Poison-antidote systems in bacteria: the co-evolution of functional counterparts

Bacterial pathogens secrete a multitude of factors, many of which exhibit enzymatic activity that is directed against host-specific targets or are activated by host-specific functions. To counter these toxic molecules, producing organisms have developed several mechanisms in order to protect themselves against enzymes that have the ability to adversely affect both the microbe and the host cell. Potentially toxic molecules are strictly controlled by several strategies often involving a functional counterpart. Through the work of ourselves and others it has recently become apparent that certain bacterial extracellular proteases require inhibition by dedicated intracellular inhibitors in order that they may function only in their specific niche and cause no harm to the producing cell. Thus, it is our contention that these protease-inhibitor couplings represent a fascinating example of functional co-evolution, and are analogous to the protective mechanisms observed with the toxin-antitoxin pairings, and other such protective systems discussed within this review.