The co-killing of penicillin sensitive and penicillin resistant bacteria at low concentrations of the antibiotic.

There are a number of reports that bacteria which have been made resistant to antibiotics may be rendered normally susceptible by contact with sensitive organisms, with extracts of such organisms, or with culture filtrates. Some of these reports are mutually contradictory in important respects, and the results have not been reproducible regularly by other workers. Voureka (1948) found that when penicillin resistant staphylococci or streptococci were grown in mixed culture with penicillin sensitive pneumococci, staphylococci, streptococci, or Corynebacterium diphtheriae and then reisolated, a large proportion of the colonies were sensitive to penicillin. Surprisingly, organisms naturally resistant to penicillin, such as Salmonella typhosa, also were capable of "sensitizing" the penicillin resistant staphylococci. Further, it was not necessary that the two bacterial strains actually grow together since exposure of resistant staphylococci to the sensitizing strains for as little as five minutes at incubator, room, or refrigerator temperatures often sufficed. Lysates produced by bacteriophage or chemicals, and autolysates of various organisms also induced penicillin sensitivity as did exposure to chloramphenicol and to antisera (Voureka, 1951), or exposure to sublethal amounts of various toxic agents including penicillin (Voureka, 1952). Although some of the early observations of Voureka were confirmied by Winner (1948) and George and Pandalai (1949), they could not be repeated by Bennison and Schwabacher (1948). Barber (1948), in commenting on Voureka's findings, pointed out that single colonies derived from a presumably resistant culture may be much less resistant than the parent culture and emphasized the necessity for careful controls on the