Three enteropathogenic Escherichia coli serotypes (PE) were compared with three non-pathogenic E. coli strains (E) and three Enterobacter aerogenes strains (A) on the basis of their biochemical, thermal death time, and low temperature growth characteristics. All nine strains were isolated from Canadian pasteurized dairy products. It was not possible to differentiate consistently between the PE and E strains from the results of any biochemical tests performed. Differentiation was achieved, however, on the basis of differences in colonial morphology of the strains when grown on Tergitol-7 agar containing 0.004% 2, 3, 5-triphenyltetrazolium chloride; the PE strains produced rough or intermediate rough colonies on this medium, whereas the E and A strains produced smooth or mucoid colonies. Thermal death time studies showed that one or more of the E strains might survive a commercial high temperature process if the number of cells present in the raw milk were unusually large; the PE and A strains were relatively heat sensitive and would be unlikely to survive pasteurization. z-Values found for the strains varied from 9.6 to 16.5, but the magnitude of these values was not directly correlated with the identity of particular groups of strains. Viable counts of the E strains suspended in skim milk at 7 C decreased with time; counts of the A strains and of one of the PE strains declined during the first 2 to 4 days of storage but thereafter increased rapidly. One PE strain grew only weakly under these conditions but the third showed a remarkable capacity for rapid initiation of fast growth. When cells were suspended in skim milk at −28 C, viable counts of all nine strains decreased during storage. It was concluded that particular significance can be attached to the capacity of the PE strains to multiply in skim milk held at 7 C if the serological evidence obtained is a valid criterion of their potential enteropathogenicity. The results emphasize the need for a simple and rapid test to detect enteropathogenic E. coli contamination of dairy products, and it is suggested that a cultural test, using Tergitol-7 agar containing 2, 3, 5-triphenyltetrazolium chloride as a differential medium, might be developed for this purpose.
1This investigation was supported by funds provided by Publilc Health Research Grant (No. 607-7-92) of the National Health Grants Program.
2Present address: Department of Microbiology, Macdonald College, Quebec, Canada.