprophyticus.

Unraveling the secret lives of bacteria: use of in vivo expression technology and differential fluorescence induction promoter traps as tools for exploring niche-specific gene expression

A major challenge for microbiologists is to elucidate the strategies deployed by microorganisms to adapt to and thrive in highly complex and dynamic environments. In vitro studies, including those monitoring genomewide changes, have proven their value, but they can, at best, mimic only a subset of the ensemble of abiotic and biotic stimuli that microorganisms experience in their natural habitats. The widely used gene-to-phenotype approach involves the identification of altered niche-related phenotypes on the basis of gene inactivation. However, many traits contributing to ecological performance that, upon inactivation, result in only subtle or difficult to score phenotypic changes are likely to be overlooked by this otherwise powerful approach. Based on the premise that many, if not most, of the corresponding genes will be induced or upregulated in the environment under study, ecologically significant genes can alternatively be traced using the promoter trap techniques differential fluorescence induction and in vivo expression technology (IVET). The potential and limitations are discussed for the different IVET selection strategies and system-specific variants thereof. Based on a compendium of genes that have emerged from these promoter-trapping studies, several functional groups have been distinguished, and their physiological relevance is illustrated with follow-up studies of selected genes. In addition to confirming results from largely complementary approaches such as signature-tagged mutagenesis, some unexpected parallels as well as distinguishing features of microbial phenotypic acclimation in diverse environmental niches have surfaced. On the other hand, by the identification of a large proportion of genes with unknown function, these promoter-trapping studies underscore how little we know about the secret lives of bacteria and other microorganisms.

Factors characterizing Staphylococcus epidermidis invasiveness determined by comparative genomics

Virulence mechanisms of the leading nosocomial pathogen Staphylococcus epidermidis are poorly understood. We used microarray-based genome-wide comparison of clinical and commensal S. epidermidis strains to identify putative virulence determinants. Our study revealed high genetic variability of the S. epidermidis genome, new markers for invasiveness of S. epidermidis, and potential targets for drug development against S. epidermidis infections.

Staphylococcus aureus virulence factors identified by using a high-throughput Caenorhabditis elegans-killing model

Staphylococcus aureus is an important human pathogen that is also able to kill the model nematode Caenorhabditis elegans. We constructed a 2,950-member Tn917 transposon insertion library in S. aureus strain NCTC 8325. Twenty-one of these insertions exhibited attenuated C. elegans killing, and of these, 12 contained insertions in different genes or chromosomal locations. Ten of these 12 insertions showed attenuated killing phenotypes when transduced into two different S. aureus strains, and 5 of the 10 mutants correspond to genes that have not been previously identified in signature-tagged mutagenesis studies. These latter five mutants were tested in a murine renal abscess model, and one mutant harboring an insertion in nagD exhibited attenuated virulence. Interestingly, Tn917 was shown to have a very strong bias for insertions near the terminus of DNA replication.

Interaction of RRF and EF-G from E. coli and T. thermophilus with ribosomes from both origins--insight into the mechanism of the ribosome recycling step

Ribosome recycling factor (RRF), elongation factor-G (EF-G), and ribosomes from Thermus thermophilus (tt-) and Escherichia coli (ec-) were used to study the disassembly mechanism of post-termination ribosomal complexes by these factors. With tt-RRF, ec-EF-G can release bound-tRNA from ec-model post-termination complexes. However, tt-RRF is not released by ec-EF-G from ec-ribosomes. This complex with tt-RRF and ec-ribosomes after the tRNA release by ec-EF-G is regarded as an intermediate of the disassembly reaction. Not only tt-RRF, but also mRNA, cannot be released from ec-ribosomes by tt-RRF and ec-EF-G. These data suggest that the release of RRF from ribosomes is coupled or closely related to the release of mRNA during disassembly of post-termination complexes. With tt-ribosomes, ec-EF-G cannot release ribosome-bound ec-RRF even though they are from the same species, showing that proper interaction of ec-RRF and ec-EF-G does not occur on tt-ribosomes. On the other hand, in contrast to a published report, tt-EF-G functions with ec-RRF to disassemble ec-post-termination complexes. In support of this finding, tt-EF-G translocates peptidyl tRNA on ec-ribosomes and catalyzes ec-ribosome-dependent GTPase, showing that tt-EF-G has in vitro translocation activity with ec-ribosomes. Since tt-EF-G with ec-RRF can release tRNA from ec-post-termination complexes, the data are consistent with the hypothesis that the release of tRNA by RRF and EF-G from post-termination complexes is a result of a translocation-like activity of EF-G on RRF.

Effect of mild acid on gene expression in Staphylococcus aureus

During staphylococcal growth in glucose-supplemented medium, the pH of a culture starting near neutrality typically decreases by about 2 units due to the fermentation of glucose. Many species can comfortably tolerate the resulting mildly acidic conditions (pH, approximately 5.5) by mounting a cellular response, which serves to defend the intracellular pH and, in principle, to modify gene expression for optimal performance in a mildly acidic infection site. In this report, we show that changes in staphylococcal gene expression formerly thought to represent a glucose effect are largely the result of declining pH. We examine the cellular response to mild acid by microarray analysis and define the affected gene set as the mild acid stimulon. Many of the genes encoding extracellular virulence factors are affected, as are genes involved in regulation of virulence factor gene expression, transport of sugars and peptides, intermediary metabolism, and pH homeostasis. Key results are verified by gene fusion and Northern blot hybridization analyses. The results point to, but do not define, possible regulatory pathways by which the organism senses and responds to a pH stimulus.

