A two-tier miniaturized scheme of eight tests was devised for biotyping strains of Escherichia coli in microwell plates. Primary biotypes were defined by positive and negative reactions in tests for fermentation of raffinose, sorbose, dulcitol and 2-deoxy-D-ribose and for decarboxylation of ornithine when read after specified periods of incubation; subtypes were identified within primary biotypes according to results in secondary tests for rhamnose fermentation, lysine decarboxylation and motility. The method gave reproducible results on different occasions of testing.
Among 100 E. coli strains from various sources, 26 of the 32 possible primary biotypes and 56 full biotypes, as defined by results in both primary and secondary tests, were identified, thus demonstrating a high index of strain discrimination (D = 0·98).
The scheme is recommended as a simple, reliable, inexpensive and efficient method of differentiating strains of E. coli.