Loss of Gramicidin Biosynthesis in Gram-Positive Biocontrol Bacterium Aneurinibacillus migulanus (Takagi et al., 1993) Shida et al. 1996 Emend Heyndrickx et al., 1997 Nagano Impairs Its Biological Control Ability of Phytophthora

The soil-borne species Aneurinibacillus migulanus (A. migulanus) strains Nagano and NCTC 7096 were shown to be potent biocontrol agents active against several plant diseases in agricultural and forest ecosystems. Both strains produce the cyclic peptide gramicidin S (GS) that was described as the main weapon inhibiting some gram-negative and gram-positive bacteria and fungus-like organ-isms along with the production of biosurfactant and hemolysis activities. However, the contribution of the cyclic peptide gramicidin S (GS) to the biocontrol ability of A. migulanus has never been studied experimentally. In this paper, using a mutant of the A. migulanus Nagano strain (E1 mutant) impaired in GS biosynthesis we evaluated the contribution of GS in the biocontrol potential of A. migulanus against Phytophthora spp. The two strains of A. migulanus, Nagano and NCTC 7096, were tested in a pilot study for the inhibition of the growth of 13 Phytophthora species in dual culture assays. A. migulanus Nagano was significantly more inhibitory than NCTC 7096 to all species. Additionally, using apple infection assays, P. rosacearum MKDF-148 and P. cryptogea E2 were shown to be the most aggressive on apple fruits displaying clear infection halos. Therefore, the three A. migulanus strains, Nagano, NCTC 7096, and E1, were used in apple infection experiments to check their effect on infection ability of these two Phytophthora species. Treatment with A. migulanus Nagano significantly reduced the severity of symptoms in apple fruits compared with NCTC 7096. A. migulanus E1 mutant showed total loss of biocontrol ability suggesting that GS is a major actor in the biocontrol ability of A. migulanus Nagano strain.