Antifungal Activity of Some Lactic Acid Bacteria Against Aflatoxin –Producing Aspergilli

Abstract

This study was performed to investigate the antifungal activity of lactic acid bacterial (LAB) strains. Seven bacterial strains were tested against four fungal isolates that were isolated from infected corn grains and identified as Aspergillus flavus, Aspergillus parasiticus, Aspergillus niger and Rhizopus nigricans. The fungal biomass inhibition method was used as a pre-screening test. The aflatoxin-producing Aspergilli were detected by the black-light method. Aspergillus parasiticus produced both B and G aflatoxins while Aspergillus flavus produced only B aflatoxins. Twelve LAB strains were screened for antifungal activity against two aflatoxin-producing Aspergilli. The tested LAB strains were Lactobacillus acidophilus ATCC 20552, Lactobacillus acidophilus NRRL-B-4495, Lactobacillus acidophilus LA3, Lactobacillus acidophilus, Lactobacillus plantarum ATCC 14917, Lactobacillus casei DSM 20011, Lactobacillus rhamnosus, Bifidobacterium bifidium, Bifidobacterium longum 15708, Bifidobacterium lactis (Bb12), Streptococcus thermophiles, Streptococcus thermophiles ATCC-3. The antifungal activity was performed by three methods; in the Overlay method, the maximum inhibitions of A. flavus and A. parasiticus were obtained by S. thermophiles ATCC-3. While, in the well diffusion method, the maximum inhibitions of A. flavus were obtained by B. lactis (38.0 mm) and B. longum (37.0 mm) and in case of A. parasiticus, the maximum inhibition was obtained by B. lactis (35.0 mm). Whereas, in the agar dilution method; the maximum inhibitions of A. flavus were shown by B. lactis (70.3%), L. casei DSM 20011 (57.77 %) and S. thermophiles ATCC-3 (57.46 %), respectively and in case of A. parasiticus the maximum inhibitions were obtained by B. bifidum (84.59%), B. lactis (82.66%) and S. thermophiles ATCC-3 (68.69%). This inhibition may be due to the production of antifungal compounds like organic acids, hydrogen peroxide, diacetyl and proteinaceous substances. HPLC analysis indicated that lactic and acetic acids represented the main organic acids in the cell-free extract of lactic acid bacteria..