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Staphylococcus aureus is an versatile pathogen that can cause life-threatening infections. Depending on the clinical setting, up to 50% of S. aureus infections are caused by methicillin-resistant strains (MRSA) that in most cases are resistant to many other antibiotics, making treatment difficult. The emergence of community-acquired MRSA drastically changed the picture by increasing the risk of MRSA infections. Horizontal transfer of genes encoding for antibiotic resistance or virulence factors is a major concern of multidrug-resistant S. aureus infections and epidemiology. We identified and characterized a type Ill-like restriction system present in clinical s. aureus strains that prevents transformation with DNA from other bacterial species. Interestingly, our analysis revealed that some clinical MRSA strains are deficient in this restriction system, and thus are hypersusceptible to the horizontal transfer of DNA from other species, such as Escherichia coil, and could easily acquire a vancomycin-resistance gene from enterococci. Inactivation of this restriction system dramatically increases the transformation efficiency of clinical S. aureus strains, opening the field of molecular genetic manipulation of these strains using DNA of exogenous origin. antibiotic resistance | transformation | targetron doi/ 10.1073/pnas.1000489107

The pathogenesis of staphylococcal infections is multifactorial. Golden pigment is an eponymous feature of the human pathogen Staphylococcus aureus that shields the microbe from oxidation-based clearance, an innate host immune response to infection. Here, we screened a collection of S. aureus transposon mutants for pigment production variants. A total of 15 previously unidentified genes were discovered. Notably, disrupting metabolic pathways such as the tricarboxylic acid cycle, purine biosynthesis, and oxidative phosphorylation yields mutants with enhanced pigmentation. The dramatic effect on pigment production seems to correlate with altered expression of virulence determinants. Microarray analysis further indicates that purine biosynthesis impacts the expression of ~400 genes involved in a broad spectrum of functions including virulence. The purine biosynthesis mutant and oxidative phosphorylation mutant strains exhibit significantly attenuated virulence in a murine abscess model of infection. Inhibition of purine biosynthesis with a known smallmolecule inhibitor results in altered virulence gene expression and virulence attenuation during infection. Taken together, these results suggest an intimate link between metabolic processes and virulence gene expression in S. aureus. This study also establishes the importance of purine biosynthesis and oxidative phosphorylation for in vivo survival. doi: 10.1128/JB.00928-09

To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1467-9566.2008.01147.x Byline: Brian Brown (1), Paul Crawford (2) Keywords: MRSA; healthcare associated infection; mutation; discourse; narrative Abstract: Abstract In this paper we will consider the question of mutation as it is manifested in press coverage of MRSA in UK hospitals. This represents a fertile field of discourse which brings into focus issues relating to microbes, people and working practices as well as the concepts of risk and vulnerability. A regular feature of reporting has been the presence of explanations for drug resistance involving repeated random mutations of the microbe to achieve progressively greater resistance and versatility, largely through a Darwinian process which is 'clever' at overcoming human attempts at elimination. More recently a discourse has emerged which foregrounds also the vulnerability of patients who are very young, old or otherwise immunocompromised, or whose own genetic makeup might put them at risk from the microbe. The hospital is decentred as a source of infection, and attention is turned instead to nursing homes and gymnasia as sources of infection in the community. This latter development mitigates the responsibilities of hospitals and statutory healthcare providers and turns the risk back towards the individual as a responsible actor in an ecology of mutation. Author Affiliation: (1)School of Applied Social Sciences, De Montfort University, Leicester (2)School of Nursing, The University of Nottingham Article note: Address for correspondence: Brian Brown, School of Applied Social Sciences, De Montfort University, Hawthorn Building, The Gateway, Leicester, LE1 9BH, e-mail: brown@brown.uk.com

Conditional mutants of pbpB with an IPTG-inducible promoter were used to compare the effects of interrupted transcription of this gene in a meticillin-sensitive (MSSA) and a meticillin-resistant (MRSA) strain of Staphylococcus aureus. After 3 h growth following the removal of IPTG, multiplication of the MSSA strain stopped abruptly, cells began to lyse, and membrane preparations showed greatly decreased quantities of penicillin-binding protein (PBP) 2. In contrast, the MRSA strain continued to grow for at least 20 h in the IPTG-free medium, but with gradually increasing doubling times, which eventually reached 180 min. The peptidoglycan produced during this period of extremely slow growth showed only minor alterations, but cells with abnormal morphology accumulated in the culture, the abundance of mecA transcript gradually declined, and the cellular amounts of PBP2A were significantly decreased. Adding back the IPTG inducer caused rapid resumption in the transcription of pbpB, followed by an increase in the transcription of mecA. No changes were detected in the transcription of pbpA, C and D, the determinant of 16S rRNA or the housekeeping gene pta. Promoter fusion experiments suggested that the transcription of the resistance gene mecA may respond to some regulatory signal generated in the bacteria during changes in the transcription of pbpB.

Inactivation of hemB in Staphylococcus aureus strain Newman resulted in a small-colony phenotype and was accompanied by an altered expression pattern of global regulators and control of virulence factor production. Transcription profiles followed over 15 h by Northern blot analyses revealed that transcripts of the global regulators arl, rot, sae, sarR, sarS, srr, svrA, and sigB disappeared after the exponential phase and that both agr transcripts were completely absent in the hemB mutant. Apart from a general concentration of transcriptional activity to the exponential phase, premature gene expression was observed for rot, hla, and spa. Nevertheless, reported [[sigma].sup.B]-dependent transcripts, such as sarC and clfA, were produced throughout the 15-h growth period monitored. The absence of these transcripts in a hemB sigB double mutant demonstrated their dependence on [[sigma].sup.B] and indicated an unexpected, permanent [[sigma].sup.B] activity in the hemB mutant. Variations in the extents of the directly [[sigma].sup.B]-controlled asp23, rsbVW-sigB, and sarC transcripts argue for additional factors modulating [[sigma].sup.B] activity. This study provides the first extended synopsis of the transcriptional patterns of different regulators over the entire growth cycle in the widely used Newman strain.