A Comparison of Membrane Filters for Counting Thermoactinomyces Endospores in Spore Suspensions and River Water

Ten types of membrane filter, most with a nominal pore size of 0.45 μm, were compared for their efficiency in recovering Thermoactinomyces endospores from water samples and yielding colonies for viable counts. The endospores were in laboratory-prepared suspensions from two strains and in river water containing a natural population of actinomycetes and bacteria. There were significant differences in recovery efficiency between membranes used with river water samples; black Sartorius membranes (SM 13006) gave highest colony counts when incubated on selective agar. All membranes except Nuclepore gave good recovery from the spore suspensions but some differences were noted in their ability to recover the two strains. Thus it is inadvisable to use the results of trials on pure cultures prepared in the laboratory to predict the performance of membranes for use with natural water samples. Czapek-Dox agar with added yeast extract and Casamino acids (CYC) was a superior recovery medium to Trypticase Soy agar (TSA). The efficiency of green Gelman membranes was improved if they were boiled in distilled water before use; in contrast, boiled Oxoid membranes (original type) carried fewer colonies.