Factors Affecting the Survival and Growth of Bacteria Introduced into Lake Water

The populations of  Pseudomonas  sp. B4,  Escherichia coli, Klebsiella pneumoniae, Micrococcus flavus , and  Rhizobium leguminosarum  biovar  phaseoli  declined rapidly in lake water. The initially rapid decline of the two pseudomonads and  R. phaseoli  was followed by a period of slow loss of viability, but viable cells of the other species were not found after 10 days. The rapid initial phase of decline was not a result of  Bdellovibrio  spp., bacteriophages, or toxins in the water since  Bdellovibrio  spp. were not present and passage of the lake water through filters that should not have removed bacteriophages or soluble toxins led to the elimination of the rapid phase of decline. The addition of 250 μg of cycloheximide and 30 μg of nystatin per ml eliminated viable protozoa form the lake water, and the population of  Pseudomonas  sp. B4 did not fall and the decline of  E. coli  and  K. pneumoniae  was delayed or slowed under these conditions.  Pseudomanos sp. L2 proliferated rapidly in lake water amended with glucose, phosphate, and NH 4 NO 3 , but its numbers subsequently fell abruptly; however, in water amended with cycloheximide and nystatin, which killed indigenous protozoa, the population density was higher and the fall in numbers was delayed. Of the nutrients, the chief response was to carbon, but when glucose was added, phosphorus and nitrogen stimulated growth further. Removing other bacteria by filtering the lake water before inoculation with  Pseudomonas  sp. L2 suggested that competition reduced the extent of response of the pseudomonad to added nutrients. We suggest that the decline in lake water of bacteria that are resistant to starvation may be a result of protozoan grazing and that the extent of growth of introduced species may be limited by the supply of available carbon and sometimes of nitrogen and phosphorus, and by predation by indigenous protozoa.