CcpA mediates the catabolite repression of tst in Staphylococcus aureus

Glucose-dependent activation of Bacillus anthracis toxin gene expression and virulence requires the carbon catabolite protein CcpA

Sensing environmental conditions is an essential aspect of bacterial physiology and virulence. In Bacillus anthracis, the causative agent of anthrax, transcription of the two major virulence factors, toxin and capsule, is triggered by bicarbonate, a major compound in the mammalian body. Here it is shown that glucose is an additional signaling molecule recognized by B. anthracis for toxin synthesis. The presence of glucose increased the expression of the protective antigen toxin component-encoding gene (pagA) by stimulating induction of transcription of the AtxA virulence transcription factor. Induction of atxA transcription by glucose required the carbon catabolite protein CcpA via an indirect mechanism. CcpA did not bind specifically to any region of the extended atxA promoter. The virulence of a B. anthracis strain from which the ccpA gene was deleted was significantly attenuated in a mouse model of infection. The data demonstrated that glucose is an important host environment-derived signaling molecule and that CcpA is a molecular link between environmental sensing and B. anthracis pathogenesis.

Control of the Staphylococcus aureus toxic shock tst promoter by the global regulator SarA

The Staphylococcus aureus SarA global regulator controls the expression of numerous virulence genes, often in conjunction with the agr quorum-sensing system and its effector RNA, RNAIII. In the present study, we have examined the role of both SarA and RNAIII on the regulation of the promoter of tst, encoding staphylococcal superantigen toxic shock syndrome toxin 1 (TSST-1). In vitro DNA-protein interaction studies with purified SarA using gel shift and DNase I protection assays revealed one strong SarA binding site and evidence for a weaker site nearby within the minimal 400-bp promoter region upstream of tst. In vivo analysis of tst promoter activation using a p(tst)-luxAB reporter inserted in the chromosome revealed partial but not complete loss of tst expression in a Δhld-RNAIII strain. In contrast, disruption of sarA abrogated tst expression. No significant tst expression was found for the double Δhld-RNAIII-ΔsarA mutant. Introduction of a plasmid containing cloned hld-RNAIII driven by a non-agr-dependent promoter, p(HU), into isogenic parental wild-type or ΔsarA strains showed comparable levels of RNAIII detected by quantitative reverse transcription-PCR (qRT-PCR) but a two-log(10) reduction in p(tst)-luxAB reporter expression in the ΔsarA strain, arguing that RNAIII levels alone are not strictly determinant for tst expression. Collectively, our results indicate that SarA binds directly to the tst promoter and that SarA plays a significant and direct role in the expression of tst.

CcpA mediates proline auxotrophy and is required for Staphylococcus aureus pathogenesis

Human clinical isolates of Staphylococcus aureus, for example, strains Newman and N315, cannot grow in the absence of proline, albeit their sequenced genomes harbor genes for two redundant proline synthesis pathways. We show here that under selective pressure, S. aureus Newman generates proline-prototrophic variants at a frequency of 3 x 10(-6), introducing frameshift and missense mutations in ccpA or IS1811 insertions in ptsH, two regulatory genes that carry out carbon catabolite repression (CCR) in staphylococci and other Gram-positive bacteria. S. aureus Newman variants with mutations in rocF (arginase), rocD (ornithine aminotransferase), and proC (Delta(1)-pyrroline 5-carboxylate [P5C] reductase) are unable to generate proline-prototrophic variants, whereas a variant with a mutation in ocd (ornithine cyclodeaminase) is unaffected. Transposon insertion in ccpA also restored proline prototrophy. CcpA was shown to repress transcription of rocF and rocD, encoding the first two enzymes, but not of proC, encoding the third and final enzyme in the P5C reductase pathway. CcpA bound to the upstream regions of rocF and rocD but not to that of proC. CcpA's binding to the upstream regions was greatly enhanced by phosphorylated HPr. The CCR-mediated proline auxotrophy was lifted when nonpreferred carbohydrates were used as the sole carbon source. The ccpA mutant displayed reduced staphylococcal load and replication in a murine model of staphylococcal abscess formation, indicating that carbon catabolite repression presents an important pathogenesis strategy of S. aureus infections.

Toxin profiling of Staphylococcus aureus strains involved in varicella superinfection

The most common complications of varicella are bacterial skin and soft tissue infections, generally due to Staphylococcus aureus and group A beta-hemolytic streptococci. The aim of this study was to characterize the toxin and antibiotic resistance profiles of S. aureus isolates involved in varicella complications. Between 2002 and 2007, the French Reference Centre for Staphylococci collected 58 S. aureus isolates involved in varicella superinfection. All the isolates were characterized by screening for 12 toxin genes, agr typing, and mecA gene detection; some isolates were also studied by spa typing, multilocus sequence typing (MLST), and resistance profiling. A major toxin gene was detected in 53% (31/58) of the isolates (genes for exfoliative toxins A and B, 17.2%; Panton-Valentine leukocidin gene, 8.6%; toxic shock syndrome toxin 1 gene, 27.6%). Most clinical manifestations were directly compatible with the classical activity of these toxins. Nineteen isolates (33%) were resistant to methicillin, and 12 of these isolates belonged to an emerging agr-2, ST5 clone that harbors the toxic shock syndrome toxin 1 gene. These data should be considered in the management and treatment of patients with varicella complicated by S. aureus superinfection. Antibiotics that decrease toxin production, such as clindamycin, may provide benefit, and their efficacy against bacterial superinfections in children with varicella should be studied.

