1980 Volume 46 Issue 5 Pages 634-638
Most media developed for enumerating soil actinomycetes do not prevent the growth of bacteria. When the pH of water agar was adjusted to 10.5, more than 90% of bacterial colonies from natural soil were suppressed, whereas the number of actinomycete colonies remained constant. All six species of bacteria tested were completely or nearly completely inhibited in alkaline water agar, whereas, colony development of all six actinomycetes was not affected. When water agar was used the numbers of actinomycetes from soil amended with glucose or alfalfa meal were very difficult to count because of the presence of numerous bacteria. In alkaline water agar more than 99.9% of bacteria were suppressed, and the numbers of actinomycetes were easily determined.