Role of a Streptococcus gordonii copper-transport operon, copYAZ, in biofilm detachment

Streptococcus gordonii is a pioneer oral bacterium that is associated with the initiation of dental plaque development. Located downstream of the S. gordonii adc operon, which is involved in competence and biofilm formation, were three open reading frames, designated copY, copA and copZ. These open reading frames were homologous to the copYAZ genes in Streptococcus mutans that are involved in copper homeostasis and biofilm detachment. This study examined whether copYAZ genes play any role in the biofilm formation and detachment of S. gordonii. The copY gene encodes a 143-amino acid protein homologous to the negative transcriptional regulator of a copper-transport operon, copA encodes a 748-amino acid copper-transporting P-type ATPase, and copZ encodes a 69-amino acid putative metallochaperone protein in S. mutans. Each open reading frame in the copYAZ operon in S. gordonii was inactivated by insertional mutation and the growth, biofilm formation and detachment of each mutant were examined. S. gordonii copY::specR, copA::specR, and copZ::specR mutants were able to form biofilms on both polystyrene and glass surfaces. However, inactivation of copZ and to a lesser extent copY resulted in phenotypes that were defective in biofilm detachment, which is consistent with previous observations in S. mutans and suggests that the trace element copper might influence biofilm detachment of bacterial biofilms.

Genetic Models in Pathogenesis

To decipher the complexity of host-pathogen interactions the widest possible range of model hosts and of analytical methods is required. As some virulence mechanisms and certain host responses have been conserved throughout evolution, even simple organisms can be used as model hosts to help our understanding of infectious diseases. The availability of molecular genetic tools and a cooperative community of researchers are pivotal to the emergence of model systems. In this review, we first summarize the genetic screens that can be used to identify pathogen virulence factors, then we present a comparative overview of existing or emerging genetically tractable host models.

Biotechnology and vaccines: application of functional genomics to Neisseria meningitidis and other bacterial pathogens

Since its introduction, vaccinology has been very effective in preventing infectious diseases. However, in several cases, the conventional approach to identify protective antigens, based on biochemical, immunological and microbiological methods, has failed to deliver successful vaccine candidates against major bacterial pathogens. The recent development of powerful biotechnological tools applied to genome-based approaches has revolutionized vaccine development, biological research and clinical diagnostics. The availability of a genome provides an inclusive virtual catalogue of all the potential antigens from which it is possible to select the molecules that are likely to be more effective. Here, we describe the use of "reverse vaccinology", which has been successful in the identification of potential vaccines candidates against Neisseria meningitidis serogroup B and review the use of functional genomics approaches as DNA microarrays, proteomics and comparative genome analysis for the identification of virulence factors and novel vaccine candidates. In addition, we describe the potential of these powerful technologies in understanding the pathogenesis of various bacteria.

Post-genomics of lactic acid bacteria and other food-grade bacteria to discover gut functionality

Recent years have seen an explosion in the number of complete or almost complete genomic sequences of lactic acid bacteria and other food-grade bacteria that are used in functional foods to increase the health of the consumer. These have been instrumental in the development of functional, comparative and other post-genomics approaches that provide the possibility to detect, unravel and understand their functionality in the human intestinal tract. In conjunction with other high-throughput approaches, these advances can be exploited in the functional food innovation cycle for developing new or designed probiotic and other bacterial products that impact gut health.

Construction of replicative and integrative plasmids for setting up the in vivo expression technology in Helicobacter pylori

Some pathogenic factors of Helicobacter pylori, a bacterium involved in peptic ulcer and gastric cancer, have already been identified using either global or particular approach, but there are still some orphan genes and unidentified pathogenic factors. One of the methods used successfully for the identification of virulence genes of many pathogens is the in vivo expression technology. We describe here the construction and sequences of three different plasmids, one integrative and two replicatives, for the identification of virulence genes by using in vivo expression technology in H. pylori, and of potential use in other bacteria such as Campylobacter spp. Moreover, the use of the green fluorescent protein could allow to classify the genes according to the strength of their expression and to identify those which are repressed upon interaction with gastric mucosa.

Staphylococcus aureus virulence genes identified by bursa aurealis mutagenesis and nematode killing

Staphylococcus aureus is the leading cause of wound and hospital-acquired infections worldwide. The emergence of S. aureus strains with resistance to multiple antibiotics requires the identification of bacterial virulence genes and the development of novel therapeutic strategies. Herein, bursa aurealis, a mariner-based transposon, was used for random mutagenesis and for the isolation of 10,325 S. aureus variants with defined insertion sites. By screening for loss-of-function mutants in a Caenorhabditis elegans killing assay, 71 S. aureus virulence genes were identified. Some of these genes are also required for S. aureus abscess formation in a murine infection model.

Enterococcal infections: host response, therapeutic, and prophylactic possibilities

The emergence of resistance against multiple antibiotics and the increasing frequency with which Enterococcus faecalis and Enterococcus faecium are isolated from hospitalized patients underscore the necessity for a better understanding of the virulence mechanisms of this pathogen and the development of alternatives to current antibiotic treatments. The genetic plasticity of enterococci and their ability to rapidly acquire and/or develop resistance against many clinically important antibiotics and to transfer these resistance determinants to other more pathogenic microorganisms makes the search for alternative treatment and preventive options even more important. A capsular polysaccharide antigen has recently been characterized that is the target of opsonic antibodies. A limited number of clinically relevant serotypes exist, and the development of an enterococcal vaccine based on capsular polysaccharides may improve our ability to prevent and treat these infections. Additional enterococcal surface antigens, including ABC transporter proteins and other virulence factors, such as aggregation substance (AS), may also be useful targets for therapeutic antibodies.

