Novel method for selective isolation of actinomycetes.

A new technique for the selective isolation of actinomycetes from natural mixed microbial populations is described. A nutrient agar medium was overlaid with a 0.22- to 0.45-microns-pore cellulose ester membrane filter, and the surface of the filter was inoculated. During incubation, the branched mycelia of the actinomycetes penetrated the filter pores to the underlying agar medium, whereas growth of nonactinomycete bacteria was restricted to the filter surface. The membrane filter was removed, and the agar medium was reincubated to allow the development of the isolated actinomycete colonies. This procedure selects actinomycetes on the basis of their characteristic mycelial mode of growth, offers a general method for their selective isolation, and does not rely on the use of special nutrient media or of antibacterial antibiotics.