Characterization of endophytic strains of Bacillus mojavensis and their production of surfactin isomers

Bacillus subtilis consists of a large collection of strains from which several cryptic species have been delineated, and most of these along with strains within the species are important biocontrol agents. Bacillus mojavensis, a species recently distinguished from this broad B. subtilis group of bacteria, was determined to be endophytic and to have biocontrol potential due to its inhibition of the maize mycotoxic fungus Fusarium verticillioides. Recently, we reported that a patented strain B. mojavensis produced the biosurfactant Leu7-surfactin that was inhibitory to fungi. In this study, the first objective was to validate the identity of strains, and analyze genotypically a collection of B. mojavensis strains, which involved analysis of repetitive-PCR amplified Bacillus DNA sequences with a PCR genotyping bar system. In an effort to understand further surfactin production, a second objective was to screen this collection of B. mojavensis strains for surfactin analogs. The results indicated that all strains are valid B. mojavensis, and that there was genotypic diversity among strains from the great deserts. Further, the study established that most strains can produce a mixture of surfactins that was comprised of acyl chain lengths ranging from C-11 to C-17. These experiments indentified high producers of C-15 surfactin, the most biologically active isoform. However, the in vitro inhibition observed did not necessarily relate to total surfactin concentrations, suggesting a complex mechanism for inhibition and/or the presence of other unknown factors.

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► We identified endophytic strains of Bacillus mojavensis by rep-PCR.
► The strains apparently exist both as clones and as distinct ecotypes.
► We demonstrated the ability of strains to produce several surfactins.
► The antagonistic effect of surfactin on Fusarium verticillioides was demonstrated.