Development and application of a dapB-based in vivo expression technology system to study colonization of rice by the endophytic nitrogen-fixing bacterium Pseudomonas stutzeri A15

Pseudomonas stutzeri A15 is a nitrogen-fixing bacterium isolated from paddy rice. Strain A15 is able to colonize and infect rice roots. This strain may provide rice plants with fixed nitrogen and hence promote plant growth. In this article, we describe the use of dapB-based in vivo expression technology to identify P. stutzeri A15 genes that are specifically induced during colonization and infection (cii). We focused on the identification of P. stutzeri A15 genes that are switched on during rice root colonization and are switched off during free-living growth on synthetic medium. Several transcriptional fusions induced in the rice rhizosphere were isolated. Some of the corresponding genes are involved in the stress response, chemotaxis, metabolism, and global regulation, while others encode putative proteins with unknown functions or without significant homology to known proteins.

Genes encoding a cellulosic polymer contribute toward the ecological success of Pseudomonas fluorescens SBW25 on plant surfaces

Pseudomonas fluorescens SBW25 is a Gram-negative bacterium that grows in close association with plants. In common with a broad range of functionally similar bacteria it plays an important role in the turnover of organic matter and certain isolates can promote plant growth. Despite its environmental significance, the causes of its ecological success are poorly understood. Here we describe the development and application of a simple promoter trapping strategy (IVET) to identify P. fluorescens SBW25 genes showing elevated levels of expression in the sugar beet rhizosphere. A total of 25 rhizosphere-induced (rhi) fusions are reported with predicted roles in nutrient acquisition, stress responses, biosynthesis of phytohormones and antibiotics. One rhi fusion is to wss, an operon encoding an acetylated cellulose polymer. A mutant carrying a defective wss locus was competitively compromised (relative to the wild type) in the rhizosphere and in the phyllosphere, but not in bulk soil. The rhizosphere-induced wss locus therefore contributes to the ecological performance of SBW25 in the plant environment and supports our conjecture that genes inactive in the laboratory environment, but active in the wild, are likely to be determinants of fitness in natural environments.

Effect of enzyme I of the bacterial phosphoenolpyruvate : sugar phosphotransferase system (PTS) on virulence in a murine model

The phosphoenolpyruvate : sugar phosphotransferase system (PTS) catalyses translocation with concomitant phosphorylation of sugars and hexitols and it regulates metabolism in response to the availability of carbohydrates. The PTS forms an interface between energy and signal transduction and its inhibition is likely to have pleiotropic effects. It is present in about one-third of bacteria with fully sequenced genomes, including many common pathogens, but does not occur in eukaryotes. Enzyme I (ptsI) is the first component of the divergent protein phosphorylation cascade. ptsI deletions were constructed in Salmonella typhimurium, Staphylococcus aureus and Haemophilus influenzae and virulence of the mutants was characterized in an intraperitoneal mouse model. The log(attenuation) values were 2.3, 1.4 and 0.9 for the Sal. typhimurium, Sta. aureus and H. influenzae ptsI mutants, respectively. The degree of attenuation is correlated with the complexity of the respective PTS, which comprises approximately 40 components in Sal. typhimurium, but only 5 in H. influenzae.

Copper homeostasis in Enterococcus hirae

Copper is an essential component of life because of its convenient redox potential of 200-800 mV when bound to protein. Extensive insight into copper homeostasis has only emerged in the last decade and Enterococcus hirae has served as a paradigm for many aspects of the process. The cop operon of E. hirae regulates copper uptake, availability, and export. It consists of four genes that encode a repressor, CopY, a copper chaperone, CopZ, and two CPx-type copper ATPases, CopA and CopB. Most of these components have been conserved across the three evolutionary kingdoms. The four Cop proteins have been studied in vivo as well as in vitro and their function is understood in some detail.

Differential expression of the enolase gene under in vivo versus in vitro growth conditions of Aeromonas hydrophila

Aeromonas hydrophila is an emerging human pathogen that leads to gastroenteritis and other invasive diseases. By using a murine peritoneal culture (MPC) model, we identified via restriction fragment differential display PCR (RFDDPCR) five genes of A. hydrophila that were differentially expressed under in vivo versus in vitro growth conditions. The gene encoding enolase was among those five genes that were differentially up regulated. Enolase is a glycolytic enzyme and its surface expression was recently shown to be important in the pathogenesis of a gram-positive bacterium Streptococcus pyogenes. By Western blot analysis and Immunogold staining, we demonstrated secretion and surface expression of enolase in A. hydrophila. We also showed that the whole cells of A. hydrophila had strong enolase activity. Using an enzyme-linked immunosorbant assay and sandwich Western blot analysis, we demonstrated binding of enolase to human plasminogen, which is involved in the fibrinolytic system of the host. We cloned the A. hydrophila enolase gene, which exhibited 62% homology at the DNA level and 57% homology at the amino acid level when compared to S. pyogenes enolase. This is a first report describing the increased expression of enolase gene in vivo that could potentially contribute to the pathogenesis of A. hydrophila infections.

