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

Authors: Bacon, Charles; Hinton, Dorothy; MITCHELL, TREVOR - University Of Georgia; SNOOK, MAURICE - Retired ARS Employee; OLUBAJO, BABATUNDE - University Of Georgia

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2012
Publication Date: 3/23/2012
Citation: Bacon, C.W., Hinton, D.M., Mitchell, T.R., Snook, M.E., Olubajo, B. 2012. Characterization of endophytic strains of Bacillus mojavensis and their production of surfactin isomers. Biological Control. 62:1-9.

Interpretive Summary: Bacterial endophytes dwell within tissues of plants and are considered in general as plant friendly. One species of bacterial endophyte, Bacillus mojavensis, was isolated and found capable of antagonizing the growth of a pathogenic fungus Fusarium verticillioides of corn. This fungus also has the ability to also dwell within corn tissue where it can produce mycotoxins that cause performance problems in livestock, poultry and is correlated with human toxicity. The toxins produced by this fungus are the fumonisins. Thus, the economic threat of this fungus on corn is severe. The discovery of a bacterial endophyte that can control this fungus suggest a novel biocontrol agent. We recently discovered that this bacterium control this fungus by the production of a potent class of antibiotics the surfactins that are produced as chemical variants or isomers. This paper characterizes all the available laboratory strains of the bacterium relative to surfactin production, both total and individual isomers of this class of antibiotics on laboratory media. The results identified several that should be better biocontrol due to both the nature and amounts of biologically active isomers of surfactin.

Technical Abstract: 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 Bacillus 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. Maize plants infected by this endophytic bacterium also displayed a marked improvement in growth and development, disease protection, and mycotoxin reduction. Recently, we reported that a patented strain B. mojavensis produced the biosurfactant Leu7-surfactin that was inhibitory to fungi. The first objective was to validate the identity of strains, and analyze genotypically the 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 a library of B. mojavensis strains for surfactin production along with the structural analogs of this biosurfactant, measure in vitro inhibition, and to analyze genotypically these strains. 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.