Application of a direct microscopic method for enumeration of substrate-responsive marine bacteria

Abstract

The suitability of a modified acridine orange epifiuorescence technique for direct enumeration of substrate-responsive marine bacteria was evaluated. Seawater samples, incubated with nalidixic acid and various growth substrates, were examined for the presence of enlarged bacterial cells. On average, direct counts of enlarged responsive cells were two orders of magnitude lower than total bacterial numbers as observed by microscopy. The number of swollen cells was found to vary with concentration and type of substrate. Largest direct counts of substrate-responsive cells were obtained in samples incubated with yeast extract or tryptone. Significantly lower counts were obtained in seawater samples incubated with simple carbon or nitrogen sources. Results indicated that growth requirements and substrate preferences of marine bacteria may affect direct microscopic counts of substrate-responsive cells.