Isolation of acid-inducible genes of Mycobacterium tuberculosis with the use of recombinase-based in vivo expression technology

A better understanding of mycobacterial gene regulation under certain stress conditions (e.g., low pH) may provide insight into mechanisms of adaptation during infection. To identify mycobacterial promoters induced at low pH, we adapted the recombinase-based in vivo expression technology (RIVET) promoter trap system for use with mycobacteria. Our results show that the TnpR recombinase of transposon gammadelta is active in Mycobacterium smegmatis and Mycobacterium tuberculosis. We developed a method to perform sequential double selection with mycobacteria by using RIVET, with a kanamycin preselection and a sucrose postselection. A library of M. tuberculosis DNA inserted upstream of tnpR was created, and using the double selection, we identified two promoters which are upregulated at low pH. The promoter regions drive the expression of a gene encoding a putative lipase, lipF (Rv3487c), as well as a PE-PGRS gene, Rv0834c, in a pH-dependent manner in both M. smegmatis and M. tuberculosis. The acid inducibility of lipF and Rv0834c was independent of the stress response sigma factor, SigF, as acid induction of the two genes in an M. tuberculosis sigF mutant strain was similar to that in the wild-type strain. No induction of lipF or Rv0834c was observed during infection of J774 murine macrophages, an observation which is in agreement with previous reports on the failure of phagosomes containing M. tuberculosis to acidify.

The effect of cell density and specific growth rate on accessory gene regulator and toxic shock syndrome toxin-1 gene expression in Staphylococcus aureus

A continuous culture technique utilising a variant of Staphylococcus aureus 8325-4 containing transcriptional gene fusions was used to investigate the relationships between cell density (OD(600)), steady-state specific growth rate (mu) and expression of both agr (accessory gene regulator) and tst (toxic shock syndrome toxin-1). The expression of these genes was assessed by two single-copy independently arranged chromosomal-based reporter systems, beta-galactosidase agr-P3 promoter fusion and a lux-tst promoter fusion. Cell density and specific agr expression were found to be positively correlated. In the model, the minimum cell density predicted to promote specific agr expression was an OD(600) of 0.14, equivalent to 1.2x10(8) CFU ml(-1). No direct relationship between cell density and specific tst expression was detected. Specific expressions of agr and tst were not correlated with specific growth rate and there appeared to be no direct link between agr and tst specific expression. The results support the hypothesis that agr is a functional unit of quorum sensing and that the amount of specific expression of tst is modulated independently of agr.

Penicillin-binding proteins in Streptococcus agalactiae: a novel mechanism for evasion of immune clearance

Group B streptococci (GBS) remain the most significant bacterial pathogen causing neonatal sepsis, pneumonia and meningitis in the USA despite CDC-recommended chemoprophylaxis strategies for preventing infection. To cause infection pathogens such as GBS must evade recognition and clearance by the host's immune system. Strategies for avoidance of opsonization and phagocytic killing include elaboration of antiopsonophagocytic capsules and surface proteins. During screening for mutants of GBS that were attenuated for virulence in a neonatal rat sepsis model, we identified a mutant with a transposon insertion in the ponA gene. ponA encodes an extra-cytoplasmic penicillin-binding protein PBP1a, a newly identified virulence trait for GBS that promotes resistance to phagocytic killing independent of capsular polysaccharide. Complementation analysis in vivo and in vitro confirmed that the altered phenotypes observed in the mutant were due to the transposon insertion in ponA. Deletion of PBP1a does not affect C3 deposition on GBS suggesting that mechanism by which PBP1a protects GBS from phagocytic killing is distinct from the antiopsonic activity of capsular polysaccharide. This is the first report describing expression of an antiphagocytic surface protein by GBS and represents a novel mechanism for evasion of immune recognition and clearance that may explain the decreased virulence observed in Gram-positive bacterial species for penicillin-binding protein mutants.

Characterization of Mycobacterium tuberculosis ribosome recycling factor (RRF) and a mutant lacking six amino acids from the C-terminal end reveals that the C-terminal residues are important for its occupancy on the ribosome

Ribosome recycling factor (RRF), coded for by the frr locus, is involved in the disassembly of post-termination complexes and recycling of the ribosomes for a fresh round of initiation in bacteria and in eukaryotic organelles. In a cross-species-complementation experiment, it was shown that the Thermus thermophilus RRF protein lacking five amino acids from its C-terminal end (deltaC5TthRRF) but not the full-length protein (TthRRF) complemented Escherichia coli for its frr(ts) phenotype. It was also shown that the Mycobacterium tuberculosis RFF protein (MtuRRF) did not complement E. coli LJ14 for frr(ts). However, simultaneous expression of elongation factor G (EFG) and RRF from M. tuberculosis resulted in complementation of E. coli LJ14. Here it is shown that unlike deltaC5TthRRF, an equivalent mutant of MtuRRF lacking six amino acids from its C-terminal end (deltaC6MtuRRF) did not complement E. coli LJ14. Surprisingly, deltaC6MtuRRF failed to complement the strain even in the presence of homologous EFG (MtuEFG). The biochemical and biophysical characterization of these proteins suggested that the mutant RRF folded properly. However, ribosome-binding assays showed that the mutant protein was compromised in its binding to E. coli ribosomes. It is suggested that the conserved amino acids at the C-terminal end of the RRFs contribute to their residency on ribosomes and that the specific interactions between RRF and EFG are crucial in the disassembly of the termination complex.

