Viability and stability of Escherichia coli and enterococci populations in fecal samples upon freezing

We studied the survival of Escherichia coli and enterococci populations in faecal samples of seven host species after storage at -20°C and -80°C for 30 days. The numbers of E. coli and enterococci in composite faecal samples of cows, chickens, horses, pigs, dogs, birds and humans were enumerated before and after storage. From each composite sample, 28 colonies of each bacterial species were typed before and after storage and the strains were assigned to different biochemical phenotypes (BPTs). A significant reduction in the number of E. coli was observed in all samples stored at -20°C, and three of the seven animal samples stored at -80°C. However, no difference in the number of enterococci was found in all stored samples (except cow and birds). The number and the distribution of E. coli and enterococci BPTs in fresh samples did not vary significantly from those stored at either temperature. Short term storage of faecal samples at -80°C does not significantly change the population structure of E. coli and enterococci populations. This study provides a viable option for storing faecal samples when resources limit immediate microbiological analyses