A modified MPN method for counting oligotrophic bacteria by using naturally occurring organics.

Abstract

A MPN method was modified to determine the number of oligotrophic bacteria in fresh water. A series of MPN media were prepared with natural lake water by either autoclaving or filter-sterili-zation, and with diluted peptone solution (0.5mg/l) as a reference. Bacterial growth-positive test tubes were judged directly by epifluorescence microscopic counting. The bacterial counts using the medium of filter-sterilized in situ lake water was significantly higher than those obtained by autoclaved in situ lake water or peptone media. The difference in counting values between auto-claving and filter-sterilization was much larger when natural lake water media were prepared after several days' incubation under light condition. In this period, the organic carbon concentration (as COD, chemical oxygen demand) increased more than twice, mainly due to the excretion by phytoplnakton. These results suggest that naturally occurring dissolved organic matter, which was much suitable for the growth of autochthonous bacteria (oligotrophs) in the corresponding period, becomes one of the best media for counting the number of oligotrophic bacteria. Seasonal fluctua-tion of bacterial counts obtained by the present method showed a good correlation to that of chl. a, suggesting that the natural population of heterotrophic bacteria was influenced by phytoplankton through its excreted organic carbon.