THE SELECTIVE ENUMERATION OF ACTINOMYCETES IN THE PRESENCE OF LARGE NUMBERS OF FUNGI

Abstract

In response to a need for an actinomycete plating medium that would effectively inhibit fungi, two substances were tested — sodium propionate and the antibiotic cycloheximide (Acti-dione). Cycloheximide proved superior to the propionate. Five micrograms of cycloheximide per ml. of agar medium greatly reduced radial growth of fungal colonies, 40 μgm. almost completely prevented fungal growth, yet even 100 μgm. showed no inhibition of any of 85 actinomycete cultures isolated from agricultural and forest soil on media free of inhibitor. The concentration selected as most suitable for routine use was 40 μgm. of cycloheximide per ml. of agar medium, added just prior to the pouring of the plates.