20 CONDITIONS MODIFYING [ch. ii 



Pathological eocvdations influence the characters of 

 bacteria growing in them to an even greater degree than 

 the natural secretions. This is particularly true of virulence 

 (vide p. 77). 



Harris (1901) examined 15 strains of B. coK from "natural" 

 sources — such as sewage, water, milk, shellfish — and also 

 11 strains from "diseased" sources, that is to say from in- 

 flammatory exudations. Of the former, only two were virulent ; 

 of the latter only one was non-virulent. 



Growth in water also influences bacteria. B. coli in river 

 water, where they are practically deprived of proteid food, 

 appear to lose their power of producing indol (Peckham, 

 1897). The same organism isolated from drinking water was 

 found by Savage (1904) to form less typical colonies than when 

 isolated from sewage or faeces, while Jenner (1898) describes its 

 morphological appearances as being diflerent, bacilli isolated 

 from water being less thick and opaque — a distinction which 

 disappeared when the strain was grown in milk. 



(d) The addition of various chemical substances, such as 

 antiseptics, to the media used for cultivation profoundly 

 modifies the development of bacteria. In the presence of 

 carbolic acid typhoid bacilli assume the form of non-motile 

 cocci and diplococci (Adami, 1892), the bacillus anthrax loses 

 virulence and also the power to form spores (Roux, 1890), 

 many bacteria no longer liquefy gelatin (Wood, 1889), while 

 others lose their power to ferment carbohydrates (Penfold, 

 1911 b). Under the influence of antiseptics B. prodigiosus 

 forms spirillae and ceases to produce pigment (Wasserzug, 

 1888). The bacillus of blue pus — normally a small short 

 bacillus — yields, on the addition of a trace of boric acid to 

 the medium, S-shaped forms and close spirals, and on the 

 addition of potassium bichromate, long undulating filaments 

 {ibid.). The presence of sodium benzoate inhibits gas pro- 

 duction in the case of B. coli (Herter, 1909). The addition of 

 glycerine prevents the liquefaction of gelatin by bacteria 

 (Adami, 1892) and also inhibits the formation of indol (Wood, 

 1889). A virulent B. diphtheriae is promptly attenuated by 

 the addition of iodine trichloride to the medium (Mohler and 



