IT IS GENERALLY supposed that bacteria which are sensitive to the action of penicillin seldom acquire resistance to this antibiotic as a result of the treatment of human infections. In most cases of infection amenable to penicillin therapy a prompt clinical response is obtained and administration is discontinued. Generally, there is little opportunity for resistant strains to develop in such cases as the period of treatment is usually of relatively short duration.
The appearance of resistant organisms while the patient was under treatment with penicillin has been reported.1 In most of the cases the sensitivities of the organisms had not been determined before treatment was started so that the strains may have been naturally resistant. In other cases the bacteria were isolated from open wounds and thus were exposed to contamination from outside sources, so that it is impossible to determine whether the resistant strains were derived from the organisms