CHARACTERIZATION OF SIDEROPHORE PRODUCTION BY THE BIOLOGICAL-CONTROL AGENT ENTEROBACTER-CLOACAE

Enterobacter cloacae EcCT-501, which suppresses Pythium damping-off of cucumber and other plant hosts, produces the hydroxamate siderophore aerobactin and a catechol siderophore tentatively identified as enterobactin. Cloned fragments of genomic DNA of E. cloacae EcCT-501 complemented the enterobactin (ent) biosynthesis mutations entA, entB, entC, entE, and entF of Escherichia coli. By complementation of ent mutations of E. coli, we localized the ent(CEB)A region to a 4.5-kb HindIII-XhoI fragment of the E. cloacae genome and the entA gene to a 1.8-kb EcoRI-XhoI fragment within the ent(CEB)A region. We deleted a 0.8-kb EcoRV fragment internal to the entA genomic region of E. cloacae EcCT-501 to construct mutants deficient in enterobactin production (Ent(-)). Introduction of this deletion into a derivative of EcCT-501 that was deficient in aerobactin production (Iuc(-)) resulted in a mutant deficient in the production of both enterobactin and aerobactin (Ent(-) Iuc(-)). Strain EcCT-501 and the Ent(-) Iuc(+) and Ent(+) Iuc(-) derivatives grew on an iron-limited medium. The Ent(-) Iuc(-) mutant, however, failed to grow on the iron-limited medium, indicating that both aerobactin and enterobactin function in iron acquisition by E. cloacae. Aerobactin and enterobactin production by E. cloacae did not contribute to its activity in biological control of Pythium damping-off of cotton or cucumber.