CHAPTER XI 



PATHOGENICITY; CHEMOTHERAPY 



From a medical point of view, bacteria are divided into 

 " pathogens " or organisms capable of causing disease in a 

 host, and " saprophytes " or organisms which are harmless 

 to other creatures. Those organisms, such as the normal 

 bacterial flora of skin, mouth, and intestine, which live in 

 constant association with man without causing any disease 

 or lesion, are called " commensals." An organism may be 

 potentially pathogenic in one situation and a commensal in 

 another as, for example, CI. welchii which is a normal harmless 

 inhabitant of the intestine, but is pathogenic should it get 

 into a deep wound. 



The majority of bacteria are saprophytic, and in this chapter 

 we intend to consider, as briefly as possible and from a bio- 

 chemical aspect, what it is that differentiates a pathogenic 

 from a saprophytic organism. 



The healthy interior tissues of animals are sterile, and are 

 maintained so by the action of both fixed and wandering cells 

 which have the power to ingest and digest bacteria by the 

 process known as phagocytosis. In mammalian blood, for 

 example, certain of the white corpuscles have this phagocytic 

 property, and if a foreign particle or organism enters the 

 blood-stream, then these phagocytes are attracted towards 

 the foreign substance, surround it, and attempt to destroy or 

 engulf it. Other phagocytic cells are fixed to the capillary 

 walls of the liver, etc., and the whole complex of fixed and 

 wandering scavenging cells is known as the " reticulo- 

 endothelial system." When an organism enters the blood- 

 stream or other tissue, a race ensues between the capacity of 

 the organism to multiply on the one hand, and the capacity 

 of the phagocytes to destroy the invading cells on the other. 

 If the phagocytes win rapidly no disease symptoms appear, 

 but if the bacteria are able to paralyse or outgrow the 

 phagocytes, then there usually follows some disturbance of 



186 



