236 



NATURE OF ANTIBIOTIC ACTION 



only 2 units of penicillin per i ml. and P. vulgaris, E. ty-phosa, Shigella, 

 Escherichia, and Aerobacter showed even greater resistance, it was still 

 believed that concentrations of penicillin in the urine can be attained to 

 inhibit the growth of these organisms (904). 



Penicillin is not active against pathogenic fungi, the growth of which 

 may actually be stimulated by this antibiotic. However, it has some 

 activity against A. bovis, the growth of which was inhibited by 0.0 1 

 O.U./ml. (496), and against certain other actinomycetes (193). 



Various forms of penicillin differ in their action upon specific bacteria. 

 Welch et al. (998) have shown that penicillin X is more effective than 

 commercial penicillin against certain bacteria but not against others, not 

 only in the test tube but also in the animal body. These results have 

 been confirmed, as shown in Table 38. S. aureus and B. subtilis are more 

 sensitive to penicillin G than to X, on a weight basis j however, peni- 

 cillin X is more effective than G on certain other bacteria. 



TABLE 38. INHIBITION OF GROWTH OF DIFFERENT BACTERIA BY TWO 

 FORMS OF PENICILLIN (MICROGRAMS OF PENICILLIN PER Ml) 



From Libby and Holmberg (568). 



Note. Unit of activity calculated on the basis of 1,650 units/mg. for pure penicillin G, and 1,000 



units/mg. for pure penicillin X. 



Different strains of the same organism show marked variations in 

 their sensitivity to penicillin. For example, a study of 40 strains of 

 hemolytic S. aureus isolated from patients in an Army hospital (725) 

 gave a range of sensitivity from complete tolerance of 4 O.U./ml. to 



