Ref ID : 1166
Larry M. Mallory, Chang-Soo Yuk, Li-Nuo Liang, and Martin Alexander; Alternative prey: A mechanism for elimination of bacterial species by protozoa. Appl.Environ.Microbiol. 46(5):1073-1079, 1983
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Antibiotic-resistant strains of Salmonella typhimurium and Klebsiella pneumoniae died readily after their addition to raw sewage, but they grew in sterilized sewage. The decline was not a result of abiotic stresses, and because the bacteria were able to survive in large numbers for at least 15 days in solutions containing no organic nutrients, it was not a result of competition. Toxin production, bacteriophages, and Bdellovibirio sp. did not cause the disappearance of the two bacterial species. A decline was also evident of the sewage was first passed through a 3-um (pore size) filter or treated with cycloheximide or cycloheximide plus nystatin, but protozoa developed under these conditions. Little or no decline occurred of the sewage was filtered and treated with the eucaryotic inhibitors before the addition of S. typhimurium or K. pneumoniae, and protozoa were not detected. S. typhimurium increased in abundance of cycloheximide, streptomycin, and erythromycin or large amounts of glucose were added to sewage. Tetrahymena thermophilus did not significantly reduce the population of S. typhimurium in buffer when the density of the bacterium was about 10E4/ml. However, when more than 10E8 Enterobacter agglomerans cells per ml were added to the buffer, T. thermophilus reduced the abundance of E. agglomerans and S. typhimurium to 10E6 and 10/ml, respectively. The density of S. typhimurium was further decreased by a second increment of E. agglomerans cells. The disappearance of S. typhimurium and K. pneumoniae from sewage thus is the result of predation by protozoa. It is proposed that predators will eliminate a prey species from a natural environment when an alternative prey is present at concentrations above the threshold number for active feeding by the predator and when the rate of growth of the prey is less than the rate of predation.