196 pathogenicity; chemotherapy 



CI. welchii a clue to the nature of the substrate existed in the 

 fact that when the organism is grown on serum-plates or in 

 egg-media, the medium becomes turbid and this turbidity is 

 due to the formation of minute fat droplets. We know now 

 that these arise from the hydrolysis of lecithin in such media. 

 Haemolysins, such as the ^-toxin of CI. welchii and the toxins 

 of S. haemolyticus, presumably act by degradation of a vital 

 constituent of the wall of the red blood cell, but there seem 

 to be no obvious clues to the chemical action of other toxins 

 such as that formed by CI. hotulinum which is probably the 

 most active of all exo- toxins. 



The toxins of certain pathogens, such as CI. tetmium (tetanus) 

 or CI. botulinum (botulism), seem to be very much more active 

 than those of CI. welchii, and it may be that they act as 

 enzyme inhibitors rather than as enzymes themselves. It 

 has been suggested that the toxin of CI. tetmium is an inhibitor 

 of choline esterase, but proof is not yet available. These 

 two toxins of CI. tetmium and CI. botulinum have been 

 obtained in a crystalline state very recently, so we may expect 

 further developments in the near future. 



THERAPY: THE COMBATING OF PATHOGENIC BACTERIA 



At the beginning of this chapter it was shown that whether 

 or not disease follows the contamination of a host's tissues 

 with a bacterium depends upon the relative activities of the 

 bacterium and of the reticulo-endothelial system. The aim 

 of medical science is to prevent the organism gaining the 

 final advantage in any infection or, referring back to Fig. 13, 

 to reduce Case 3 to Case 2 and, if possible, to Case 1. The 

 most successful ways of accomplishing this are based on two 

 fundamental principles : 



1. Immunological defence 



A property of a toxin or any foreign protein in the blood- 

 stream is to stimulate the formation of an antitoxin or anti- 

 body by the bone-marrow. The chemistry of antigens and 



