aureus adhesion to and invasion of human osteoblasts. MG-63 osteoblastic cells were infected for 2 h at approximately 50 bacteria/cell with S. aureus strain 8325-4, pre-treated or not (untreated control) with 1/2 MIC linezolid, oxacillin or rifampicin, and S. aureus strain DU5883 this website lacking

fnbA and fnbB (negative control). To enumerate cell-associated bacteria, infected cells were washed twice to discard unbound bacteria and analysed by osmotic shock in pure water, and then, suitable dilutions of the lysates were plated on agar. The same procedure was used to quantify intracellular bacteria, except that the cells were incubated for 1 h with 200 mg/L gentamicin before the lysis step to kill extracellular bacteria. Adherent bacteria were calculated by subtracting intracellular bacteria from cell-associated bacteria. The results were expressed as the means +/- standard deviation of the percentage of recovered internalised (a) or adherent (b) bacteria with respect to inoculated bacteria derived from four independent experiments performed in duplicate. Asterisk = significantly different from the control (corresponding isolate grown without antibiotic), with a P value

of 0.05 by one-way analysis of variance followed by a posteriori Dunnett’s test. Discussion Several Selleck Bafilomycin A1 major findings emerge from this investigation of the impact of sub-inhibitory concentrations of anti-staphylococcal drugs on S. aureus adhesion and invasion phenotypes. S. aureus binding to human fibronectin and the transcriptional levels of the fnbA/B genes encoding the fibronectin-binding proteins were differentially modulated by antimicrobial agents. Oxacillin, moxifloxacin and linezolid treatment led to the development of a hyper-adhesive phenotype, along with an increase in fnbA/B mRNA levels relative to the gyrB tetracosactide internal standard. The same hyper-adhesive phenotype was induced by clindamycin treatment, although no significant change in fnbA/B mRNA levels was observed. Rifampin was the only antimicrobial agent among

those tested that significantly inhibited S. aureus binding to fibronectin without affecting relative fnbA/B transcription profiles. Vancomycin and gentamicin induced no change in either the adhesion phenotype or the fnbA/B transcription. S. aureus adhesion to and invasion of live eukaryotic cells was also assessed after oxacillin, linezolid or rifampin treatment in an ex vivo infection model of cultured human osteoblasts. Oxacillin treatment significantly increased S. aureus adhesion but not invasion, while no significant change in adhesion or invasion levels was observed after linezolid or rifampin treatment. Several recent studies have focused on the influences of sub-inhibitory concentrations of antimicrobial agents on the expression of various virulence factors produced by S. aureus and on the various regulation mechanisms involved in this modulation [6, 8, 17].