Abstract
A membrane filtration technique was developed for the enumeration of B. fragilis group (BFG) organisms from sewage and natural waters The method uses the agar medium of Wilkins and Chalgren supplemented with gentamicin, penicillin, esculin and ferric ammonium citrate. Membrane filters with 0.22 .mu.m pores were significantly more efficient than those with 0.45 .mu.m pores in the isolation of BFG. A preliminary incubation period of 4 h at 30.degree. C prior to 44 h at 37.degree. C yielded significantly higher numbers of BFG than direct incubation at 37.degree. C for 48 h.

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