Influence of Competitor Niche on the Ability of Enterobacter cloacae to Suppress the Germination Globisporangium ultimum Sporangia

Enterobacter cloacae is a bacterium that can prevent seedling damping-off by Globisporangium ultimum (formerly Pythium ultimum ). This bacterium suppresses damping-off by metabolizing the fatty acid cue released by germinating seeds that initiates G. ultimum sporangia germination and infection. However, this suppression mechanism has been unreliable because it only occurs only when carbohydrates are scarce. We hypothesized that niche preemption by competitor bacteria in carbohydrate rich environments could enhance the pathogen suppression success by E. cloacae by narrowing its realized niche to consist mainly of fatty acids. We tested this hypothesis through pair-wise competition experiments between E. cloacae and bacteria with varying niche overlap profiles and measured the germination of G. ultimum sporangia. After these trials, we postulated that other chemical substrates may influence the behavior of G. ultimum and E. cloacae based on patterns witnessed; namely amygdalin, hydroxybenzoic acid, palatinose, D-gluconic acid, and L-glutamic acid. We then exposed G. ultimum and E. cloacae to these compounds to determine how they influenced the suppression behavior of E. cloacae and the germination of G. ultimum sporangia. We found that although niche preemption does not predict pathogen suppression patterns between E. cloacae and competitor bacteria, niche overlap was inversely correlated with sporangia germination for gram-negative bacteria; the opposite pattern predicted by niche overlap. We also found several taxa that significantly enhanced the pathogen suppression by E. cloacae . Finally, we found that amygdalin and 4-hydroxybenzoic acid inhibited the suppression performance E. cloacae and found that 4-hydroxybenzoic acid, palatinose, and L-glutamic acid inhibited sporangia germination.