Identification of Staphylococcus aureus genes expressed during growth in milk: a useful model for selection of genes important in bovine mastitis?

Symposium review: Features of Staphylococcus aureus mastitis pathogenesis that guide vaccine development strategies

Bovine mastitis affects animal health and welfare and milk production and quality, and it challenges the economic success of dairy farms. Staphylococcus aureus is one of the most commonly found pathogens in clinical mastitis but it also causes subclinical, persistent, and difficult-to-treat intramammary infections. Because of the failure of conventional antibiotic treatments and increasing pressure and concern from experts and consumers over the use of antibiotics in the dairy industry, many attempts have been made over the years to develop a vaccine for the prevention and control of Staph. aureus intramammary infections. Still, no commercially available vaccine formulation demonstrates sufficient protection and cost-effective potential. Multiple factors account for the lack of protection, including inadequate vaccine targets, high diversity among mastitis-provoking strains, cow-to-cow variation in immune response, and a failure to elicit an immune response that is appropriate for protection against a highly complex pathogen. The purpose of this review is to summarize key concepts related to the pathogenesis of Staph. aureus, and its interaction with the host, as well as to describe recent vaccine development strategies for prevention and control of Staph. aureus mastitis.

The expression of a putative exotoxin and an ABC transporter during bovine intramammary infection contributes to the virulence of Staphylococcus aureus

Staphylococcus aureus is a leading cause of intramammary infections (IMI) and bovine mastitis is an important disease for the dairy industry. As this bacterium probably expresses specific genes for establishment of IMI, we studied the transcriptional profile of four S. aureus strains recovered from experimentally infected cows. Microbial RNA was extracted from bacteria isolated from milk, reverse-transcribed and labeled for hybridization to sub-genomic microarrays to detect candidate genes for further investigations. Several S. aureus genes were expressed during IMI; some were detected in samples from more than one strain, more than one cow and at more than one time point during infection. A selection of four genes showing strong expression and with putative functions in pathogenesis was further studied by qPCR. By comparing the expression in different media in vitro, we found that gene SACOL2171 was induced by iron restriction whereas the expression of the transcriptional regulator SACOL2325 and the ABC transporter SACOL0718-720 (vraFG) were induced by milk. In addition, the putative exotoxin SACOL0442 seemed to require the intramammary environment for expression. Gene-disrupted mutants for SACOL0720 and SACOL0442 showed no growth defect in vitro but were attenuated during bovine IMI, causing infections with significant reductions in bacterial and somatic cell counts. The milk from the mammary quarters infected with these mutants also showed better appearance and composition than milk from quarters infected with the wild type. In conclusion, we have identified genes that are most likely important for S. aureus IMI. These represent novel candidates to include in a vaccine.

Staphylococcus aureus virulence and metabolism are dramatically affected by Lactococcus lactis in cheese matrix

In complex environments such as cheeses, the lack of relevant information on the physiology and virulence expression of pathogenic bacteria and the impact of endogenous microbiota has hindered progress in risk assessment and control. Here, we investigated the behaviour of Staphylococcus aureus, a major foodborne pathogen, in a cheese matrix, either alone or in the presence of Lactococcus lactis, as a dominant species of cheese ecosystems. The dynamics of S. aureus was explored in situ by coupling a microbiological and, for the first time, a transcriptomic approach. Lactococcus lactis affected the carbohydrate and nitrogen metabolisms and the stress response of S. aureus by acidifying, proteolysing and decreasing the redox potential of the cheese matrix. Enterotoxin expression was positively or negatively modulated by both L. lactis and the cheese matrix itself, depending on the enterotoxin type. Among the main enterotoxins involved in staphylococcal food poisoning, sea expression was slightly favoured in the presence of L. lactis, whereas a strong repression of sec4 was observed in cheese matrix, even in the absence of L. lactis, and correlated with a reduced saeRS expression. Remarkably, the agr system was downregulated by the presence of L. lactis, in part because of the decrease in pH. This study highlights the intimate link between environment, metabolism and virulence, as illustrated by the influence of the cheese matrix context, including the presence of L. lactis, on two major virulence regulators, the agr system and saeRS.