A combination of independent transcriptional regulators shapes bacterial virulence gene expression during infection

Transcriptional regulatory networks are fundamental to how microbes alter gene expression in response to environmental stimuli, thereby playing a critical role in bacterial pathogenesis. However, understanding how bacterial transcriptional regulatory networks function during host-pathogen interaction is limited. Recent studies in group A Streptococcus (GAS) suggested that the transcriptional regulator catabolite control protein A (CcpA) influences many of the same genes as the control of virulence (CovRS) two-component gene regulatory system. To provide new information about the CcpA and CovRS networks, we compared the CcpA and CovR transcriptomes in a serotype M1 GAS strain. The transcript levels of several of the same genes encoding virulence factors and proteins involved in basic metabolic processes were affected in both DeltaccpA and DeltacovR isogenic mutant strains. Recombinant CcpA and CovR bound with high-affinity to the promoter regions of several co-regulated genes, including those encoding proteins involved in carbohydrate and amino acid metabolism. Compared to the wild-type parental strain, DeltaccpA and DeltacovRDeltaccpA isogenic mutant strains were significantly less virulent in a mouse myositis model. Inactivation of CcpA and CovR alone and in combination led to significant alterations in the transcript levels of several key GAS virulence factor encoding genes during infection. Importantly, the transcript level alterations in the DeltaccpA and DeltacovRDeltaccpA isogenic mutant strains observed during infection were distinct from those occurring during growth in laboratory medium. These data provide new knowledge regarding the molecular mechanisms by which pathogenic bacteria respond to environmental signals to regulate virulence factor production and basic metabolic processes during infection.

At the crossroads of bacterial metabolism and virulence factor synthesis in Staphylococci

Bacteria live in environments that are subject to rapid changes in the availability of the nutrients that are necessary to provide energy and biosynthetic intermediates for the synthesis of macromolecules. Consequently, bacterial survival depends on the ability of bacteria to regulate the expression of genes coding for enzymes required for growth in the altered environment. In pathogenic bacteria, adaptation to an altered environment often includes activating the transcription of virulence genes; hence, many virulence genes are regulated by environmental and nutritional signals. Consistent with this observation, the regulation of most, if not all, virulence determinants in staphylococci is mediated by environmental and nutritional signals. Some of these external signals can be directly transduced into a regulatory response by two-component regulators such as SrrAB; however, other external signals require transduction into intracellular signals. Many of the external environmental and nutritional signals that regulate virulence determinant expression can also alter bacterial metabolic status (e.g., iron limitation). Altering the metabolic status results in the transduction of external signals into intracellular metabolic signals that can be "sensed" by regulatory proteins (e.g., CodY, Rex, and GlnR). This review uses information derived primarily using Bacillus subtilis and Escherichia coli to articulate how gram-positive pathogens, with emphasis on Staphylococcus aureus and Staphylococcus epidermidis, regulate virulence determinant expression in response to a changing environment.

Effect of a glucose impulse on the CcpA regulon in Staphylococcus aureus

Background

The catabolite control protein A (CcpA) is a member of the LacI/GalR family of transcriptional regulators controlling carbon-metabolism pathways in low-GC Gram-positive bacteria. It functions as a catabolite repressor or activator, allowing the bacteria to utilize the preferred carbon source over secondary carbon sources. This study is the first CcpA-dependent transcriptome and proteome analysis in Staphylococcus aureus, focussing on short-time effects of glucose under stable pH conditions.

Results

The addition of glucose to exponentially growing S. aureus increased the expression of genes and enzymes of the glycolytic pathway, while genes and proteins of the tricarboxylic acid (TCA) cycle, required for the complete oxidation of glucose, were repressed via CcpA. Phosphotransacetylase and acetate kinase, converting acetyl-CoA to acetate with a concomitant substrate-level phosphorylation, were neither regulated by glucose nor by CcpA. CcpA directly repressed genes involved in utilization of amino acids as secondary carbon sources. Interestingly, the expression of a larger number of genes was found to be affected by ccpA inactivation in the absence of glucose than after glucose addition, suggesting that glucose-independent effects due to CcpA may have a particular impact in S. aureus. In the presence of glucose, CcpA was found to regulate the expression of genes involved in metabolism, but also that of genes coding for virulence determinants.

Conclusion

This study describes the CcpA regulon of exponentially growing S. aureus cells. As in other bacteria, CcpA of S. aureus seems to control a large regulon that comprises metabolic genes as well as virulence determinants that are affected in their expression by CcpA in a glucose-dependent as well as -independent manner.

The effect of photodynamic treatment combined with antibiotic action or host defence mechanisms on Staphylococcus aureus biofilms

Staphylococcus aureus is one of the most important etiological agents of infections associated with medical devices. This is in part due to the ability of the organism to form biofilm, which provides a microenvironment that protects from attack by the host's immune system and by antibiotics. In this study we examined the structure of polysaccharide intercellular adhesin (PIA)-dependent or protein-based S. aureus biofilms. We defined new strategies aimed at treatment of mature established biofilms using photodynamic treatment (PDT) combined with chemotherapy or phagocytosis. Significant inactivation of bacteria was observed when structurally distinct biofilms were exposed to the cationic porphyrin, tetra-substituted N-methyl-pyridyl-porphine (TMP), and simultaneously to visible light. Moreover, PDT-treated biofilms exposed to vancomycin or subjected to the phagocytic action of whole blood resulted in their almost complete eradication. The drastic reduction in staphylococcal survival and the disruption of biofilms were confirmed by confocal laser scanning microscopy and scanning electron microscopy. The results suggest that PDT combined with vancomycin and the host defences may be a useful approach for the inactivation of staphylococcal biofilms adhering to medical implant surfaces.