Model systems for microbial interactions, bacterial commensalism, and mutualism in continuous culture

Abstract

A chemically defined medium was selected which supported the growth of Bacillus cereus but not Proteus vulgaris, in pure culture, however, P. vulgaris grew in mixed culture with B. cereus. Steady-state populations of each organism in mixed culture at various dilution rates were enumerated with Petroff-Sausser chamber. An essential niacin-like factor elaborated by cereus and required by P. vulgaris explains the interdependence. The numbers of B. cereus were lower in mixed culture than in a pure culture; thus the interaction is not a strict commensalism. A single addition of niacin to a steady-state mixed culture upset the dependence of P. vulgaris on B. cereus, and the numbers of P. vulgaris rose until continued pumping diluted the added niacin and restored the initial steady-state. Neither P. vulgaris nor B. polymyxa will grow alone on a medium deficient in niacin and biotin, but symbiotic growth occurs in batch or in continuous cultures. Proteus vulgaris will grow alone if niacin is added, and B. polymxa will grow alone but poorly if biotin is added to the basal medium. At several dilution rates, symbiotic populations in continuous culture oscillated in numbers, and no steady-states were achieved. Proteus vulgaris produces a substance, perhaps a bacteriocin, which inhibits B. polymyxa. Accumulation of this inhibitor as both species multiply in continuous culture stops B. polymyxa growth, and through the vitamin interdependency P. vulgaris growth also ceases abruptly. Both species wash-out of the vessel until the inhibitor is diluted, and rapid growth of each resumes to start a new cycle of the oscillation. The inhibitory factor was roughly characterized as unstable at room temperature, highly labile in acid or base, nondialyzable, and degraded by proteolytic enzymes. Pulse addition of chymotrypsin to a continuous culture with oscillatory populations stopped the oscillations and gave an increase in each species. Continued pumping washed out the enzyme and restored the.·. lower populations which again oscillated. Chymotrypsin added to the feed reservoir gave a steady-state increased population of each organism.