Relationship between virulence factor genes in bovineStaphylococcus aureussubclinical mastitis isolates and binding to anti-adhesin antibodies

Staphylococcus aureusis the most common aetiologic agent of contagious bovine mastitis. It is characterized by a wide array of virulence factors. The differences among strains jeopardize the development of effective vaccines againstStaph. aureusmastitis. We tested the immunogenicity of a peptide subunit vaccine coding for three different adhesion factors, fibrinogen-binding protein (Efb), fibronectin-binding protein A (FnbpA) and clumping factor A (ClfA). Then we evaluated the influence of some virulence factors on the ability of specific anti-adhesin antibodies to react with sixteenStaph. aureusstrains isolated from bovine subclinical mastitis. Immunization with the recombinant adhesins stimulated a strong humoural (IgG and IgA) and mucosal IgA immune response in all animals tested. Hyperimmune serum recognized with diverse efficiency the sixteenStaph. aureusstrains and this circumstance correlated well with the level of expression of adhesins. Among the different virulence factors considered to classify strains,spagene polymorphisms showed the strongest influence on isolate reactions to hyperimmune serum. Our results indicate the importance of a disease- and environment-specific analysis of isolates. Thus, as opposed to other pathogens to obtain an effective vaccine we should characterize multiple strains and identify the prevalent virulence factors expressed.

Development of serological proteome analysis of mastitis by Staphylococcus aureus in ewes

Staphylococcus aureus is a major agent of mastitis in ruminants worldwide. So far, efficient measures for its prophylaxis (including vaccination) have proven to be unsuccessful and there is a need for a better understanding of the host response to udder infection by S. aureus. Serological proteome analysis (SERPA) is a promising technique that can be used to identify S. aureus immuno-dominant determinants providing that bacterial culture conditions used to grow S. aureus strains for protein sample preparation mimic the context of mastitis. A S. aureus strain was used in experimental mastitis to generate sheep serum used to determine the best growth conditions for SERPA. Sera collected in the field from different ewes suffering from mastitis by S. aureus were used to confirm experimental observations. Three different culture media (BHI, whey and iron-depleted RPMI) were tested. The influence of aeration and growth phase on protein production was also evaluated by immuno-detection of protein samples prepared from cultures grown in different conditions and obtained from different culture fractions (supernatant, cell wall, and total lysates). Our results showed that culturing in iron-depleted RPMI with (secreted proteins, prepared from stationary phase) or without aeration (cell wall proteins, prepared from early stationary phase, and total proteins, prepared from exponential phase) is the condition that best mimics growth in vivo during mastitis and this in vitro growth condition is to be used henceforth in experiments involving SERPA.

Antibacterial effect of plant-derived antimicrobials on major bacterial mastitis pathogens in vitro

The objective of this study was to investigate the antimicrobial effect of plant-derived antimicrobials including trans-cinnamaldehyde (TC), eugenol, carvacrol, and thymol on major bacterial mastitis pathogens in milk. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the aforementioned compounds on Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, Staphylococcus aureus, and Escherichia coli were determined. In addition, the bactericidal kinetics of TC on the aforementioned pathogens and the persistence of the antimicrobial activity of TC in milk over a period of 2 wk were investigated. All 4 plant-derived molecules exhibited antimicrobial activity against the 5 mastitis pathogens tested, but TC was most effective in killing the bacteria. The MIC and MBC of TC on Staph. aureus, E. coli, and Strep. uberis were 0.1 and 0.45%, respectively, whereas that on Strep. agalactiae and Strep. dysgalactiae were 0.05 and 0.4%, respectively. The MIC and MBC of the other 3 molecules ranged from 0.4 to 0.8% and 0.8 to 1.5%, respectively. In time-kill assays, TC at the MBC reduced the bacterial pathogens in milk by 4.0 to 5.0 log(10) cfu/mL and to undetectable levels within 12 and 24 h, respectively. The antimicrobial effect of TC persisted for the duration of the experiment (14 d) without any loss of activity. Results of this study suggest that TC has the potential to be evaluated as an alternative or adjunct to antibiotics as intramammary infusion to treat bovine mastitis.

