MODE OF ACTION AND CONSTITUTION 183 



grow in the presence of penicillin, the immediate survivors of short exposures to 

 penicillin have not gained in penicillin-resistance. Working with Staph, aureus, 

 Foster and Wilker (1943) could find no change in susceptibiUty of survivors taken 

 at all stages during growth-inhibition, and concluded that inhibition was due to 

 a prolongation of the average generation time of the culture. With B. adhcerens, 

 aeration of the culture, which presumably increases the rate of growth of the 

 bacillus, increased the rate of killing. 



A B 



Fig. 29. — Str. pyogenes grown in media containing : A, no penicillin, and B, sub- 

 bacteriostatic concentrations of penicillin ( X 1000). 

 (From photographs kindly supphed by Prof. A. D. Gardner.) 



Gardner (1940) studied the changes induced in bacteria by growth in a con- 

 centration of peniciUin 8 to 30 times lower than that causing inhibition. Spherical 

 enlargement and imperfect fission were observed in staphylococci and streptococci, 

 (Fig. 29) and Gram-positive bacilli ; and swelling and sometimes bursting in 

 Gram-negative bacilH like Bact. coli and Salmonella bacilU. No morphological 

 changes were seen in penicillin-susceptible meningococci. Gardner concluded that 

 penicilUn, like the sulphonamides, caused a failure of fission which led to cellular 

 enlargement and, in some cases, to autolysis. Macroscopically, visible lysis occurs 

 with some cultures of Staph, aureus (Fleming 1929, Smith and Hay 1942, Kantz 

 and Kirby'19446), and is probably a feature of strains possessing a powerful auto- 



