Penicillin-induced Penicillin Resistance 79 



lation has returned to the original state of (relative) penicillin 

 sensitivity. The phenomenon of induced penicillinase form- 

 ation occurs in most strains of B. cereus, and in Bacillus 

 suhtilis and Bacillus megaterium, and has been studied in 

 great detail in B, cereus strain 569 (Pollock, 1953). It is not 

 proposed to discuss it in much further detail here except to 

 point out that even the uninduced cells (untreated with peni- 

 cillin) produce small quantities of penicillinase ("basal" 

 enzyme) and that maximal induction results in its formation 

 at about 300 times the basal rate. It has not hitherto been 

 found possible to test the penicillin-sensitivity of cells in the 

 absence of penicillinase induction. This is probably because 

 penicillin, except in very high concentrations, does not itself 

 appear to inhibit induced penicillinase synthesis, which takes 

 place very rapidly after addition of penicillin in sensitivity 

 tests. Thus although the increase in penicillin resistance of 

 individual cells on penicillinase induction is probably con- 

 siderable, only a relatively slight increase can be actually de- 

 monstrated. It could, however, be shown (by the experiment 

 summarized in Table I) that preinduction of cells with peni- 

 cillin at subinhibiting doses does considerably increase their 

 chances of survival when treated later with much higher con- 

 centrations of the drug. Spores of strain 569 incubated for 

 5 hours in plain nutrient agar (i.e. having grown to produce 

 microcolonies of 16 to 32 cells) were unable to survive addition 

 of penicillin to a concentration above 60 units/ml., while the 

 same inoculum grown in penicillin-agar suffered only slight 

 reduction in viability after addition of 100 units/ml., and 

 a considerable proportion of microcolonies (11 out of 42) were 

 still able to grow into normal colonies after addition of 160 

 units penicillin/ml. (No such protective effect of induction 

 occurs with the penicillinase-constitutive mutant strain 569/H 

 which forms penicillinase at maximal rate without need for 

 previous treatment with penicillin.) 



The second process by which B. cereus populations may 

 acquire the penicillinase type of penicillin resistance is the 

 (perhaps) more familiar one of mutation followed by selection. 



