A method for estimating viability of aquatic bacteria by slide culture

To estimate the viability of freshwater bacteria, slide cultures were prepared by spreading 10 μl of water, concentrated by centrifugation, over 1 cm 2 of agar. After drying, a cover‐slip was placed on the agar. Following incubation, microcolonies and single cells were counted under a phase contrast microscope, and the viability estimated. Incubation at 10°C on casein‐peptone‐starch agar for 24 or 72 h provided the highest estimates of viability ( ca 45%). Most microcolonies developed under these conditions and the total number of microcolonies and single cells did not decrease significantly during incubation. Several morphological types of bacteria were observed as both microcolonies and single cells. Most microcolonies consisted of two cells, although some larger ones were present. The method could be used as an alternative to the spread plate and other methods which assess viability from bacterial growth on agar.