Programmed cell death in bacteria

Abstract

Programmed death (PDC) of individual cells is a genetically controlled biological process related to the development of multicellular organisms. It proceeds in most cases as apoptosis characterized by DNA degradation and breakdown of dying cells to apoptotic bodies, and ending by their phagocytosis by macrophages or by the surrounding tissue. Unlike apoptosis, necrosis is a genetically unregulated sudden death of a group of cells caused by a severe damage of membranes and other cell components. In bacteria, programmed cell death is mostly related to population development. This holds mainly for sporulation of bacilli where the process is best understood at the morphological, physiological and genetic level. Sporulation of bacilli begins by an asymmetric division of the nongrowing cell into two parts—the mother and the forespore compartment, whose fate is different. Whereas the smaller compartment develops into the spore, the function of the larger is twofold. It participates in the spore development mainly by forming spore coast but it also synthesizes or activates the autolytic apparatus which lyzes the sporangium cell wall at the end of the process. Some phases of the development of myxobacteria and streptomycetes also have characteristic features of programmed death. Unlike sporulation of bacilli, the autolysis of a portion of population of myxobacteria or hyphae of streptomycetes proceeds in the middle of their developmental cycle. Extensive turnover of cell membranes in growing myxobacteria results in the formation of a fatty acid mixture—theautocide—which kills a smaller or greater portion of the myxobacterial population. The dead cells are digested by extracellular enzymes released by myxobacteria and the digest is used as nutrient for completion of the developmental cycle of the remaining living population. Similar events take place also during the formation of aerial mycelium in streptomycetes. Here the autolysis of a portion of vegetative (substrate) mycelium supplies amino acids for the formation of aerial mycelium. The recently discovered programmed death of plasmid-free descendants of a plasmid-bearing population of different bacteria is based on the loss of control of toxin activity by its antidote. Both substances are encoded by plasmid DNA and the loss of the plasmid results in an “enforced suicide” of the host cell because the effective concentration of the antidote decreases more rapidly than that of the toxin. The mechanisms of this suicide can vary. In addition to the above mentioned kinds of programmed death, other events of developmentally regulated death of prokaryotes probably exist. Some bacteria contain “death genes” in their chromosome which trigger cell death at the onset of the stationary phase. The physiological function of this kind of suicide is not known. However, most nonsporulating bacteria developed a strategy of surviving at the nongrowing stage by transforming the growing cell to a more resistant dormant (cryptobiological) form.