Penicillin-induced Penicillin Resistance 81 



same population as that which undergoes penicilHnase- 

 induction under the direct influence of penicilhn (Kogut, 

 Pollock and Tridgell, 1956). However, a culture of strain 569 

 cannot be induced to evolve into a culture of 569/H by growth 

 in the liquid penicillin-containing medium. There are three 

 possible reasons for this. (1) The margin of diff'erence between 

 the penicillin resistance of 569 and of 569/H is not large 

 because (as explained above) increased penicillinase forma- 

 tion is induced so rapidly in 569 that cells take only a few 

 minutes before they are able to produce the enzyme almost 

 as fast as 569/H. (2) 569/H grows more slowly than 569. Its 

 specific growth rate in broth or casein hydrolysate is about 

 80 per cent that of 569 in the same media. (3) Penicillinase is 

 a communal bacterial weapon against penicillin, especially 

 because 85-90 per cent of the total enzyme is liberated into 

 the medium by both strains and therefore its production by 

 one section of a mixed population of cells might be expected 

 also to protect, to some extent, other individuals in the 

 culture. 



The mutation-plus-selection method of developing peni- 

 cillin resistance can, however, be studied satisfactorily in 

 another group of B, cereus strains, none of which are suscept- 

 ible to penicillinase induction. These will be referred to as 

 the "5" group. B. cereus strain 5 was found (Sneath, 1955) 

 to be unique amongst Q5 strains oiB. cereus tested, in having 

 a very high penicillin sensitivity (the growth of single spores 

 being inhibited by only 0-01 units penicillin/ml.). No peni- 

 cillinase production [as tested by the standard manometric 

 method of Henry and Housewright (1947)] could be detected 

 in strain 5, either before or after treatment with penicillin. 

 After overnight growth of a fairly large inoculum (approx. 

 10^ cells per ml.) into broth containing 100 units penicillin/ml., 

 strain 5 was found to evolve into a practically homogeneous 

 population of penicillin-resistant cells (5/B) capable of form- 

 ing penicillinase spontaneously at very high rate. By applica- 

 tion of the velvet pad technique of Lederberg and Lederberg 

 (1952), Sneath was able to demonstrate very beautifully that 



