Influence of host seed on metabolic activity of Enterobacter cloacae in the spermosphere

Little is known regarding the influences of nutrients released from plants on the metabolic activity of colonizing microbes. To gain a better understanding of these influences, we used bioluminescence- and oxygen consumption-based methods to compare bacterial metabolic activity expressed during colonization of two different seed types. Metabolic activity expressed by Enterobacter cloacae during colonization of pea seeds, which exude high levels of reduced carbon nutrients, was compared with that during colonization of cucumber seeds, which exude orders of magnitude less reduced carbon nutrients. Metabolic activity levels expressed by E. cloacae populations were much higher throughout a 72 h colonization period on pea seed compared with those observed on cucumber seed, directly correlating metabolic activity level with amounts of nutrients released by seeds. In vitro studies indicated E. cloacae cells expressed different levels of metabolic activity when incubated with different individual carbohydrates commonly found in cucumber and pea seed exudates. The addition of exogenous carbohydrate to cucumber seed increased metabolic activity expressed by colonizing E. cloacae; with the level of increase dependent on both quantity and type of carbohydrate supplement. Subtraction of carbohydrate available to E. cloacae on cucumber and pea seeds through mutation in pfkA decreased metabolic activity by this bacterium. Results presented here provide strong evidence that metabolic activity of spermosphere-colonizing bacteria is host dependent, and that levels of activity are based largely on both quantitative and qualitative composition of host exudate released during seed germination.