Document: GRE-3-46-21

The importance of allelopathy in mediating competition between naturally co-occurring bacterial strains.

KRUKONIS, G.P.* and F.M.COHAN

Wesleyan University, Middletown, CT 06459,U.S.A. ¹

Abstract:
Bacterial strains from many taxa can kill or inhibit the growth of other bacterial strains. This allelopathy may occur by several mechanisms, including production of primary metabolites, production of antibiotics, and the induction bacteriophage. In general individual strains are unaffected by their own inhibitory substances. The ubiquity of allelopathy has led to the suggestion that such interactions may be important in bacterial competition, but there is a paucity of data from collections of naturally sympatric isolates. We examined the allelopathic interactions of wild isolates of Bacillus subtilis. The bacteria were collected from a small (several grams) soil sample, and we examined the effects of direct competition between pairs of bacterial strains in three experimental environmentspetri plates, liquid culture, and soil microcosms. We determined the prevalence of allelopathy between stains, and measured the magnitude of the interaction. We varied the relative and absolute densities of the competing strains to determine how a numerical advantage of one strain over the other would effect the outcome and to determine to what extent the relative competitive abilities of the pair changed as the absolute densities were altered. Allelopathy was common between bacteria from our sample community, and bacterial pairs, even those with demonstrably different substrate utilization preferences, usually achieved lower total density in mixed culture than either strain attained in monoculture. Competition mediated through resource acquisition is not the rule for a typical pair of wild bacterial isolates. We found orders of magnitude differences in the relative densities at which two strains achieved allelopathic parity, while within pairs the superiority of the more toxic strain was often constant across a wide range of absolute starting densities. Finally, we observed numerous cases where, at equal densities, Strain A could kill Strain B, which could in turn kill Strain C, yet Strain C would kill strain A. This suggests that the outcome of competition in natural communities may be highly dependent on initial densities of the competing strains irrespective of resource availability.

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This abstract is being presented at: 8:15 AM in session:
Oral Session #23: Soil Ecology.