Four automatic colony counters (ACC), 3M Model 620 and Artek Models 480, 870 and 880, were evaluated for their precision and accuracy in counting bacterial colonies in pour-plates prepared using raw and pasteurized milk samples. The automatic colony counters were precise, labor saving devices. but not one of the ACC units approached our acceptability criterion that 90% of the ACC counts fall within ± 10% of the corresponding manual count. Some parameters of experimental design and instrument calibration which may significantly influence the response and performance of the automatic counters are discussed.

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