Dilution culture, a method for growing the typical small bacteria from natural aquatic assemblages, has been developed. Each of 11 experimental trials of the technique was successful. Populations are measured, diluted to a small and known number of cells, inoculated into unamended sterilized seawater, and examined three times for the presence of 10 or more cells per ml over a 9-week interval. Mean viability for assemblage members is obtained from the frequency of growth, and many of the cultures produced are pure. Statistical formulations for determining viability and the frequency of pure culture production are derived. Formulations for associated errors are derived as well. Computer simulations of experiments agreed with computed values within the expected error, which verified the formulations. These led to strategies for optimizing viability determinations and pure culture production. Viabilities were usually between 2 and 60% and decreased with >5 mg of amino acids per liter as carbon. In view of difficulties in growing marine oligobacteria, these high values are noteworthy. Significant differences in population characteristics during growth, observed by high-resolution flow cytometry, suggested substantial population diversity. Growth of total populations as well as of cytometry-resolved subpopulations sometimes were truncated at levels of near 10 cells per ml, showing that viable cells could escape detection. Viability is therefore defined as the ability to grow to that population; true viabilities could be even higher. Doubling times, based on whole populations as well as individual subpopulations, were in the 1-day to 1-week range. Data were examined for changes in viability with dilution suggesting cell-cell interactions, but none could be confirmed. The frequency of pure culture production can be adjusted by inoculum size if the viability is known. These apparently pure cultures produced retained the size and apparent DNA-content characteristic of the bulk of the organisms in the parent seawater. Three cultures are now available, two of which have been carried for 3 years. The method is thus seen as a useful step for improving our understanding of typical aquatic organisms.
