The antifungal activity of natamycin toward molds isolated from commercially manufactured poultry feed

The antifungal activity of natamycin, a polyene antifungal compound, was ev aluated on molds isolated from commercial poultry feed. The antifungal acti vity was measured by determination of the minimal inhibitory concentration (MIC) for natamycin on molds growing on semisolid microbiological medium (p otato dextrose agar) containing pure natamycin at concentrations ranging fr om 0 to 200 mg/liter. Natamycin exhibited a high degree of antifungal activ ity against the 191 isolates of aspergilli used in this study, with average MIC values ranging from 5.08 to 40.1 mg/liter for Aspergillus fumigatus an d Aspergillus parasiticus, respectively. Natamycin was also equally effecti ve in inhibiting the growth of nonaflatoxigenic compared with aflatoxigenic isolates of Aspergillus flavus and A. parasiticus. Natamycin was also effi cacious against molds other than aspergilli, with MIC values ranging from 2 .15 to 5.80 mg/liter for Paecilomyces and Rhizopus spp., respectively. Nata mycin exhibited apparent sporicidal activity against spores of toxigenic st rains of Fusarium moniliforme and A. parasiticus but not Penicillium rubrum . This sporicidal activity was evident only when spores were exposed to an in vitro concentration of natamycin of 25 mg/liter or higher for a period o f time of at least 12 hr. The growth inhibiting activity of natamycin was m ore pronounced compared with the sporicidal activity.