Bacterial Populations in Sea Water as Determined by Different Methods of Enumeration1

Five different cultural and two direct microscopic methods were employed for estimating the abundance of bacteria in samples of sea water collected from both oceanic and neritic areas. The cultural methods included macrocolony counts on nutrient agar, silica gel, and membrane filters, microcolony counts on membrane filters, and the extinction dilution method. Direct microscopic counts were made of microbes on membrane filters and of microbes transferred from membrane filters to glass slides. Direct counts showed the presence of from 13 to 9,700 times as many bacteria as cultural methods. The extinction dilution method and the microcolony membrane filter method gave counts 20 and 35 times higher, respectively, than did any of the macrocolony methods. Direct microscopic counts on membrane filters were approximately 150 times higher than plate counts, and the numbers of microbes transferred from membrane filters to glass slides were approximately 2,000 times higher than plate counts. In all of the cultural methods, a peptone‐yeast extract medium was used. Differences in the abundance of microorganisms obtained by the various methods are attributed to a variety of factors: the presence of bacteria in aggregates, selective effects of the media, and the presence of inactive cells. A marked decrease in bacterial numbers was observed just below the thermocline as reflected in the macrocolony methods but not by the direct microscopic methods. A considerable population of spirilli‐like forms was noticed directly under the microscope but not after cultivation.