18 CONDITIONS MODIFYING [CH. n 



and B. subtilis pass in a few hours from a bacillary motile 

 stage with cilia, to one of filamentous growth preceded by the 

 casting off of cilia (Marshall Ward and Blackman, 1910). 



This factor influences the characters not only of bacteria 

 known to be "pleomorphic" but of others also. Young tubercle 

 bacilli are said not to be "acid-fast" (Hamer, 1900). 



Young cultures of B. diphtheriae more often show branched 

 and clubbed forms (Kanthack and Andrewes, 1905) and solid 

 staining bacilli (Denny, 1903) than do older cultures; at the 

 same time they are unable to ferment glycerine and lactose 

 though older cultures usually ferment both (Muir and Ritchie). 



A young culture of B. coli does not yield indol (MacConkey, 

 1909). Wood (1889) found that an old culture of cholera failed 

 to liquefy gelatin but did so readily after subculturing, and 

 that a young culture of the same organism was much more 

 susceptible to the action of antiseptics than a younger one. 



Old cultures of pigment forming bacteria are often colour- 

 less (Adami, 1892). 



Arkwright (1909) found bacillary forms of the meningo- 

 coccus in old cultures. The tubercle bacillus also in old 

 cultures displays elongated and even branched forms. 



8. The character of the culture medium employed may 

 influence bacteria in many ways. 



(a) The age of the medium. S. pyogenes normally does 

 not ferment saccharose, raffinose or salicin, but if old media be 

 used this organism will ferment all three substances (S. Martin, 

 1908-9). On the other hand B. diphtheriae, which in fresh 

 beef serum gives its characteristic "sugar" reactions, fails to 

 do so if this medium is old (Fisher, 1909). 



(b) Changes in the reaction of the medium employed is in 

 many cases accompanied by changes in the morphological 

 characters of organisms growing in it, bacilli giving place to 

 cocci and diplococci, and vice versa, and pigment formation 

 being modified or lost (Adami, 1892). The reaction also affects 

 the vitality of many bacteria (Wood, 1889), and their virulence 

 (Peckham, 1897). 



(c) The nature of the medium is important. Individual 

 morphology, the appearance of colonies, fermenting power, 



