Drug Resistance of Staphylococci 269 



containing the same amount of penicillin and plated out on 

 ditch-plates. The latter were incubated for 7-8 days, when it 

 was frequently noted that there was a ridge of increased 

 growth, in which case subcultures were made from this into 

 similar fluid medium. 



Penicillin-destroying activity was tested at intervals by 

 growing the cultures in the presence of larger amounts of 

 penicillin and testing for penicillin activity after various 

 periods of incubation by the agar-cup method. After over 

 two months of passage, two strains — the Oxford staphylo- 

 coccus and a penicillin-sensitive mutant (D3S) derived from 

 a penicillinase-producing culture — yielded staphylococci with 

 very weak penicillin-destroying activity. By further transfer 

 in penicillin broth (0-01 [i/ml.) and selection of likely colonies 

 from penicillin ditch-plates this activity was increased, but 

 even after more than a year the penicillin-destroying capacity 

 of these strains was less than one-hundredth that of a typical 

 penicillin-resistant culture of Staph, aureus isolated from an 

 infective process. 



Characteristics of penicillin-destroying cultures isolated 

 in vitro from two strains of penicillin -sensitive 



Staph, aureus 



Penicillin- destroying activity. After 2-5 months, strains 

 were isolated from both the Oxford staphylococcus and D3S 

 which inactivated 2-5 (x/ml. penicillin in a week, but not in 

 48 hours. After 8 months, 2 [jt/ml. penicillin was inactivated 

 in 3 hours by a strain isolated from the Oxford staphylococcus ; 

 and in 24, but not in 3, hours by one from D3S. After 16 

 months, 2 (jt/ml. penicillin was inactivated by cultures from 

 both parent strains in one hour, and 20 pi/ml. was inactivated 

 in 3 hours by two strains derived from the Oxford staphylo- 

 coccus; and in 24, but not in 3, hours by two from D3S. 

 Under similar experimental conditions, clinically isolated 

 penicillin-resistant strains inactivated from 100 to 500 {x/ml. 

 penicillin in less than one hour. 



