Humic acid-vitamin agar, a new medium for the selective isolation of soil actinomycetes

A new medium, designated HV agar, containing soil humic acid as the sole source of carbon and nitrogen was developed. The HV agar was superior to other currently used media, including colloidal chitin agar, glycerol-arginine agar and starch-casein-nitrate agar, for the isolation and enumeration of soil actinomycetes: It allowed the growth of the largest numbers of actinomycete colonies belonging to each genus of Streptomyces, Micromonospora, Microbispora, Streptosporangium, Nocardia, Dactylosporangium, Microtetraspora and Thermomonospora on the plate, while restricting the development of true bacteria. The HV agar supported adequate growth and good sporulation for these actinomycetes. Even when spore suspensions were used as the inoculum, the HV agar produced remarkably larger numbers of actinomycetes, especially strains of the genera Micromonospora, Microbispora, Streptosporangium, Dactylosporangium and Saccharomonospora, than did glycerol-arginine agar. It was found that the spores of these actinomycetes were activated upon germination by treatment at 20°C for 30 min with a O.2% solution of humic acid prior to incubation.