ANTIBIOTICS AS CHEMOTHERAPEUTIC AGENTS 267 



found practical application in chemotherapy. Among these, penicil- 

 lin, streptomycin, and tyrothricin occupy a leading place. In a study of 

 the relative susceptibility of different staphylococci to the bacteriostatic 

 action of three different antibiotics it was found (676) that no one agent 

 parallels necessarily the action of another j a strain resistant to one anti- 

 biotic may be sensitive to another. Also, different staphylococcus strains 

 may vary in sensitivity to the same antibiotic, some being inhibited by 

 0.1 unit of penicillin and others being resistant even to 10 units. Hence 

 a knowledge not only of species sensitivity but also of strain sensitivity 

 is essential in selecting an antibiotic for the treatment of a given infec- 

 tion (310). 



In any attempt to evaluate an antibiotic substance as a chemothera- 

 peutic agent, one must consider the interaction between the drug and 

 the parasite, on the one hand, and the drug and the host, on the other 

 (601). 



Penicillin 



Nature of Action. Since the publication of the first edition of this 

 book, much work has been done on the in vivo activity of penicillin and 

 the chemotherapeutic use of this important antibiotic agent. The ap- 

 pearance of the monographs by Herrell (410), Kolmer (522), and 

 Fleming (262a) make a detailed discussion of this problem unneces- 

 sary here J discussion is therefore limited to the fundamental principles 

 involved. 



The antibacterial action of penicillin is selective, susceptibility being, 

 however, one of degree rather than of kind. Some bacteria are in- 

 hibited by very low concentrations of the drug} others are not in- 

 hibited at all or only by high concentrations. On the basis of clinical 

 experience (494), it has been suggested that organisms inhibited by o.i 

 O.U. or less per ml. are susceptible to penicillin therapy j some organ- 

 isms do not respond at all or respond only on occasions. Pyogenic cocci, 

 anaerobic Clostridia, and certain pathogenic gram-negative cocci {Gono- 

 coccusy Meningococcus y and Micrococcus catarr kalis') are sensitive, 

 whereas the colon-typhoid, hemophilic chromogenic bacilli and certain 

 micrococci {Micrococcus favus) are resistant to its action j it has no 

 effect upon M, tuberculosis (863), Trypanosoma equiferdumy and the 



