Evaluation of the Autoanalysis Colilert test for detection and enumeration of total coliforms.

The Autoanalysis Colilert (AC) test was compared with the membrane filter (MF), 10-tube multiple-tube fermentation (MTF) technique, and the presence-absence test as described in Standard Methods for the Examination of Water and Wastewater for the detection and enumeration of total coliforms in water. The methods were evaluated with 31 samples from seven different sources. Each sample was analyzed by each of the techniques, using replicate 100-ml sample volumes. A total of 582 confirmed tubes were positive by the MTF test, and 533 tubes were positive by the AC test. Statistical analysis of the most-probable-number comparability data showed a statistically significant difference in the number of positive tubes, with the MTF test resulting in more positive tubes. There were no statistically significant differences in precision between the two methods. All the methods were comparable in detection of total coliforms. Levels of heterotrophic bacteria generally encountered in drinking water did not interfere with detection or enumeration of coliforms by the AC test.

Factors affecting virus plaque confirmation procedures.

Direct plaque counts obtained by using the monolayer cell culture assay technique reliably confirmed the number of viruses isolated. Analysis revealed some significant differences in the false-positive rate, depending on the test method used or virus samples evaluated. Plaques from laboratory stock viruses showed a higher confirmation rate than sewage plaque isolates. Test results with laboratory stock viruses suggested that confirmation rates may be affected by virus types present in the sample. Plaques picked by the stab and scrape method and immediately passed into cell culture tubes produced the most reliable counts as compared to those picked by the stab only method or those stored at -70 degrees C in Earle's balanced salt solution with or without fetal calf serum. Plaque confirmation using this method was 90% or better. Although the term 'false positive plaque' has been applied to a particular plaque that was not confirmed, five of ten plaques picked by the stab and scrape method in one series of experiments were confirmed when repicked by the same method from original plaque bottles, indicating that a substantial number of unconfirmed plaques may be caused by plaque transfer techniques.