CH. II] THE CHARACTERS OF BACTERIA 19 



indol formation, pigment production, and virulence, all vary 

 with the kind of medium used. 



Gordon (1900-1) states that the streptococcus of scarlatina 

 may form, on serum, rods which closely resemble B. diph- 

 theriae but in a liquid medium it grows in a typical strepto- 

 coccal form. 



B. diphtheriae does not form toxins readily if there is much 

 carbohydrate present in the medium (Fisher, 1909). 



Many media contain substances derived from the living 

 body such as serum, or blood, and to this extent are "natural" 

 rather than "artificial" media and the alteration in the 

 character of organisms growing in them, particularly as regards 

 virulence, is possibly to be attributed to this factor. 



Penfold (1914) mentions the fact that vaccination with a 

 plague strain grown on agar will protect rats against itself 

 but not against the same strain grown on serum. 



If the ordinary artificial media are replaced by the natural 

 secretions of the body the modifications in character on the 

 part of the organisms growing in them may be even more 

 marked. 



Rosenow (1912-13) found that a streptococcus which 

 presented certain morphological and cultural characters on 

 ordinary media, underwent a profound modification in respect 

 to both as a result of growth in unheated milk. 



Horrocks (1911) found that a strain oi B. typhosus, obtained 

 from the urine of a "carrier," lost virulence on ordinary media 

 (broth, and agar) in a few days but maintained it for over a 

 year in urine. 



Both in milk and in urine B. coli may form a dense network 

 of branching filaments (Revis, 1908, Wilson, 1908) and give 

 atypical fermenting reactions {ibid.). 



In the presence of saliva B. coli yields leptothrix forms 

 (Adami, Abbott and Nicholson, 1899), while 8. mastitidis is 

 deprived of virulence (Savage, 1908-9). 



Diplococcic forms of B. coli occur in bile (Adami, Abbott 

 and Nicholson), while in ascitic fluid the same organism 

 undergoes profound changes in respect both to its morphology 

 and its fermenting power (ibid.). 



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