Inhibition of Penicillium nordicum in MRS medium by lactic acid bacteria isolated from foods

During a screening procedure for lactic acid bacteria exhibiting antifungal activity against an ochratoxin-producing Penicillium nordicum (BFE 487), numerous strains were observed to produce zones of inhibition against the mould on MRS agar. For the detection of antifungal activity in the culture supernatants, an assay was used where P. nordicum was spotted on agar plates prepared by mixing malt extract agar with the filter-sterilized supernatant. In this “mould agar spot assay” growth of the Penicillium was prevented only with supernatants containing the sodium acetate concentration of normal MRS medium and showing a low pH indicating that acetic acid is the main factor involved in inhibition. The comparison of the antifungal effect of culture supernatants from selected LAB strains with un-inoculated MRS medium acidified to the respective pH by addition of HCl or lactic acid showed that the culture supernatants were more effective in inhibiting P. nordicum growth than the acidified MRS medium, indicating that besides acetic and lactic acid other metabolic products of the LAB contribute to the inhibition.