At 37°C no significant difference was observed when comparing the growth curves of the wild type strain Newman and the mutant (Figure 1B). However, colonies of secDF mutants were smaller on blood agar compared to the wild type (83% ± 5.1 of the wild type’s cross section). TEM pictures were prepared from exponentially growing cells. In contrast to the wild type (Figure 2A) and the complemented mutant (Figure 2C), displaying normally shaped cells with a maximum of one septum, the secDF

mutant had difficulties in separating daughter cells (Figure 2B and 2D). This resulted in clusters with sometimes multiple and wrongly placed septa. At least 400 cells per BAY 57-1293 purchase strain were analyzed, showing that 20.4 ± 8.7% of the mutant cells could not divide correctly whereas this was only the case in 0.3 ± 0.7% for the wild type and 0.9 ± 1.3% for the complemented mutant. Figure 2 Cell morphology. TEM pictures from thin sections of strains (A) Newman pCN34, (B and D) ΔsecDF pCN34 and (C) ΔsecDF pCQ27 during exponential phase (OD600 0.5). As secDF knock out mutants in B. subtilis and E. coli show a cold sensitive phenotype [6, 24], growth of the S. aureus secDF mutant was tested at 15°C. The temperature drop affected the secDF mutant approximately after two generations, causing a notably reduced growth rate with a subsequent halt in growth after 24 h. The plasmid pCQ27, but not the empty

vector pCN34, significantly restored growth at 15°C (Figure 1B). Increased susceptibility of the secDF mutant towards RND-substrates, β-lactam Doxorubicin ic50 and glycopeptide antibiotics Reverse transcriptase Since multidrug resistance can be mediated unspecifically by RND exporters [21, 25], we characterized the

resistance profile of the secDF mutant by testing several different classes of antibiotics and typical RND-substrates [26, 27]. The secDF mutant showed increased susceptibility to the RND substrates acriflavine, ethidium bromide and sodium dodecyl sulfate (SDS) on gradient plates (Figure 3). Furthermore, a distinct increased susceptibility to the β-lactam oxacillin and the glycopeptide vancomycin was observed (Figure 3). The reduction of oxacillin resistance was even more apparent in the presence of mecA, the gene encoding the penicillin binding protein 2a (PBP2a), mediating methicillin resistance, as shown for the methicillin resistant S. aureus (MRSA) strain pair Newman pME2 and Newman secDF pME2 (Figure 3) [28]. Reduction of oxacillin resistance in MRSA by secDF inactivation was confirmed in strains of different genetic backgrounds or SCCmec types, such as the clinical isolate CHE482 [29] and RA2 [30] or RA120 [31] (data not shown). The complementing plasmid pCQ27 was able to restore the wild type resistance levels. Figure 3 Effect of secDF inactivation on resistance profiles. (A) Gradient plates with increasing concentrations of β-lactam and glycopeptide antibiotics.