Lactose-fermenting (lac) mutants were isolated from a non-lactose-fermenting (lac) strain of , 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 , 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.


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