Streptococcus pneumoniae (pneumococcus) is a major cause of morbidity and mortality world-wide. The initial event in invasive pneumococcal disease is the attachment of encapsulated pneumococci to epithelial cells in the upper respiratory tract. This work provides evidence that initial bacterial adhesion and subsequent ability to cause invasive disease is enhanced by pili, long organelles able to extend beyond the polysaccharide capsule, previously unknown to exist in pneumococci. These adhesive pili-like appendages are encoded by the pneumococcal rlrA islet, present in some, but not all, clinical isolates. Introduction of the rlrA islet into an encapsulated rlrA-negative isolate allowed pilus expression, enhanced adherence to lung epithelial cells, and provided a competitive advantage upon mixed intranasal challenge of mice. Furthermore, a pilus-expressing rlrA islet-positive clinical isolate was more virulent than a nonpiliated deletion mutant, and it out-competed the mutant in murine models of colonization, pneumonia, and bacteremia. Additionally, piliated pneumococci evoked a higher TNF response during systemic infection, compared with nonpiliated derivatives, suggesting that pneumococcal pili not only contribute to adherence and virulence but also stimulate the host inflammatory response.inflammation ͉ Streptococcus pneumoniae ͉ pili ͉ adhesion ͉ pathogenicity islet

Deformylase inhibitors belong to a novel antibiotic class that targets peptide deformylase, a bacterial enzyme that removes the formyl group from N-terminal methionine in nascent polypeptides. Using the bacterium Salmonella enterica, we isolated mutants with resistance toward the peptide deformylase inhibitor actinonin. Resistance mutations were identified in two genes that are required for the formylation of methionyl (Met) initiator tRNA (tRNAi) fMet : the fmt gene encoding the enzyme methionyl-tRNA formyltransferase and the folD gene encoding the bifunctional enzyme methylenetetrahydrofolate-dehydrogenase and -cyclohydrolase. In the absence of antibiotic, these resistance mutations conferred a fitness cost that was manifested as a reduced growth rate in laboratory medium and in mice. By serially passaging the low-fitness mutants in growth medium without antibiotic, the fitness costs could be partly ameliorated either by intragenic mutations in the fmt͞folD genes or by extragenic compensatory mutations. Of the extragenically compensated fmt mutants, approximately one-third carried amplifications of the identical, tandemly repeated metZ and metW genes, encoding tRNAi. The increase in metZW gene copy number varied from 5-to 40-fold and was accompanied by a similar increase in tRNAi levels. The rise in tRNAi level compensated for the lack of methionyl-tRNA formyltransferase activity and allowed translation initiation to proceed with nonformylated methionyl tRNAi. Amplified units varied in size from 1.9 to 94 kbp. Suppression of deleterious mutations by gene amplification may be involved in the evolution of new gene functions. compensatory evolution ͉ gene amplification ͉ Salmonella typhimurium ͉ actinonin ͉ peptide deformylase

In most Staphylococcus aureus strains, inactivation of sarA increases hla transcription, indicating that sarA is a repressor. However, in S. aureus NCTC 8325 and its derivatives, used for most studies of hla regulation, inactivation of sarA resulted in decreased hla transcription. The disparate phenotype of strain NCTC 8325 seems to be associated with its rsbU mutation, which leads to B deficiency. This has now been verified by the demonstration that sarA repressed hla transcription in an rsbU ؉ derivative of strain 8325-4 (SH1000). That sarA could act as a repressor of hla in an 8325-4 background was confirmed by the observation that inactivation of sarA in an agr sarS rot triple mutant dramatically increased hla transcription to wild-type levels. However, the apparent role of sarA as an activator of hla in 8325-4 was not a result of the rsbU mutation alone, as inactivation of sarA in another rsbU mutant, strain V8, led to increased hla transcription. Northern blot analysis revealed much higher levels of sarS mRNA in strain V8 than in 8325-4, which was likely due to the mutation in the sarS activator, tcaR, in 8325-4, which was not found in strain V8. On the other hand, the relative increase in sarS transcription upon the inactivation of sarA was 15-fold higher in 8325-4 than in strain V8. Because of this, inactivation of sarA in 8325-4 means a net increase in repressor activity, whereas in strain V8, inactivation of sarA means a net decrease in repressor activity and, therefore, enhanced hla transcription.Staphylococcus aureus is a common human pathogen which colonizes the nares and skin of about one-third of all healthy people. The types of infection caused by this organism range from superficial cutaneous infections to life-threatening bacteremias. The pathogenesis of S. aureus is very complex, and virulence depends on the production of an array of extracellular toxins and enzymes, as well as adhesins, which are regulated by a number of global regulators, e.g., agr (accessory gene regulator) and the sarA (staphylococcal accessory regulator) family of regulators (reviewed in references 1, 9, and 50). In addition, the alternative sigma factor, sigma B ( B ), seems to be involved in virulence gene expression in addition to its role in regulating stress responses and intermediary metabolism (4,12,37,38). Notably, among the genes controlled by sigma B are the virulence regulators sarA and sarS (4,20,67). The activity of B is regulated by the anti-sigma factor RsbW, which binds B , thereby inhibiting its association with the RNA polymerase (3, 46). The ability of RsbW to bind B depends on the phosphorylation status of RsbV, which is determined by the activity of the phosphatase RsbU (22,68,70).Most S. aureus strains produce ␣-hemolysin (alpha-toxin, hla), which is a 33-kDa pore-forming protein that can lyse a wide range of human cells, including lymphocytes and keratinocytes (21,31,69). In addition, ␣-hemolysin may induce apoptosis in T lymphocytes (31) and has also been shown to be required for biofilm formation ...

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