SummaryRNAIII is a 514 nt regulatory RNA that is the effector molecule of the staphylococcal agr quorum-sensing system, regulating a large set of virulence and other accessory genes at the level of transcription. RNAIII was discovered nearly 20 years ago and we long ago hypothesized that it would function by regulating the synthesis or activity of one or more intermediary transcription factors. We have finally confirmed this hypothesis, showing that Staphylococcus aureus RNAIII regulates the synthesis of a major pleiotropic transcription factor, Rot, by blocking its translation. RNAIII has a complex secondary structure with several stable hairpins that have highly C-rich end loops, unusual in an AT-rich organism. We noted that these loops are complementary to two G-rich stem loops of the rot mRNA translation initiation region (TIR). Pairing of the complementary RNAs would be predicted to occlude the rot Shine-Dalgarno (SD) site and to block rot translation. Through a combination of transcriptional and translational fusions and Northern and Western blot hybridization analyses, we show that RNAIII does, indeed, block rot translation. Through alterations in the C-rich loops of RNAIII and the G-rich loops of rot, we show that the sequences of these loops are critical for inhibition of rot translation and suggest that this inhibition is affected by pairing between the complementary stem loops, followed by the cleavage of rot mRNA. We propose that the RNAIII-rot mRNA interaction plays a key role in agr regulation of staphylococcal virulence.

This paper describes an investigation of the complex internal regulatory circuitry of the staphylococcal sae locus and the impact of modifying this circuitry on the expression of external genes in the sae regulon. The sae locus contains four genes, the saeR and S two-component signalling module (TCS), and saeP and Q, two upstream genes of hitherto unknown function. It is expressed from two promoters, P A sae, which transcribes only the TCS, and P C sae, which transcribes the entire locus. A bursa aurealis (bursa) transposon insertion in saeP in a derivative of Staphylococcus aureus NCTC 8325 has a profound effect on sae function. It modifies the activity of the TCS, changing the expression of many genes in the sae regulon, even though transcription of the TCS (from P A sae) is not interrupted. Moreover, these effects are not due to disruption of saeP since an in-frame deletion in saeP has essentially no phenotype. The phenotype of S. aureus strain Newman is remarkably similar to that of the saeP : : bursa and this similarity is explained by an amino acid substitution in the Newman saeS gene that is predicted to modify profoundly the signalling function of the protein. This concurrence suggests that the saeP : : bursa insertion affects the signalling function of saeS, a suggestion that is supported by the ability of an saeQR clone, but not an saeR clone, to complement the effects of the saeP : : bursa insertion.

Background. Staphylococcus aureus has numerous virulence factors, including exotoxins that may increase the severity of infection. This study was aimed at assessing whether preexisting antibodies to S. aureus toxins are associated with a lower risk of sepsis in adults with S. aureus infection complicated by bacteremia. Methods. We prospectively identified adults with S. aureus infection from 4 hospitals in Baltimore, MD, in 2009-2011. We obtained serum samples from prior to or at presentation of S. aureus bacteremia to measure total immunoglobulin G (IgG) and IgG antibody levels to 11 S. aureus exotoxins. Bacterial isolates were tested for the genes encoding S. aureus exotoxins using polymerase chain reaction (PCR). Results. One hundred eligible subjects were included and 27 of them developed sepsis. When adjusted for total IgG levels and stratified for the presence of toxin in the infecting isolate as appropriate, the risk of sepsis was significantly lower in those patients with higher levels of IgG against α-hemolysin (Hla), δ-hemolysin (Hld), Panton Valentine leukocidin (PVL), staphylococcal enterotoxin C-1 (SEC-1), and phenol-soluble modulin α3 (PSM-α3). Conclusions. Our results suggest that higher antibody levels against Hla, Hld, PVL, SEC-1, and PSM-α3 may protect against sepsis in patients with invasive S. aureus infections.

Transformation of a type I SCCmec element into Staphylococcus aureus yielded highly oxacillin-resistant transformants with a reduced growth rate. Faster-growing variants could again be selected at the cost of reduced resistance levels, demonstrating an inverse correlation between oxacillin resistance levels and growth rate.

Mutations in the staphylococcal virulence regulator gene agr frequently occur during Staphylococcus aureus infection. Whether agr-defective strains are fit for colonization, an important prerequisite for infection, is unknown. Screening by means of assays to detect delta-hemolysin activity and agr autoinducing peptide production indicated that 15 ( approximately 9%) of 160 healthy human subjects were colonized with an agr-defective strain or a mixture of agr-positive and -defective S. aureus strains. The presence of identical agr-defective strains in family members suggests that these strains are transmissible. Additionally, carriage of an agr-defective strain was associated with hospitalization, raising the possibility that such strains may be selected in a nosocomial setting.