Factors involved in the early pathogenesis of bovine Staphylococcus aureus mastitis with emphasis on bacterial adhesion and invasion. A review

Staphylococcus aureus is the most important and prevalent contagious mammary pathogen; it causes clinical and subclinical intramammary infection with serious economic loss and herd management problems in dairy cows. In vitro studies have shown that Staphylococcus aureus adheres to mammary epithelial cells and extracellular matrix components and invades into mammary epithelial as well as other mammary cells. Staphylococcus aureus strains from intramammary infection produce several cell surface-associated and extracellular secretory products. The exact pathogenic roles of most of the products and their effects on adhesion and invasion are not well evaluated. It is also known that mammary epithelial cell-associated molecules and extracellular matrix components interact with S. aureus during the pathogenesis of mastitis, but their roles on adhesion and invasion have not been characterized. The adhesion of S. aureus to epithelial cells may involve non-specific physicochemical interactions and/or specific interactions between bacterial cell-associated ligands and host cell surface receptors. In vitro adhesion depends on the S. aureus strain, the growth phase of the bacteria, the growth medium and the origin of the epithelial cells. Adhesion is hypothesized to be a prerequisite and crucial early step for mammary gland infection. Staphylococcus aureus invades mammary epithelial cells. It also invades other cells such as endothelial cells and fibroblasts. Bacteria are found enclosed in membrane bound vacuoles in the cytoplasm of mammary epithelial cells. Recent observations indicate that S. aureus escapes from the phagosome into the cytoplasm and induces apoptosis. The invasion into mammary epithelial cells may occur through an endocytic process that requires involvement of elements of the cytoskeleton or by direct binding of bacteria to epithelial cells through a process mediated by specific receptors that needs de novo protein synthesis by both cells. Thus, the recurrent subclinical infection may result from this intracellular existence of bacteria that are protected from host defenses and effects of antibiotics. This review emphasizes on recent findings on S. aureus adhesion to mammary epithelial cells and extracellular matrix components and invasion into mammary epithelial cells.

Is the GehD lipase from Staphylococcus epidermidis a collagen binding adhesin?

The opportunistic human pathogen Staphylococcus epidermidis is the major cause of nosocomial biomaterial infections. S. epidermidis has the ability to attach to indwelling materials coated with extracellular matrix proteins such as fibrinogen, fibronectin, vitronectin, and collagen. To identify the proteins necessary for S. epidermidis attachment to collagen, we screened an expression library using digoxigenin-labeled collagen as well as two monoclonal antibodies generated against the Staphylococcus aureus collagen-adhesin, Cna, as probes. These monoclonal antibodies recognize collagen binding epitopes on the surface of S. aureus and S. epidermidis cells. Using this approach, we identified GehD, the extracellular lipase originally found in S. epidermidis 9, as a collagen-binding protein. Despite the monoclonal antibody cross-reactivity, the GehD amino acid sequence and predicted structure are radically different from those of Cna. The mature GehD circular dichroism spectra differs from that of Cna but strongly resembles that of a mammalian cell-surface collagen binding receptor, known as the alpha(1) integrin I domain, suggesting that they have similar secondary structures. The GehD protein is translated as a preproenzyme, secreted, and post-translationally processed into mature lipase. GehD does not have the conserved LPXTG C-terminal motif present in cell wall-anchored proteins, but it can be detected in lysostaphin cell wall extracts. A recombinant version of mature GehD binds to collagens type I, II, and IV adsorbed onto microtiter plates in a dose-dependent saturable manner. Recombinant, mature GehD protein and anti-GehD antibodies can inhibit the attachment of S. epidermidis to immobilized collagen. These results provide evidence that GehD may be a bi-functional molecule, acting not only as a lipase but also as a cell surface-associated collagen adhesin.

Staphylococcus aureus aconitase inactivation unexpectedly inhibits post-exponential-phase growth and enhances stationary-phase survival

Staphylococcus aureus preferentially catabolizes glucose, generating pyruvate, which is subsequently oxidized to acetate under aerobic growth conditions. Catabolite repression of the tricarboxylic acid (TCA) cycle results in the accumulation of acetate. TCA cycle derepression coincides with exit from the exponential growth phase, the onset of acetate catabolism, and the maximal expression of secreted virulence factors. These data suggest that carbon and energy for post-exponential-phase growth and virulence factor production are derived from the catabolism of acetate mediated by the TCA cycle. To test this hypothesis, the aconitase gene was genetically inactivated in a human isolate of S. aureus, and the effects on physiology, morphology, virulence factor production, virulence for mice, and stationary-phase survival were examined. TCA cycle inactivation prevented the post-exponential growth phase catabolism of acetate, resulting in premature entry into the stationary phase. This phenotype was accompanied by a significant reduction in the production of several virulence factors and alteration in host-pathogen interaction. Unexpectedly, aconitase inactivation enhanced stationary-phase survival relative to the wild-type strain. Aconitase is an iron-sulfur cluster-containing enzyme that is highly susceptible to oxidative inactivation. We speculate that reversible loss of the iron-sulfur cluster in wild-type organisms is a survival strategy used to circumvent oxidative stress induced during host-pathogen interactions. Taken together, these data demonstrate the importance of the TCA cycle in the life cycle of this medically important pathogen.

Acute septic arthritis

Acute septic arthritis may develop as a result of hematogenous seeding, direct introduction, or extension from a contiguous focus of infection. The pathogenesis of acute septic arthritis is multifactorial and depends on the interaction of the host immune response and the adherence factors, toxins, and immunoavoidance strategies of the invading pathogen. Neisseria gonorrhoeae and Staphylococcus aureus are used in discussing the host-pathogen interaction in the pathogenesis of acute septic arthritis. While diagnosis rests on isolation of the bacterial species from synovial fluid samples, patient history, clinical presentation, laboratory findings, and imaging studies are also important. Acute nongonococcal septic arthritis is a medical emergency that can lead to significant morbidity and mortality. Therefore, prompt recognition, rapid and aggressive antimicrobial therapy, and surgical treatment are critical to ensuring a good prognosis. Even with prompt diagnosis and treatment, high mortality and morbidity rates still occur. In contrast, gonococcal arthritis is often successfully treated with antimicrobial therapy alone and demonstrates a very low rate of complications and an excellent prognosis for full return of normal joint function. In the case of prosthetic joint infections, the hardware must be eventually removed by a two-stage revision in order to cure the infection.

