Phase-variable expression of the biofilm-associated protein (Bap) in Staphylococcus aureus

A process of phase variation is described that affects the expression of Bap, (biofilm-associated protein) in Staphylococcus aureus. Upon subculture of the Bap-positive S. aureus strain V329 on Congo red agar, spontaneous smooth biofilm-negative colonies appeared at a low frequency (5x10(-4)). Northern blot analysis of these variants with a bap-specific gene probe showed that transcription of the bap gene did not occur. However, DNA typing, Southern blot hybridization and DNA sequencing did not show any differences between the parent V329 strain and the biofilmnegative variants. The biofilm-negative phenotype reverted to wild-type at a similar frequency upon subculture of Bap-negative variants in liquid media. Experimental infection of ovine mammary glands with Bap-negative variants showed that phase variation occurred in vivo, because Bap-expressing, biofilm-positive revertants were isolated from infected mammary glands. The absence of Bap correlated with increased adherence to fibrinogen and fibronectin. It is possible that S. aureus can detach from a biofilm by switching to a Bap-negative state.