Direct counts of bacteria by a modified acridine orange method in relation to their heterotrophic activity

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Abstract
Modifying the acridine orange staining procedure used for direct counting of aquatic bacteria, by staining the bacteria after filtering instead of in solution, gave bright fluorescence and eliminated the need for filtering stained samples. Millipore, Sartorius, and Nucleopore filters all gave good results with the staining method. Counts on the Nucleopore polycarbonate filters were about twice as high as on Millipore and Sartorius cellulose filters. Bacteria were counted and their heterotrophic activity measured as uptake and respiration of 14C‐glucose in 111 freshwater and marine samples. For standing waters which were not grossly polluted, bacterial counts per millilitre, using Millipore filters, varied between narrow limits (1.16–17.4 × 103), but the mean heterotrophic activity per bacterium showed a much wider range of values (0.6–111 × 10−11 μg.h−1).