Inhibiting factors produced by lactic acid bacteria. 1. Oxygen metabolites and catabolism end-products

Lactic acid bacteria can produce a variety of antimicrobial compounds which may affect both the lactic acid bacteria themselves as weil as undesirable or pathogenic strains. In this first section, we describe the biosynthesis, mode of action and activity spectra of these inhibitors. Metabolites of oxygen (hydrogen peroxide and free radicals) exhibit a bacteriostatic or bactericidal activity against lactic or non-Iactic flora. When associated with the lactoperoxidase/thiocyanate system, hydrogen peroxide leads to the formation of inhibitory compounds which are bacteriostatic for lactic acid bacteria and bactericidal for Gram-negative bacteria. The antimicrobial activity of organic acids (Iactic, acetic and formic) and of pH are closely linked. It appears that the non-dissociated fraction of these acids is the major inhibitory form. Thus, acetic acid whose pKa is higher than that of lactic acid exhibits the highest inhibitory activity. The antimicrobial activities of diacetyl, acetaldehyde and of the D isomers of amino acids are also described, although their effects are slight in usual lactic fermentations. 8acteriocins are the last type of inhibitory substance produced by some lactie acid bacteria. They will be described in a second section. pH, diacetyl, acetaldehyde, 0 isomers of amino acids). In the second section, we will examine inhibitions due to small antimicrobial molecules or to bacteriocins. Inhibition resulting from inter-strain competition