Purpose: In this study, we address the possible development of tolerance against, the antifungal food preservative, natamycin.
Methods: A selection of 20 fungal species, originating from a medical, as well as a food product context, was subjected to increasing concentrations of natamycin for prolonged time, a procedure designated as "training".
Results: The range of Minimum Inhibitory Concentrations (M.I.C.) before (1.8-19.2 µM) and after (1.8-19.8 µM) training did not change significantly; but, natamycin exposure caused an increase of M.I.C. in 13 out of 20 tested strains. The average M.I.C. increased from 6.1 to 8.6 µM and four strains showed a two-fold increase of tolerance after training. One strain of Aspergillus ochraceus, also, showed increased tolerance to amphotericin B and nystatin. However, two Fusarium strains showed similar or decreased tolerance for these other polyene antifungals.
Significance: The work reported here shows that a continuous and prolonged increasing selection pressure induced natamycin tolerance in individual strains. This implies that such a selection pressure should be avoided in the technical application of natamycin to continue to ensure its safe use as a food preservative.