Antifungal activity of lactic acid bacteria: Factors affecting production and stability of antifungal compounds of Lactobacillus plantarum, and effects of the antifungal compounds on growth and aflatoxin production by Aspergillus spp.

Increased concern over chemical food additives has prompted searches for more natural antifungal agents. One potential source of these agents are lactic acid bacteria. To better understand the factors involved in the antifungal activity of these bacteria, this research studied the organic acid profile of Lactobacillus plantarum, the factors influencing the production of active compounds by these bacteria, the stability of the antifungal compounds produced and their effect on aflatoxin production by Aspergillus spp. The organic acids produced included lactic, acetic, pyruvic, 3-phenyllactic, p-hydroxyphenyllactic, and pyroglutamic acids. The production of antifungal compounds by the bacteria was increased with incubation time up to 96 hours and temperatures of 30 and 35oC. During bacterial growth, the best results were obtained when pH was maintained at 7.0. Cultures grown under agitation showed higher antifungal activity than those grown under static conditions, with 10% oxygen saturation favoring the production of antifungals. The bacterial supernatant lost activity at pH 4.0 or higher; while temperature and digestive enzymes did not affect the activity. When the bacterial supernatant was fractionated, the presence of 3-phenyllactic, 3-hydroxyphenylacetic, and pyroglutamic acids in some of the active fractions were indicated by HPLC-PDA and HPLC-MS. A study on the effect of bacterial supernatant on the production of aflatoxins by Aspergillus spp. gave results that varied with mold strain and aflatoxins produced, with some strains or types of aflatoxin being not affected by the bacterial metabolites, some being inhibited and others stimulated. Information provided by this research helps to better understand the factors that may be used to enhance the production of antifungals by Lactobacillus plantarum, the factors affecting the stability of the antifungals and also indicate that an assessment of the antifungal activity of these agents, along with their effect on toxin production, should be part of a risk assessment plan for any food system where they may be utilized