Comparison of four membrane filter methods for fecal coliform enumeration

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Abstract
Four membrane filter methods for fecal coliform enumeration were evaluated and compared in 6 laboratories based on determination of accuracy, specificity, upper counting limit and recovery comparability. Recovery accuracy with pure cultures ranged from 89-100% for m-FC, mTEC (a procedure developed for thermotolerant Escherichia coli) and m-FC2 methods (the latter incorporating a 2-h, 35.degree. C resuscitation period), but was < 60% for the MacConkey membrane broth method. These figures dropped by .apprx. 40-55% when the cultures were subjected to temperature (10.degree. C) stress. Close to 800 colonies were verified to determine specificity. False-positive colonies occurred most frequently with the m-FC2 method (18%); false-negative colonies were most common on MacConkey membrane broth (26%). In counting range experiments using a variety of samples, the highest upper counting limit was 130 colonies/filter with the mTEC procedure. Recovery comparisons were based on over 130 samples including raw surface waters, raw sewage, and chlorinated and unchlorinated sewage effluents. In general, recoveries were significantly higher with the m-FC2 and mTEC methods; on m-FC2, growth of nontarget background organisms was also higher in most cases. Highest recoveries from chlorinated sewage effluents were obtained by the mTEC method, and the addition of a similar resuscitation period to the m-FC procedure (m-FC2) improved fecal coliform recovery from such samples. The best overall performance for fecal coliform enumeration was obtained with the mTEC method with high recovery and low levels of background colonies, good specificity and accuracy, and a high upper counting limit. This procedure also offers the advantage of enumerating E. coli within 24 h.