text: Pseudomonas aeruginosa is a known opportunistic pathogen. One of its virulence factors is its ability to produce pyocyanin, a toxin released to kill both microbes and mammalian cells alike. The pyocyanin production occurs when activated by PhoB. This implies that P. aeruginosa uses the low Pi as a signal that the host has been damaged and to start producing toxin to improve chances of its survival.
  In contrast to P. aeruginosa, Vibrio cholerae has its toxin genes repressed by PhoB. It is thought that PhoB in V. cholerae is activated when Pi is low to prevent the production of toxins. It could be activated by other signals in the environment, but it has been shown that PhoB directly inhibits the toxins production by binding to the tcpPH promoter and stopping the ToxR regulon from being activated. Evidence supporting Pi as the signal is given by how the regulon is not repressed under high Pi conditions. The regulatory cascade is only repressed under low Pi conditions.