Abstract
Interactive phenomena among pairs of microorganisms were studied by overlapping drop pairs of the species on nutrient agar plates over a 103-fold range of concentration. Changes in the number and size of the colonies in and beyond the zone of overlap were recorded after incubation for 24–48 hours at 32C. Pseudomonas fluorescens, Pseudomonas fragi, and Alcaligenes viscolactis had no visible effect on coliform bacteria and certain species of Micrococcus, Serratia, Sarcina, and Gaffkya. The inhibitory action of Escherichia coli and Aerobacter aerogenes was limited to a partial or complete reduction of growth of the test species in the zone of overlap and seemed independent of the concentration of the test species. Mutual inhibition was evident when Streptococcus faecalis var. liquefaciens or Streptococcus faecalis was tested against E. coli, A. aerogenes, or Serratia marcescens. The extent of inhibition of each species depended on the ratio of concentrations of the species in the drop pair. The partial to complete and diffusible inhibition of Micrococcus flavus, Micrococcus candidus, Micrococcus freudenreichii, Sarcina flava, and Gaffkya tetragena by certain concentrations of S. faecalis var. liquefaciens, S. faecalis, and S. marcescens, seemed to be affected by the concentration of the test species.
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