80 VARIATIONS IN VIRULENCE [ch. vi 



numbers in the healthy intestine, develops in many inflam- 

 matory conditions of the intestinal mucous membrane the 

 property of virulence. Sanarelli (1894) caused the colon 

 bacillus to become virulent in experimental typhoid fever 

 by producing an inflammation with typhoid toxin. Acute 

 peritonitis due to B. coli, following strangulation of the gut — 

 a sequence frequently observed clinically — would appear to 

 depend upon something more than a lowering of the vitality 

 of the tissues, for De Klecki (quoted by Peckham, 1897) found, 

 by experimenting on dogs, that the colon bacillus acquired 

 virulence in the lumen of a strangulated coil of intestine. 

 Dreyfuss has described the increased virulence of B. coli in 

 intestinal disease, and Fermi and Salto (quoted, Peckham) 

 a similar increased virulence in inflamed conditions of the 

 intestines due to cold, bad food, etc. 



Harris (1901) tested the toxicity of 29 strains of B. coli 

 communis derived from various sources. Out of 15 strains 

 obtained from "natural sources" (healthy faeces, sewage, 

 water, milk, shellfish) only two were virulent (one of these very 

 slightly so) whereas out of 11 strains derived from pathological 

 secretions (pus and the stools in epidemic cholera, cholera 

 nostras and summer diarrhoea) only one (from the last-named 

 source) was non- virulent. 



The acquirement of virulence by saprophytic organisms in 

 the cavity of an inflamed uterus during the puerperium, is 

 another case in point, 



9. In many cases the development of virulence by organisms 

 growing in pathological exudations is due not only to the 

 presence of inflammatory products or to the absence of sub- 

 stances normally found in the healthy secretions but also to 

 some extent, as we have already said, to the presence of other 

 bacteria and their products. 



In discussing the efiect of symbiosis on organisms, reference 

 was made {vide p. 22) to the fact that substances excreted by 

 one species of bacteria may markedly influence the growth of 

 another species. An example was there given of a strain of 

 B. influenzae which would grow on a sterilised medium 

 previously impregnated with the products of a staphylococcal 



