80 



M. R. Pollock 



Very rarely there may appear mutant cells, which give rise 

 to clones able to form penicillinase spontaneously at high 

 rates ("penicillinase-constitutive" mutants). These muta- 

 tions occur by a process which is independent of the presence 

 of penicillin. Their cause in unknown. Penicillin, however, 

 is necessary for the full expression of resistance within the 



Table I 



Bacillus cereus : Effect of previous treatment with penicillin on the 



PENICILLIN resistance OF (o) THE PENICILLINASE-INDUCIBLE STRAIN 

 569 (b) THE CONSTITUTIVE MUTANT STRAIN 569/H. 



No. of colonies/ plate after 16 hr. incubation at 35° 



Spores of 569 and 569/H were inoculated into a series of nutrient agar plates 

 with and without added penicillin at 1 unit/ml. After 5 hr. incubation at 35° 

 layers of agar containing different concentrations of penicillin were poured on 

 the surface such that final concentrations of the penicillin, after diffusion 

 through the plate, ranged from to 160 units/ml. Incubation at 35° was then 

 continued and the colonies counted after a further 16 hr. 



culture, in order to permit the selective growth of the few 

 resistant cells and so allow the evolution of the population 

 towards one in which all or most of the individuals form the 

 enzyme spontaneously at high rate. Resistance developed in 

 this way is genetically stable and may persist undiminished 

 during repeated subcultures in the absence of penicillin. Such 

 "spontaneous" penicillinase-constitutive mutants (569/H) 

 can be detected at an incidence of about 1 in 10^ in cultures 

 of strain 569 (derived from a single spore) — i.e. amongst the 



