Automated Counting of Bacterial Colonies on Spread Agar Plates and Non-Gridded Membrane Filters

The Biotran II automated colony counter was compared with a manual procedure for accuracy in counting bacterial colonies using both spread agar plate and membrane filter techniques. Comparative total bacterial counts of 250 samples (14 food, 124 water and 112 raw milk) were analyzed using the spread agar plate technique. Compared to manual enumeration, the Biotran II was found to be inaccurate for counting bacterial colonies on spread agar plates. Only 60 (24%) and 79 (31.6%) Biotran II counts fell within 10 and 20%, respectively, of the corresponding manual counts. Two samples from each of three river and four effluent sources were analyzed for total aerobic, total coliform, fecal coliform, fecal streptococci and total staphylococci bacterial counts using non-gridded membrane filters. A yellow acetate filter was used to mask the background growth and enhance the target colonies on the membrane filter. However, the method had limited success. Only 12 (20.7%) and 20 (34.5%) Biotran II counts fell within 10 and 20%, respectively, of the corresponding manual counts. Until the effect of background growth can be eliminated, the Biotran II cannot be relied upon to accurately count bacterial colonies on membrane filters.