Molecular basis of florfenicol-induced increase in adherence of Staphylococcus aureus strain Newman

Objectives: The aim of this study was to determine the molecular basis of the florfenicol-dependent increased adherence of Staphylococcus aureus strain Newman to HEp-2 cells. Methods and results: Northern slot blot analysis showed that mRNA expression of fnbA, fnbB, coa, emp and eap, coding for adhesins, was increased in the presence of 0.5 x MIC of florfenicol. Under the same conditions expression of cap5, coding for type 5 capsular polysaccharides, was distinctly decreased. Since global regulatory systems can modulate the expression of adhesins, their role in this process was investigated by including three isogenic mutants with functionally inactive global regulator systems, agr, sar or sae. Growth in the presence of 0.5 x MIC of florfenicol significantly increased the adherence to HEp-2 cells, fibronectin and fibrinogen of the Delta agrand Delta sarmutant strains, but not that of the Delta saemutant strain. In contrast to components of the agr or sar system, expression of saeRS was increased, suggesting a potential sae-directed decrease in the expression of cap5 and increase in the expression of genes coding for adhesins under the influence of florfenicol. Analysis of RNA stability revealed that the increased amount of transcripts of saeRS and adherence-associated genes was due to a stabilization of the respective mRNAs by florfenicol. Conclusions: Our data provide evidence that an activation of the global regulator sae and a stabilization of mRNA coding for specific adhesins seem to act synergically in generating a more adherent phenotype in the presence of a high subinhibitory concentration of florfenicol.