Possible odour-mediated attraction of flies to Bacillus subtilis NRS-762 stationary phase culture

Background and purpose: Volatile compound secreted by Proteus mirabilis mediates interkingdom signalling between the bacterium and flies, and activation of swarming motility. The above observation helps bring broader meaning to serendipitous observation of flies moving on or around the cotton plugs of aerobic Bacillus subtilis NRS-762 stationary phase cultures in a volatile compound-dependent phenomenon, leading to the formulation of a conceptual model on interkingdom signalling mediated cell population survival. Specifically, long-range dispersal of the bacterium might constitute one arm of a dual strategy enabling B. subtilis and possibly, more generally, Bacteria, to cope with environmental stressors occurring over a range of timescales.

Main conclusion(s): Pungent odours emanated from B. subtilis stationary phase cultures coincident with observations of drastic optical density decline and flies gathering on and around the cotton plugs. More important, flies numbers was odour pungency dependent, and greater pungency correlated with more intense attempts by flies in gaining entry into the cotton plug matrix; thereby, suggesting possible behaviour modification effects of the volatile compounds. Lack of cellular debris in autoclave decontaminated cultures further suggested that sporulation (an important cellular differentiation mechanism for responding to nutritional stress) did not occur to a significant extent. Hence, cell lysis could be mediated by either cannibalism or entry into the lytic cycle by resident prophages, although, in the current context, cannibalism would be more likely given its utility in releasing cytoplasmic content as carbon and nitrogen sources for the remaining population. Synthesising the observations into a conceptual model points to possible roles of volatile compound secretion as mediators of interkingdom signalling, where flies are attracted and enlisted as transport vectors for long-range dispersal of B. subtilis in return for possible protection of the flies’ gut microbiome or supply of metabolites. Useful for coping with irreversible decline in a microcosm’s habitability, long-range cell dispersal could be part of a broader strategy by which B. subtilis respond to nutritional stress, in which, well-characterized cell dormancy (e.g., persister formation and stringent response), and cellular differentiation programmes (e.g., competence, cannibalism and sporulation) facilitate adaptation to short- and medium-term environmental fluctuations. Given the importance, from the organismal perspective, of preserving the clonal population, such a dual-pronged strategy may be generic across the domain, Bacteria, and of a deep evolutionary origin. Interested researchers are invited to extend the preliminary findings reported.