Repeat-based subtyping and grouping of Staphylococcus aureus from human infections and bovine mastitis using the R-domain of the clumping factor A gene

Staphylococcus aureus has become an emerging public health concern. Markers capable of differentiating separate host-specific lineages are needed for tracing strain sources. Thus, a coding variable number tandem repeat-based typing was explored in this study, based on R-domain of clumping factor A (clfA) gene. DNA from isolates and strains of human infections and bovine mastitis were amplified and sequenced. Sequences of clfA from published strains were also analyzed. Results indicate that except one with 36 copies, 44 of the 55 R-domains had repeat copies between 44 and 57, whereas the remaining 10 had 59.5 to 73 copies. Furthermore, human isolates were polymorphic, while mastitis isolates were clonal. Phylogenetic grouping assigned host-specific strains into respective clusters. The repeats were stable during passages in milk, nutrient broth, and invasion of mammary cells showing suitability for typing. Our data show that the R-domain can be useful for typing and grouping host-specific lineages. Moreover, existence of variant repeats in human strains and the dominance of a clonal motif in mastitis may imply that a specific selection has occurred in the mammary gland.

Sequence analysis of the Staphylococcus aureus srrAB loci reveals that truncation of srrA affects growth and virulence factor expression

The SrrAB system regulates metabolism and virulence factors in Staphylococcus aureus. We sequenced the srrAB loci of 21 isolates and performed a phylogenetic analysis. Vaginal and bovine isolates clustered together, while skin isolates were genetically diverse. Few nucleotide polymorphisms were observed, and most were synonymous. Two strains (N2 and N19) with N-terminal truncations in SrrA displayed defects in growth and abnormally upregulated virulence factor expression under low-oxygen conditions.

Characterization of cell wall associated proteins of a Staphylococcus aureus isolated from bovine mastitis case by a proteomic approach

Staphylococcus aureus causes different pathologies in humans and animals. In particular, it is involved in intramammary infections in cows, causing economic losses and milk-safety problems. Although it is well-known that surface components (proteins and capsular polysaccharides) and exotoxins are virulence factors involved in the pathogenesis of bovine mastitis, less is known about the precise biochemical identity of such molecules. Therefore, mapping of surface proteins using specific disease- and environment-isolates provides a benchmark for strain comparison of pathogens with different pathogenic characteristics and antibiotic resistance mechanism and can aid in defining specific vaccine and therapeutic targets. In this study, we used a proteomic approach on protein extracts of lysostaphin-treated S. aureus in isotonic conditions, to produce a reproducible and well resolved 2-D electrophoresis (2-DE) reference map of surface associated proteins of isolated S. aureus from a case of bovine mastitis. The most abundant protein components were identified by Matrix assisted laser desorption ionisation-time of flight (MALDI-TOF) mass spectrometry.

Investigations into sigmaB-modulated regulatory pathways governing extracellular virulence determinant production in Staphylococcus aureus

The commonly used Staphylococcus aureus laboratory strain 8325-4 bears a naturally occurring 11-bp deletion in the sigmaB-regulating phosphatase rsbU. We have previously published a report (M. J. Horsburgh, J. L. Aish, I. J. White, L. Shaw, J. K. Lithgow, and S. J. Foster, J. Bacteriol. 184:5457-5467, 2002) on restoring the rsbU deletion, producing a sigmaB-functional 8325-4 derivative, SH1000. SH1000 is pleiotropically altered in phenotype from 8325-4, displaying enhanced pigmentation, increased growth yields, and a marked decrease in secreted exoproteins. This reduction in exoprotein secretion appears to result from a sixfold reduction in agr expression. In this study we have undertaken transposon mutagenesis of SH1000 to identify components involved in the modulation of extracellular proteases and alpha-hemolysin compared to 8325-4. In total, 13 genes were identified displaying increased alpha-hemolysin transcription and extracellular proteolysis. Phenotypic analysis revealed that each mutant also had decreased pigmentation and a general increase in protein secretion. Interestingly this phenotype was not identical in each case but was variable from mutant to mutant. None of the genes identified encoded classic regulatory proteins but were predominantly metabolic enzymes involved in amino acid biosynthesis and transport. Further analysis revealed that all of these mutations were clustered in a 35-kb region of the chromosome. By complementation and genetic manipulation we were able to demonstrate the validity of these mutations. Interestingly transcriptional analysis revealed that rather than being regulated by sigmaB, these genes appeared to have a role in the regulation of sigmaB activity. Thus, we propose that the loss of individual genes in this chromosomal hot spot region results in a destabilization of cellular harmony and disruption of the sigmaB regulatory cascade.

