Isolation of Lac+ Mutants from a Lac- Strain of Escherichia coli, by the Replica Platinǵ Technique

Lactose-fermenting (lac+) mutants were isolated from a non-lactose-fermenting (lac-) strain of Escherichia coli, strain O 110, by the replica plating technique. When high dilutions of a 24 hr. broth culture of the strain were plated on Endo agar, to give discrete colonies, secondary lac+ colonies began to appear in the initially lac- colonies after 3 days' incubation; after 10 days, over 50% of colonies contained secondary lac+ colonies. Twenty serial subcultures were made in broth containing lactose, each subculture being incubated for only 6 hr. to prevent selection of lac+ mutants from exhaustion of other nutrients while exposing the bacteria to lactose during many generations. When heavy inocula from the final subcultures were plated on Endo agar, no more lac+ colonies appeared than from control platings of broth cultures which had not been exposed to lactose. An R variant of the lac- strain of E. coli, strain O 110, was found to assimilate lactose to a considerable extent without fermentation; the original S form possessed this capacity only to an insignificant extent.