Biochemical genetics of a natural population of Escherichia coli: seasonal changes in alleles and haplotypes.

The level of diversity, degree of enzyme polymorphism, effective population size, and the relative roles of drift and selection were examined in a cross-section of a natural Escherichia coli population based on random samples of haplotypes of E. coli isolated from sewage. The population studied contained E. coli strains derived from a human population of approximately 16,000 individuals, as well as from other sources. Three sample sets were taken between May and August. Each set consisted of 100 E. coli clones. Six enzyme loci [GPI (5 alleles), GPD (5 alleles), PGD (10 alleles), ADH (8 alleles), IDH (6 alleles), PGM (6 alleles)] were surveyed electrophoretically for each clone; 159 different haplotypes were obtained and it is likely that all possible combinations are present in the population sampled. The large numbers of different haplotypes observed were distributed as a series of four genetically similar families of clones. The large estimated effective population size (Ne = 10(10)) means that the observed large and highly significant changes in allele frequencies with time are not due to genetic drift. Selection, though not necessarily at the loci studied, is considered the only likely explanation.