Identification of ABC transporters in vancomycin-resistant Enterococcus faecium as potential targets for antibody therapy

The occurrence of an outbreak of septicaemias due to vancomycin-resistant Enterococcus faecium (VRE), in Manchester, UK, provided an opportunity to examine the antibody responses in patients infected by the same strain. Immunoblotting sera from 24 cases, six of whom died, showed an immunodominant cluster of antigens at 34, 54 and 97 kDa, with a statistically significant correlate between survival and immunoglobulin G to the 34 and 97 kDa bands (P<0.05). Screening a genomic expression library of VRE with seropositive serum and peritoneal dialysate from a survivor gave a recombinant clone with two contiguous open reading frames, the derived amino acid sequences of which both showed sequence homologue with ABC transporters, with a Walker A and Walker B motif and the signature sequence LSGGQ. The first open reading frame (putative VRE ABC1) showed 57% homologue with YbxA from Bacillus subtilis. A partial sequence (putative VRE ABC2) was also obtained, in the same recombinant clone, of a second ABC transporter with 72% homologue with ybaE from B. subtilis. Affinity selection with the seropositive serum and peritoneal dialysate used to screen the library showed that the eluted antibody bound to the 97, 54, 34 and 30 kDa bands. Direct amino acid sequencing identified this as a possible ABC transporter. Rabbit antiserum against peptides representing Walker A and an area adjacent to the Walker B site cross-reacted with bands at 34, 54, 97, 110 kDa and at 30, 34 and 54 kDa respectively. This therefore appeared to be an immunodominant complex of ABC transporters of which the smallest was the 30 kDa antigen. Epitope mapping of this antigen with seropositive patients' sera delineated three linear epitopes (KVGIV, FGPKNF and RVAI). The Walker A site represented by peptide 1 (GHNGSGKSTLAKTIN), epitope RVAI represented by peptides 2 (MRRVAIAGVLAMPRE) and 3 (ELSGGQMRRVAIAGV), epitope KVGIV represented by peptide 4 (LKPIRKKVGIVFQFP), and recombinant VRE ABC1 and VRE ABC2 expressed in Escherichia coli pBAD were then used to isolate human genetically recombinant antibodies from a phage antibody display library. An assessment of the protective potential of these antibodies was carried out in a mouse model of the infection. This study suggests that an ABC transporter homologue could be a target for antibody therapy against VRE infections.

Identification of Escherichia coli genes that are specifically expressed in a murine model of septicemic infection

Identification and characterization of bacterial genes that are induced during the disease process are important in understanding the molecular mechanism of disease and can be useful in designing antimicrobial drugs to control the disease. The identification of in vivo induced (ivi) genes of an Escherichia coli septicemia strain by using antibiotic-based in vivo expression technology is described. Bacterial clones resistant to chloramphenicol in vivo were recovered from the livers of infected mice. Most of the ivi clones were sensitive to chloramphenicol when grown in vitro. Using reverse transcription-PCR, it was demonstrated that selected ivi clones expressed cat in the livers of infected mice but not during in vitro growth. A total of 750 colonies were recovered after three successive rounds of in vivo selection, and 168 isolated ivi clones were sequenced. The sequence analysis revealed that 37 clones encoded hypothetical proteins found in E. coli K-12, whereas 10 clones contained genes that had no significant homology to DNA sequences in GenBank. Two clones were found to contain transposon-related functions. Other clones contained genes required for amino acid metabolism, anaerobic respiration, DNA repair, the heat shock response, and the cellular repressor of the SOS response. In addition, one clone contained the aerobactin biosynthesis gene iucA. Mutations were introduced in to seven of the identified ivi genes. An in vivo mouse challenge-competition assay was used to determine if the mutants were attenuated. The results suggested that these ivi genes were important for survival in vivo, and three of the seven mutant ivi clones were required for successful infection of mice.

Pathogenesis of streptococcal and staphylococcal endocarditis

Although streptococcal and S. aureus IE share the same primary site of infection, their pathogenesis and clinical evolution present several major differences. Streptococci adhere to cardiac valves with pre-existing endothelial lesions. In contrast, S. aureus can colonize either damaged endothelium or invade physically intact endothelial cells. These interactions are mediated by multiple surface adhesins, some of which have been only partially characterized. Streptococci produce surface glucans (gtf and ftf), ECM adhesins (e.g., fibronectin-binding proteins, FimA), and platelet aggregating factors (phase I and phase II antigens, pblA, pblB, and pblT), all of which have been.

Virulence gene identification by differential fluorescence induction analysis of Staphylococcus aureus gene expression during infection-simulating culture

We have employed a strategy utilizing differential fluorescence induction (DFI) in an effort to identify Staphylococcus aureus genes whose products can be targeted for antimicrobial drug development. DFI allows identification of promoters preferentially active under given growth conditions on the basis of their ability to drive expression of a promoterless green fluorescent protein gene (gfp). A plasmid-based promoter trap library was constructed of 200- to 1,000-bp fragments of S. aureus genomic DNA fused to gfp, and clones with active promoters were isolated under seven different in vitro growth conditions simulating infection. Six thousand two hundred sixty-seven clones with active promoters were screened to identify those that exhibited differential promoter activity. Bioinformatic analysis allowed the identification of 42 unique operons, containing a total of 61 genes, immediately downstream of the differentially active putative promoters. Replacement mutations were generated for most of these operons, and the abilities of the resulting mutants to cause infection were assessed in two different murine infection models. Approximately 40% of the mutants were attenuated in at least one infection model.

