The virulent or carrier state of Salmonella is determined by the phosphorylation status of SsrB, a transcriptional regulator protein. While the virulent lifestyle is regulated through phosphorylation of SsrB by the SsrA kinase, the biofilm state is determined by unphosphorylated SsrB in an SsrA-independent manner.
This was revealed in a study published in Elife in 2016.
Desai SK, Winardhi RS, Periasamy S, Dykas MM, Jie Y, Kenney LJ (2016) The horizontally-acquired response regulator SsrB drives a Salmonella lifestyle switch by relieving biofilm silencing. Elife. 2;5. pii: e10747. doi: 10.7554/eLife.10747.
More information on the Kenney lab and the Yan lab.
Figure: Top panel shows the role of SsrB in Salmonella virulence- 1 & 2. The membrane bound SsrA kinase senses environmental conditions such as acidity and phospohorylates SsrB. 3 & 4) Phosphorylated SsrB binds the SPI-2 pathogenicity island to relieve the silencing of virulence genes by H-NS and to activate their transcription and release of effectors through T3SS. Bottom panel shows the role of SsrB in biofilm formation- 1 & 2. In the absence of SsrA, SsrB is unphosphorylated . 3 & 4. Unphosphorylated SsrB binds DNA and removes H-NS mediated silencing of CsgD and activation of genes coding for biofilm components. (Note that Melanie is to work on an improved version of this image)