A bacterial species is operationally difficult to define. There is considerable phenotypic variation in some species, such as Escherichia coli; thus, the limits of the species are difficult to determine. Other species such as Edwardsiella tarda and Serratia marcescens are “tighter” because the strains which comprise these species show little phenotypic variation. The latter species are easier to define biochemically.

Newer taxonomic techniques have been helpful in defining bacterial species. Often, the results have agreed with those obtained by conventional methods. However, species such as Enterobacter agglomerans have been shown by DNA-DNA hybridization to be composed of over twelve different hybridization groups which could be considered “new species.”

Many different bacterial species are washed from the soil into water. Some of these are soil bacteria; others make up the gut flora of insects, invertebrates, and lower vertebrates. Often, these environmental species are closely related to species known to be pathogenic for man. Sometimes environmental isolates can be differentiated from the human pathogens by simple tests; S. liquefaciens and S. rubidaea are phenotypically different from the human pathogen S. marcescens and can be recognized easily. Environmental strains of Klebsiella, for which the taxonomy has not been completely defined, cannot yet be differentiated from the human pathogens. Similarly, in Enterobacter, Citrobacter, Proteus, Vibrio, Pseudomonas, Aeromonas, and other groups, there are taxonomic gaps where species lines are not completely clear. Many isolates that have been classified in these genera may not be pathogenic for man. Consequently, to ensure that standards will have relevance in public health, writers of microbiological standards for water should proceed very carefully in their treatment of indicator organisms. Many species in water are probably not pathogenic for humans but are closely related to species which are pathogenic. Even with a complete battery of biochemical and serological tests, isolates cannot always be assigned to their correct species. We see many isolates which probably belong to new species which have not been defined.