[[sigma].sup.B] regulates IS256-mediated Staphylococcus aureus biofilm phenotypic variation

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 IS256 in biofilm-essential genes. In this study, we present evidence suggesting that the absence of the [[sigma].sup.] 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 IS256 copies in the chromosomes of biofilm-negative variants, as well as with an augmented IS256 insertion frequency into the icaC and the sarA genes. IS256-mediated biofilm switching is reversible, and biofilm-positive variants could emerge from biofilm-negative [[sigma].sup.B] mutants. Analysis of the chromosomal insertion frequency using a recombinant IS256 element tagged with an erythromycin marker showed an almost three-times-higher transposition frequency in a [DELTA][[sigma].sup.B] strain. However, regulation of IS256 activity by [[sigma].sup.B] appears to be indirect, since transposase transcription is not affected in the absence of [[sigma].sup.B] and IS256 activity is inhibited to wild-type levels in a [DELTA][[sigma].sup.B] strain under NaCl stress. Overall, our results identify a new role for [[sigma].sup.B] as a negative regulator of insertion sequence transposition and support the idea that deregulation of IS256 activity abrogates biofilm formation capacity in S. aureus.