Identification and testing of Porphyromonas gingivalis virulence genes with a pPGIVET system

An in vivo expression technology (IVET) system was designed to identify previously unknown virulence genes of Porphyromonas gingivalis. Fourteen ivi (for in vivo induced) genes that are induced during infection in a mouse abscess model were identified in our study. Of these, seven had homology to genes in the NCBI database, and the rest had no homology to reported DNA sequences. In order to determine virulence-related properties of these genes, three mutant strains, deleted of ivi8 (no homology to genes in the database), ivi10 (homologous to a putative TonB-dependent outer membrane receptor protein), and ivi11 (an immunoreactive 33-kDa antigen PG125 in P. gingivalis), were created. The mutants were tested in a mouse abscess model for alterations in virulence relative to the wild type by a competition assay in BALB/c mice. After 5 days we observed the enrichment of the wild-type strain over mutant strains Deltaivi10 and Deltaivi11, which indicated that mutant strains Deltaivi10 and Deltaivi11 are less able to survive in this model than the wild-type strain, while Deltaivi8 survives as well as the wild-type strain. We propose that knockout of these ivi genes reduced the ability of the mutated P. gingivalis to survive and cause infection compared to the wild-type strain at the site of injection. Also, in separate experiments, groups of mice were challenged with subcutaneous injections of each individual mutant strain (Deltaivi8, Deltaivi10, and Deltaivi11) or with the wild-type strain alone and were then examined to assess their general health status. The results showed that knockout of these ivi genes conferred a reduction in virulence. The ability of the mutants to invade KB cells compared to the wild type was also determined. Interestingly, the CFU counts of the mutant strain Deltaivi10 recovered from KB cells were eight times lower than those of the wild type, indicating that this mutant has a lower capacity for invasion. These results demonstrate that IVET is a powerful tool in discovering virulence genes and the significant role that ivi genes play in the pathogenesis of this species.

Identification of genes induced in vivo during Klebsiella pneumoniae CG43 infection

A novel in vivo expression technology (IVET) was performed to identify Klebsiella pneumoniae CG43 genes that are specifically expressed during infection of BALB/c mice. The IVET employed a UDP glucose pyrophosphorylase (galU)-deficient mutant of K. pneumoniae which is incapable of utilizing galactose and synthesizing capsular polysaccharide, as demonstrated by its low virulence to BALB/c mice and a white nonmucoid colony morphology on MacConkey-galactose agar. By using a functional galU gene as the reporter, an IVE promoter could render the galU mutant virulent while maintaining the white nonmucoid colony phenotype. A total of 20 distinct sequences were obtained through the in vivo selection. Five of them have been identified previously as virulence-associated genes in other pathogens, while another five with characterized functions are involved in regulation and transportation of nutrient uptake, biosynthesis of isoprenoids, and protein folding. No known functions have been attributed to the other 10 sequences. We have also demonstrated that 2 of the 20 IVE genes turn on under iron deprivation, whereas the expression of another five genes was found to be activated in the presence of paraquat, a superoxide generator.

Interaction of the CopZ copper chaperone with the CopA copper ATPase of Enterococcus hirae assessed by surface plasmon resonance

Intracellular copper routing in Enterococcus hirae can be accomplished by the CopZ metallochaperone. Using surface plasmon resonance analysis, we show here that CopZ interacts with the CopA copper ATPase. The binding affinity of CopZ for CopA was increased in the presence of copper, due to a 15-fold lower dissociation rate constant. Mutating the N-terminal copper binding motif of CopA from CxxC to SxxS abolished this copper-induced effect. Moreover, CopZ failed to show an interaction with an unrelated copper binding protein used as a control. These results show that (i) the CopA copper ATPase specifically interacts with the CopZ chaperone, (ii) this interaction is based on protein-protein interaction, and (iii) surface plasmon resonance is a novel tool for quantitative analysis of metallochaperone-target interactions.

Reassessing the role of Staphylococcus aureus clumping factor and fibronectin-binding protein by expression in Lactococcus lactis

Since Staphylococcus aureus expresses multiple pathogenic factors, studying their individual roles in single-gene-knockout mutants is difficult. To circumvent this problem, S. aureus clumping factor A (clfA) and fibronectin-binding protein A (fnbA) genes were constitutively expressed in poorly pathogenic Lactococcus lactis using the recently described pOri23 vector. The recombinant organisms were tested in vitro for their adherence to immobilized fibrinogen and fibronectin and in vivo for their ability to infect rats with catheter-induced aortic vegetations. In vitro, both clfA and fnbA increased the adherence of lactococci to their specific ligands to a similar extent as the S. aureus gene donor. In vivo, the minimum inoculum size producing endocarditis in > or =80% of the rats (80% infective dose [ID80]) with the parent lactococcus was > or =10(7) CFU. In contrast, clfA-expressing and fnbA-expressing lactococci required only 10(5) CFU to infect the majority of the animals (P < 0.00005). This was comparable to the infectivities of classical endocarditis pathogens such as S. aureus and streptococci (ID80 = 10(4) to 10(5) CFU) in this model. The results confirmed the role of clfA in endovascular infection, but with a much higher degree of confidence than with single-gene-inactivated staphylococci. Moreover, they identified fnbA as a critical virulence factor of equivalent importance. This was in contrast to previous studies that produced controversial results regarding this very determinant. Taken together, the present observations suggest that if antiadhesin therapy were to be developed, at least both of the clfA and fnbA products should be blocked for the therapy to be effective.

Identification and analysis of Staphylococcus aureus components expressed by a model system of growth in serum

A model system mimicking Staphylococcus aureus bacteremia was developed by growth in serum under microaerobic conditions. Eight genes induced by growth in serum were identified, including an antimicrobial peptide biosynthesis locus, amino acid biosynthetic loci, and genes encoding putative surface proteins. Nine independent insertions were found in the major lysine biosynthesis operon, which encodes eight genes, is repressed by lysine in vitro, and is expressed in vivo.

