124 BACTERIOLOGICAL CHEMISTRY 



like phenol, inercuric chloride, or chlorine compounds 

 are as toxic to animal cells as to micro-organisms and 

 obviously cannot be used internally. The more damaging 

 the drug to the parasite compared with its toxicity to 

 the host the more useful is it likely to be. The ratio 



toxicity to micro-organism 



, often called the chemothera- 



toxicity to host 



peutic index, is frequently used as a measure of the value 

 of a drug, the higher the ratio the more useful is the 

 substance likely to be, other things being equal. The 

 chemotherapeutic index is sometimes expressed as the 

 ratio between the smallest amount of drug which, when 

 injected in one dose, will effect cure and the largest 

 amount tolerated by the host. 



The route of injection of a drug may influence its 

 apparent efficacy, as a result of differences in absorption 

 or excretion. Thus intramuscular injection of a rela- 

 tively insoluble drug will produce a depot of the drug 

 which will maintain a more or less uniform concentration 

 of the drug in the circulation for a considerable time, 

 whereas intravenous injection is followed by fairly rapid 

 excretion. As an example salvarsan, when injected 

 intramuscularly into fowls, protects them against infec- 

 tion by spirochsetes for several weeks, although they 

 become susceptible again within six days of an intra- 

 venous injection. Penicillin is of little value when taken 

 orally because it is destroyed by the acid conditions 

 prevailing in the stomach, and is administered intra- 

 venously. On the other hand, sulphaguanidine is effective 

 against intestinal organisms because it is only slowly 

 absorbed from the gut, whilst the rapidly absorbed 

 sulphanilamide is almost useless for such infections, but 

 very effective against the bactersemia type of infection. 



A micro-organism may be killed in one host by a drug 

 but may be resistant to the same drug in another host. 



