Use of Antibiotics for Selective Isolation and Enumeration of Actinomycetes in Soil.

Several antibiotics were tested against a range of actinomycetes, bacteria and fungi representing types found in soil. From these tests four antibiotics, nystatin (50 pg./ml.), actidione (50 pg./ml.), polymyxin B sulphate (5.0 pg./ml.) and sodium penicillin (1.0 ,ug./ml.), were selected for incorporation into a starch +casein medium to achieve selective growth of actinomycetes on soil dilution plates. This mixture of antibiotics was tested with a number of soils and its efficiency compared with several other methods for selective development of actinomycete colonies. The most suitable mixture for the enumeration of soil actinomycete colonies was starch + casein medium with the two antifungal antibiotics (nystatin, actidione) ; for isolation of actinomycete colonies the same medium with all four antibiotics was the most satisfactory.