94 MICRO-ORGANISMS AND DISEASE [chap. 



take bacillus anthracis, this bacillus in nutritive gelatine 

 (beef broth, peptone, gelatine) often forms already during the 

 first twerrty-four to forty-eight hours torula-like chains and 

 filaments, of which the elements are spindle-shaped or pear- 

 shaped. Bacillus diphtheriae grown on nutritive Agar, 

 forms already after forty-eight hours longish jointed filaments, 

 whereas in gelatine most of the bacilli are short cylindrical. 

 The cholera vibrios forms in fluid media in a few days longish 

 spirals, on solid media it sometimes takes weeks. I have 

 isolated from the human tonsils a microbe which, grown on 

 blood serum or Agar forms exquisite commas, semicircular 

 forms, and spiral and wavy threads, grown on gelatine 

 the microbes are rod-shaped with just a faint indication of 

 curvature, transferred back on to scrum or Agar they 

 promptly yield commas, semicircles, and spirals. Some species 

 of streptococci — e.g. streptococcus pyogenes and erysipelas 

 form long and exquisite chains in fluid, very short chains 

 and diplococci on solid media, &c. And the same applies 

 to physiological action ; thus the tubercle bacillus grown on 

 glycerine Agar after a series of transfers on glycerine Agar 

 loses considerably in virulence when tested on the guinea- 

 pig, while when grown on serum for many generations retains 

 its full virulence. When it is kept growing on glycerine Agar 

 for a considerable series of generations it loses almost 

 entirely its virulence, but when so weakened it is replanted 

 in glycerine serum it soon regains it. 



Many instances can be mentioned when similar alterations 

 in physiological functions of bacteria take place differing with 

 the medium in which they are grown. 



The temperatures at which bacteria best grow show con- 

 siderable differences : {a) while some grow best at tempera- 

 tures at or below 20 or 21° C, and do not grow at all, or 

 only very feebly, above these— f.^^. bacillus prodigiosus, vibrio 



