These factors contribute to the virulence of the microorganisms and to their survival in the hostile envirnoment within the body of their host.
Various signals control the expression of the virulence factors.
Oxygen, temperature, concentration of ions, and pH are some of the known signals that change the bacterial virulence. Pettersson et al and Zhang and Normark, provide the evidence in the August issue of Science that bacteria-host cell contact plays a major role in controlling the virulence of bacteria by changing the expression of virulence factors. Pettersson et al examined the control of the virulence factor in the Yersinia pseudotuberculosis . The virulence of these microorganisms is due to the secretion of virulence factors that are called Yops. YpkA, a kinase homologus to eukaryotes serine and threonine kinases 2.