Antimicrobial-producing lactic acid bacteria isolated from raw barley and sorghum

From a total of four thousand presumed lactic acid bacteria, obtained from raw, unmalted sorghum and barley, 308 isolates were shown to exhibit inhibitory activity against the indicator strain Listeria innocua 4202. Six of these inhibitor-producing isolates were selected for further study on the basis of their relatively wide antimicrobial spectrum, which showed that these producers inhibited several Gram-positive bacteria, including a range of beer spoiling bacteria. The proteinaceous nature, anti-microbial activity against closely related species, heat resistance and pH stability of the inhibitory substances produced by these six bacteria identified these compounds as bacteriocins. All six isolates were shown to secrete the inhibitory compounds into the cell free supernatants. Bacteriocins produced by five of the six producers were purified to homogeneity. Further analytic data was obtained for three of the inhibitory compounds by means of mass spectroscopy and/or N-terminal amino acid sequencing.