The recA gene in Porphyromonas gingivalis is expressed during infection of the murine host

The recA gene in Porphyromonas gingivalis is involved in DNA repair. To further elucidate the importance of the recA locus in the pathogenesis of P. gingivalis, we assessed its ability for expression in an animal host. The promoterless xa-tetA(Q)2 cassette was used in heterodiploid mutants to study recA promoter activity during infection. P. gingivalis FLL118.1 had the xa-tetA(Q)2 cassette under the control of recA promoter whereas P. gingivalis FLL119 had the cassette in the opposite orientation. xa encodes a bifunctional xylosidase/arabinosidase enzyme (XA) and the tetA(Q)2 gene product confers tetracycline resistance. Intramuscular infection in a mouse model allowed the recovery of the bacteria from inguinal lymph nodes. Infusion of tetracycline in the animals permitted the enrichment P. gingivalis FLL118.1 over the wild-type strain, during a mixed infection. The xylosidase activity of FLL118.1 could be detected on agar plates in the presence of 5-methylumbellifiry-beta-D-xyloside. No such enrichment for xylosidase activity was detected when the mixture of P. gingivalis W83 and P. gingivalis FLL119 was used to infect the mouse or cultured in vitro. These results indicated that recA promoter was transcriptionally active during the infection of the murine host and further support the importance of this locus during the P. gingivalis infection process.

Selection for in vivo regulators of bacterial virulence

We devised a noninvasive genetic selection strategy to identify positive regulators of bacterial virulence genes during actual infection of an intact animal host. This strategy combines random mutagenesis with a switch-like reporter of transcription that confers antibiotic resistance in the off state and sensitivity in the on state. Application of this technology to the human intestinal pathogen Vibrio cholerae identified several regulators of cholera toxin and a central virulence gene regulator that are operative during infection. These regulators function in chemotaxis, signaling pathways, transport across the cell envelope, biosynthesis, and adherence. We show that phenotypes that appear genetically independent in cell culture become interrelated in the host milieu.

Selection for in vivo regulators of bacterial virulence

We devised a noninvasive genetic selection strategy to identify positive regulators of bacterial virulence genes during actual infection of an intact animal host. This strategy combines random mutagenesis with a switch-like reporter of transcription that confers antibiotic resistance in the off state and sensitivity in the on state. Application of this technology to the human intestinal pathogen Vibrio cholerae identified several regulators of cholera toxin and a central virulence gene regulator that are operative during infection. These regulators function in chemotaxis, signaling pathways, transport across the cell envelope, biosynthesis, and adherence. We show that phenotypes that appear genetically independent in cell culture become interrelated in the host milieu.

Regulation of virulence determinants in Staphylococcus aureus

The pathogenicity of Staphylococcus aureus depends on the combined action of more than 40 different extracellular toxins, enzymes and cell surface proteins. A global regulator agr controls the production of many of these virulence factors by a regulating RNA molecule, RNAIII. Most of the virulence genes regulated by RNAIII are also regulated by SarA and a family of homologous proteins. The Sar proteins appear to repress transcription of individual virulence genes or sets of genes. As some Sar proteins also repress one or more sar homologous genes an increased production of a single Sar protein can result in decreased expression of some virulence genes, and an increased expression of others. Results are presented suggesting that RNAIII might function as an antirepressor, binding one or more of the Sar proteins.

In vivo-expressed genes of Pasteurella multocida

Pasteurella multocida is the causative agent of infectious diseases of economic importance such as fowl cholera, bovine hemorrhagic septicemia, and porcine atrophic rhinitis. However, knowledge of the molecular mechanisms and determinants that P. multocida requires for virulence and pathogenicity is still limited. To address this issue, we developed a genetic expression system, based on the in vivo expression technology approach first described by Mahan et al. (Science 259:686--688, 1993), to identify in vivo-expressed genes of P. multocida. Numerous genes, such as those encoding outer membrane lipoproteins, metabolic and biosynthetic enzymes, and a number of hypothetical proteins, were identified. These may prove to be useful targets for attenuating mutation and/or warrant further investigation for their roles in immunity and/or pathogenesis.

Combating Gram-positive pathogens: emerging techniques to identify relevant virulence targets

Recent progress in microbial genome sequencing, along with functional genomics technologies based on gene expression, proteomics and genetics have facilitated the identification of significant numbers of Gram-positive virulence genes. These genes represent a novel and heterogeneous class of targets for antimicrobial drug development. This review will concentrate of the contribution of two functional genomics technologies, in vivo expression technology (IVET) based on gene expression and signature-tagged mutagenesis (STM), a genetics based technology to the identification of virulence genes in Gram-positive pathogens.

Assessment of bacterial pathogenesis by analysis of gene expression in the host

A number of techniques have been developed to assess the expression of microbial virulence genes within the host (in vivo). These studies have shown that bacteria employ a wide variety of mechanisms to coordinately regulate the expression of these genes during infection. Two tenets have emerged from these studies: bacterial adaptation responses are critical to growth within the host, and interactions between microorganisms and the microenvironments of their hosts cannot be revealed from in vitro studies alone. Results that support these tenets include (i) the prevalent class of in vivo expressed genes are involved in adaptation to environmental stresses, (ii) pathogens recovered from host tissues (versus laboratory growth) are often more resistant to host killing mechanisms, and (iii) virulence gene expression can differ in the animal compared to laboratory media. Thus, pathogenicity comprises the unique ability to adapt to the varied host milieus encountered as the infection proceeds.