ANTIBACTERIAL ACTION 203 



divided into two groups, on the basis of the action of penicillin: first, 

 Neisseria gonorrhoea. Neisseria intracellular y and Neisseria catarrhalisy 

 which are sensitive j and second, A^. flavus and other nonpathogenic 

 Neisseriae, which are not sensitive (261, 266, 267). 



Different strains of S. aureus vary little in their susceptibility to the 

 action of penicillin j however, by growing the organism in increasing 

 concentrations of this substance, it is possible to obtain more resistant 

 cultures. Strains of staphylococci possessing increasing resistance to peni- 

 cillin were also isolated from infections treated with the substance 



(727)- 



The oxygen uptake of suspensions of staphylococci was not inhibited 

 to any extent by the action of penicillin for 3 hours. In a concentration 

 of 1 : 1,000, after incubation for 24 hours at 37° C, the bacteria gave 

 larger numbers of colonies on plating (7). Although 0.0 1 to o.i mg. of 

 penicillin per milliliter was found (418-424) to be sufficient to inhibit 

 the growth of 2,500,000 hemolytic streptococci (Group A), no conclu- 

 sion could be reached as to whether its action is truly bactericidal or 

 bacteriostatic. 



A comparison was made of the amounts of penicillin and gramicidin 

 required to bring about total inhibition of growth of bacteria, on the 

 basis of micrograms per milliliter of culture medium (rabbit's plasma 

 and a serum extract of chick embryo). The results were as follows: 



* Inhibition not complete at these figures. 



The two substances appeared to be as effective against bacteria in cul- 

 tures containing growth tissue as in cultures in which no tissue was 

 present (388, 389). 



Inhibition of growth of 2 to 4 million hemolytic streptococci was ob- 

 tained by the use of 0.03 y penicillin with an activity of 240 to 250 Ox- 

 ford units per milligram (418-424). Peptone, ^-amino-benzoic acid, 



