Figure 9.

The AggR-dependent virulence switch: a feed forward loop with dampening. In the absence of triggering, AggR-independent expression from the aggR and aar promoters maintains a balanced level of AggR and Aar, keeping the virulence switch in an off state. AggR expression is controlled positively (+) by Fis and repressed (-) by H-NS [25]. Triggering of the AggR-dependent virulence switch, either by a specific signal or a stochastic event, leads to the increased expression of AggR, amplifying the response (i.e. a feed forward loop), and the production of specific virulence determinants involved in host attachment, biofilm formation and protein secretion. Aar expression is also increased, inhibiting AggR-dependent transcription activation, dampening the AggR feed forward loop, and interfering with biofilm formation, possibly in concert with H-NS at specific promoters. The virulence switch will be reset by the removal of the triggering signal and Aar-mediated repression of AggR. Green arrows denote activation (+), red lines repression (-), and dotted blue arrows indicate the processes of transcription and translation for aggR and aar.