Drug Resistance of Staphylococci 271 



increased drug-tolerance. From such plates, therefore, sub- 

 cultures were taken from the area of increased growth. 



Fig. 4 shows a more active penicillin-destroying culture 

 isolated from the Oxford staphylococcus after more than 

 12 months. It will be seen that after 24 hours of incubation 

 colonies of the variant strain are smaller than those of the 

 parent culture, but after 7 days the reverse is the case. The 

 resistance, except in degree, resembles that of a typical 

 penicillinase-producing strain. 



Figs. 5 and 6 show variants derived from D3S, after 8 or 

 more than 12 months respectively, on one side, and the 

 parent culture on the other. The resistance of the two 

 variant cultures is similar, except in degree, and with the same 

 reservation resembles that of D3R. Again it will be seen that 

 the colonies of the variant strains are smaller than those 

 of the parent after 24 hours of incubation, but larger after 

 7 days. 



Finally, Fig. 7 shows a ditch-plate on which a number of 

 overnight broth cultures have been streaked at right-angles 

 to the ditch. The top three are typical penicillinase-producing 

 strains and the next two are penicillin-sensitive; streak 6 is 

 a semi-transparent penicillin-tolerant staphylococcus; streaks 

 7-10 are weakly penicillin-destroying strains isolated from the 

 Oxford staphylococcus (7 and 9) and D3S (8 and 10). The 

 penicillin-destroying strains show thick luxuriant growth at 

 the penicillin end of the streak, whereas growth of the peni- 

 cillin-sensitive and penicillin-tolerant strains fades away as the 

 concentration of penicillin becomes bacteriostatic. 



Conclusions 



Staphylococci with weak penicillin-destroying activity 

 have been isolated from two penicillin-sensitive strains of 

 Staph, aureus after prolonged exposure to penicillin. Whether 

 this be due to a penicillinase similar to that of strains isolated 

 from cases of penicillin-resistant infection remains to be deter- 

 mined. At present the two types of resistant culture differ in 



