The biosynthesis of ochratoxin A by Penicillium as one mechanism for adaptation to NaCl rich foods.

Penicillium.nordicum is an ochratoxin A producing filamentous fungus, which is adapted to sodium chloride and protein rich food environments like certain cheeses or dry cured meats. Penicillium.verrucosum usually occurs on cereals but can also be isolated from brined olives. It could be shown that sodium chloride has a profound influence on the regulation of ochratoxin A biosynthesis in both Penicillium species. High amounts of ochratoxin A are produced by P. nordicum over a wide concentration range of NaCl (5-100 g/l) with a weak optimum at about 20 g/l after growth on YES medium. P. verrucosum shifts secondary metabolite biosynthesis after growth on YES medium from citrinin at low to ochratoxin at elevated NaCl concentrations. The ochratoxin A biosynthesis of P. nordicum is accompanied by an induction of the otapksPN gene, the gene of the ochratoxin A polyketide synthase. A mutant strain unable to produce ochratoxin showed a drastic growth reduction under high NaCl conditions. Determination of the dry weight and the chloride content in the mycelium of the P. nordicum wild type strain and a non-ochratoxin A producing mutant strain showed a much higher increase of both parameters in the mutant compared to the wild type. These results suggest, that the constant biosynthesis and excretion of ochratoxin A, which itself contains a chloride atom, ensures a partial chloride homeostasis in the fungal cell. This mechanism may support the adaptation of ochratoxin A producing Penicillia to NaCl rich foods.