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Fungal strains and the development of tolerance against natamycin

Streekstra, Hugo, Verkennis, Alex E.E., Jacobs, Robbert, Dekker, Angelina, Stark, Jacques, Dijksterhuis, Jan
International journal of food microbiology 2016 v.238 pp. 15-22
Aspergillus ochraceus, Fusarium, amphotericin B, antibiotic resistance, food preservatives, foods, fungi, minimum inhibitory concentration, natamycin, nystatin, selection pressure
Antimicrobial resistance is a relevant theme with respect to both antibacterial and antifungal compounds. In this study we address the possible development of tolerance against the antifungal food preservative natamycin. 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”. The range of Minimum Inhibitory Concentrations (M.I.C.) before (1.8–19.2μM) and after (1.8–19.8μM) 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 8.6μM and 4 strains showed a >2-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 even decreased tolerance for these other polyene antifungals.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 ensure its continued safe use as a food preservative.