Possible cannibalism in Bacillus subtilis NRS-762 stationary phase culture

Abstract

Present in a variety of environmental niches ranging from soil to leaves, Bacillus subtilis is a common soil bacterium endowed with a versatile metabolism and an expansive cellular differentiation programme. Specifically, the bacterium is capable of entering into cannibalism, sporulation, filamentous growth, swarming motility, stringent response, persister state, and competence, upon nutrient starvation. But, given the multiple strains present in the species, there remains a need to characterize the growth behaviour of individual strains, where differences in genetic repertoire may potentiate differences in growth and response to environmental stressors. Serendipitous observations of drastic decline in optical density in stationary phase cultures of B. subtilis NRS-762 (ATCC 8473) cultivated in LB Lennox medium at 25 o C and 250 rpm rotational shaking in aerobic shake flasks pointed to the possibility of cannibalism or prophage induced cell lysis as mechanisms responsible for cell lysis. More importantly, further evidence of widespread cell lysis in the culture came from the lack of cellular debris in the shake flask after autoclave decontamination at 121 o C for 20 minutes. With optical density declining gradually from a maximal of 5.4 at 22.5 hours post inoculation to 2.5 after 38 hours of incubation, it was more probable that cannibalism lysed a significant fraction of the cell population. Specifically, a subpopulation of resistant cannibalistic cells might have secreted cell lysis factors that lysed non-resistant cells, with the released cellular content serving as nutrients to support the remaining population. Prophage induced cell lysis was not probable given that such a process would result in total population collapse within 2 to 3 hours instead of the observed gradual decline in optical density. Thus, cannibalism is likely to be the mechanism responsible for observed drastic decline in optical density of stationary phase cultures of B. subtilis NRS-762, which was further accompanied by evolution of pungent odour and presence of houseflies circulating the cotton plugs of the cultures. Hence, given that drastic decline in optical density of B. subtilis NRS-762 stationary phase cultures could confound experiment readout of studies aimed at assessing the survivability of microbes under differing environmental stressors, the bacterium is not suitable as model organism for microbial survivability studies.