198 pathogenicity; chemotherapy 



upon the antibody response being effective over a reasonably 

 long period, as it is undesirable to repeat the treatment 

 frequently. 



2. Chemotherapeutic intervention 



If the immunological method is ineffective or difficult, an 

 alternative method of therapy is to prevent the growth of 

 the organisms by chemical means. We must distinguish 

 between bactericidal agents, which actually kill the organisms, 

 and bacteriostatic agents which do not kill but prevent 

 multiplication of the bacterial cells and so allow the reticulo- 

 endothelial system to attack and remove the invaders. The 

 main chemotherapeutic agents in use to-day are: 



Natural antibiotics: substances produced by micro-organisms 

 and which are naturally bactericidal or bacteriostatic. 



1. Penicillin: a substance produced by various moulds, 

 particularly Penicillium notatum. It is bactericidal in very 

 high dilution against Gram-positive bacteria — Staph, aureus 

 being inhibited in vitro by 1 part penicillin in 3 x 10^ parts 

 water or medium. PenicilKn is exceptional in that it is non- 

 toxic to man so that large amounts can be injected to deal 

 with established or stubborn infections. One of its actions is 

 to prevent the assimilation of glutamic acid, and possibly 

 other amino-acids, needed by Gram-positive species for the 

 synthesis of bacterial protein (see p. 99). Sensitive organisms 

 continue to grow for a time after the addition of penicillin to 

 the medium, but the growth produces abnormally large and 

 distorted cells. After this short period of growth the cells 

 become non-viable and eventually undergo lysis. Penicillin has 

 no effect on the respiration of washed suspensions of Staph, 

 aureus. The chemistry of penicillin has received intensive 

 attention during recent years but there is still some doubt 

 about its structure. There are several substances produced 

 by moulds which have the properties of penicillin and 

 differ in the chemical structure of the group R in the 

 following formulae. 



