Enumeration of Scientific Populations by Immunofluorescence

Abstract

The fluorescent antibody (FA) staining technique is a powerful tool in microbial ecology since it makes possible the direct identification of microbial populations in mixed communities. A number of quantitative and qualitative immunofluorescence studies have great advanced our knowledge of the autecology of many microorganisms in diverse habitats. To illustrate the quantitative application of the technique in an aquatic habitat, the distribution of Nitrobacter, a chemoautotrophic nitrite-oxidizing bacterium, was determined in a Minnesota lake. Population densities of 2 to 10 × 102 cells/ml were relatively uniform in the upper 7 m of the water column but decreased to 0.1 to 1 × 102 in the bottom 3 m, which suggested that conditions in the upper layers were more favorable for nitrification. The distribution with depth on three consecutive days was quite similar, as were duplicate subsamples from the same sample. These results indicate that the method accurately measured the nitrobacter population density. Interference from debris and other cells was presumably the reason for nonproportionality between the sample volume assayed and the population density calculated from larger sample volumes. This interference determines the minimum population density which can be measured accurately by FA staining