σ B Regulates IS 256 -Mediated Staphylococcus aureus Biofilm Phenotypic Variation

Abstract

ABSTRACT Biofilm formation in Staphylococcus aureus is subject to phase variation, and biofilm-negative derivatives emerge sporadically from a biofilm-positive bacterial population. To date, the only known mechanism for generating biofilm phenotypic variation in staphylococci is the reversible insertion/excision of IS 256 in biofilm-essential genes. In this study, we present evidence suggesting that the absence of the σ B transcription factor dramatically increases the rate of switching to the biofilm-negative phenotype in the clinical isolate S. aureus 15981, under both steady-state and flow conditions. The phenotypic switching correlates with a dramatic increase in the number of IS 256 copies in the chromosomes of biofilm-negative variants, as well as with an augmented IS 256 insertion frequency into the icaC and the sarA genes. IS 256 -mediated biofilm switching is reversible, and biofilm-positive variants could emerge from biofilm-negative σ B mutants. Analysis of the chromosomal insertion frequency using a recombinant IS 256 element tagged with an erythromycin marker showed an almost three-times-higher transposition frequency in a Δσ B strain. However, regulation of IS 256 activity by σ B appears to be indirect, since transposase transcription is not affected in the absence of σ B and IS 256 activity is inhibited to wild-type levels in a Δσ B strain under NaCl stress. Overall, our results identify a new role for σ B as a negative regulator of insertion sequence transposition and support the idea that deregulation of IS 256 activity abrogates biofilm formation capacity in S. aureus .