Characteristics of bacterial communities having different colony forming time on agar plates.

Bacterial communities in epilithon on the river bed were examined at the Tamagawa River by a plate count method. Colonies developed on 1/2 PYG agar plates were classified into four groups according to the colony forming time. Incubation periods which were needed for the colony formation were between 0 and 48 hours for group A, 24 and 72 hours for group B, 120 and 168 hours for group C, and 288 and 336 hours for group D. Generic compositions and diversity indexes of the four groups were similar each other. On the other hand, positive reaction against morphological and physiological characterization (specific total activity) of these groups were quite different each other. Group A, which contains the organisms having the fastest colony forming time, had high specific total activity. On the other hand, group D, which is constituted with the bacteria having the slowest colony forming time, had lowest specific total activity. Most bacterial strains of the group A grew rapidly on the 1/2 PYG agar medium, too. However, the group D included organisms which grew both rapidly and slowly on this medium. Based on these results, the colony formation of the bacteria seemed to prescribe largely by the physiological conditions of the cells at the time when cells were placed on the agar plates.