Development of Yeast Inhibitory Compounds for Incorporation into Silage Inoculants

Abstract

Microorganisms cause considerable losses of ensiled animal fodder. Anaerobic deterioration is mediated principally by bacteria but this can be minimised by careful practice during silage making. Aerobic deterioration is however difficult to control, especially when ambient temperatures are high and feeding rates are low as in a mild winter for example. The principal cause of this type of spoilage is the growth of yeast and, to a lesser extent, moulds. The development of strategies to control this would thus be advantageous. Previously we have screened several strains of lactic acid bacteria for production of antifungal compounds without success. In this study, strains of Bacillus were screened for the production of compounds inhibiting growth of yeast. Many species of Bacillus produce antibiotics of which Gramicidin S and Tyrocidine are the best characterised. Antifungal peptides are also known to be produced. The major family of these are lipopeptides which comprise iturins (Delcambe & Devignat, 1957), surfactins (Kluge et al., 1988), bacillomycins (Peypouxet al., 1981) and mycosubtilins (Besson & Michel, 1990). Most of these compounds are cyclic peptides containing a sequence of D and L amino acids closed by a beta-amino acid carrying a long aliphatic chain. They all appear to be active against fungi and yeasts. Although there are numerous lipopeptidic antibiotics produced by B.subtilis strains only few hydrophilic and antifungal metabolites have been reported in the literature, including the dipeptides bacilysin (Walker & Abraham, 1970) and chlorotetain (Rapp et al., 1988). Another similar substance, previously named Rhizoctonia factor (Michener & Snell, 1949), has recently been identified as a hydrophilic phosphono-oligopeptide usually existing as a dipeptide and named rhizocticin (Rapp et al., 1988). Some isolates of B.licheniformis have been found to produce bacilysin (Kugler et al., 1990), but there are no other reports on the production of di- and tri-peptides in this organism.