16 AIDS TO BACTERIOLOGY 



in some way that we do not yet understand, adds greatly 

 to the virulence of the pathogenic organism which it 

 accompanies. Attenuated cultures of B. anthracis may 

 reacquire virulence if injected simultaneously with a 

 culture of B. prodigiosus, and attenuated tetanus bacilli 

 become greatly exalted in virulence when cultivated with 

 the Proteus vulgaris. 



Streptococci and colon bacilli are not uncommonly con- 

 cerned in cystitis. When the urine is alkaline, the strep- 

 tococci attain predominance, while an acid urine sup- 

 presses them and allows the colon bacilli to get the upper 

 hand. 



Antagonism of Species. One species suppresses another 

 by exhausting the food material or by excreting metabolic 

 products detrimental to the growth of the other. 



Immunity. Insusceptibility to the attack of a patho- 

 genic organism may be natural to a tribe (racial immunity), 

 or to a person (individual immunity). 



The animal economy stubbornly resists attack by 

 pathogenic bacteria. If any be swallowed, the acidity of 

 the gastric juice will probably destroy them, and the normal 

 flora of the intestine tend to suppress harmful organisms. 

 The nasal secretion entangles and destroys organisms that 

 have been inspired. The tonsillar epithelium acts as a 

 bacterial filter, preventing passage of bacteria at times, 

 but allows free entry into the lymph-channels at 

 others. Wright regards this as a physico-chemical pro- 

 cess 'affecting the surface tension of colloids, of which the 

 cells and bacteria are composed. Even milk, when first 

 drawn, has a germicidal action. When infection takes place 

 through skin or mucous membrane, the phagocytes can 

 dispose of many alien bacteria (see below). Resistance 

 is also offered to toxins, which are destroyed or eliminated 

 by various processes, such work being a notable function 

 of the liver. All these processes are non-specific, with more 

 or less restricted capacity for eliminating bacteria and their 

 toxic products. When, owing to the number of bacteria 

 or amount of their toxins offered, they are overcome, the 

 infection will obtain a hold unless a degree of immunity 

 specific for the particular organism or toxin is available. 

 Artificial immunity may be active or passive. 



Active immunity may be produced by one of the follow- 

 ing methods: 