The pathogenesis and control of Staphylococcus aureus-induced mastitis: study models in the mouse

The intramammary colonisation by Staphylococcus aureus provokes mastitis in the cow. Once established, the infection is difficult to eradicate with available therapies and may become chronic. The present article focuses on the use of the experimental mouse model of S. aureus-induced mastitis as a practical approach for the study of bovine mastitis. Results obtained regarding the pathogenesis of S. aureus and the development of new therapeutic approaches are discussed.

Large-scale identification of genes required for full virulence of Staphylococcus aureus

Gene products required for in vivo growth and survival of microbial pathogens comprise a unique functional class and may represent new targets for antimicrobial chemotherapy, vaccine construction, or diagnostics. Although some factors governing Staphylococcus aureus pathogenicity have been identified and studied, a comprehensive genomic analysis of virulence functions will be a prerequisite for developing a global understanding of interactions between this pathogen and its human host. In this study, we describe a genetic screening strategy and demonstrate its use in screening a collection of 6,300 S. aureus insertion mutants for virulence attenuation in a murine model of systemic infection. Ninety-five attenuated mutants were identified, reassembled into new pools, and rescreened using the same murine model. This effort identified 24 highly attenuated mutants, each of which was further characterized for virulence attenuation in vivo and for growth phenotypes in vitro. Mutants were recovered in numbers up to 1,200-fold less than wild type in the spleens of systemically infected animals and up to 4,000-fold less than wild type in localized abscess infections. Genetic analysis of the mutants identified insertions in 23 unique genes. The largest gene classes represented by these mutants encoded enzymes involved in small-molecule biosynthesis and cell surface transmembrane proteins involved in small-molecule binding and transport. Additionally, three insertions defined two histidine kinase sensor-response regulator gene pairs important for S. aureus in vivo survival. Our findings extend the understanding of pathogenic mechanisms employed by S. aureus to ensure its successful growth and survival in vivo. Many of the gene products we have identified represent attractive new targets for antibacterial chemotherapy.

The fibronectin-binding proteins of Staphylococcus aureus may promote mammary gland colonization in a lactating mouse model of mastitis

The fibronectin-binding proteins (FnBPs) of Staphylococcus aureus are believed to be implicated in the pathogen's adherence to and colonization of bovine mammary glands, thus leading to infectious mastitis. In vitro studies have shown that FnBPs help the adhesion of the pathogen to bovine mammary epithelial cells. However, the importance of FnBPs for the infection of mammary glands has never been directly established in vivo. In this study with a mouse model of mastitis, the presence of FnBPs on the surface of S. aureus increased the capacity of the bacterium to colonize mammary glands under suckling pressure compared to that of a mutant lacking FnBPs.

In vitro growth of mastitis-inducing Escherichia coli in milk and milk fractions of dairy cows

The outcome of E. coli mastitis in cows ranges from mild to severe in individual animals. This study explored the hypothesis that milk from individual cows differs in its growth medium properties for E. coli, and whether possible variation could be related to specific milk constituents. To mimic the early phase of intramammary E. coli infection, a low inoculum size and a short incubation period were used. Cell-reduced, cell- and fat-free (skim) and cell- and fat-free and protein-reduced (whey) fractions were prepared from whole milk samples (n=18). Ten ml of whole milk, milk fractions and brain heart infusion broth (BHI) were inoculated with approximately 100cfu E. coli. After 6h of incubation, bacterial counts were assessed by dilution plating in triplicate. Bacterial counts in whole milk differed up to a 100-fold between cows, which was not associated with SCC. Bacterial counts were significantly higher in whey fractions than in whole milk, cell-reduced and skim fractions and variation in whey was smaller, indicating that the acid-precipitable protein fraction contains the milk constituents of major relevance for inhibition of and variation in bacterial growth. The presence of fat and cells added to bacterial growth inhibition to a lesser extent. In conclusion, in vitro growth of E. coli in milk differs substantially between individual cows within an incubation period comparable with the early phase of intramammary infection. This suggests that the growth medium properties of milk could be of importance in the pathogenesis of E. coli mastitis and subsequent outcome of disease.

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.

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.