THE EFFECTS OF BACTERIA. 119 



drop by drop. Tlie appearance of a rose color indi- 

 cates the presence of peptone.* 



The formation of proteolytic ferments varies in 

 many, perhaps in all, species to a much greater extent 

 than we would imagine from the ordinary descrip- 

 tions. In the case of two phosphorescent vibriones 

 Beyerinck found that one which at first liquefied 

 gelatin very slowly, did so more rapidly after longer 

 culture, while the other variety acted in the opposite 

 way. Katz made a similar observation in experi- 

 ments on Australian phosphorescent bacteria. Max 

 Gruber and Firtsch have watched very closely the 

 development of feebly liquefying races in vibrio pro- 

 teus (A. H., VIII., 369), and similar statements have 

 been made concerning cholera vibrio, bacterium 

 vulgare, and micrococcus pyogenes. Indeed, some 

 observers have even seen a liquefying streptococcus 

 pyogenes. 



We have also observed in many varieties that on 

 thin plates the individual distinctly visible, super- 

 ficial colonies exhibit very different degrees of lique- 

 faction. In fact a beginner would be convinced that 

 he had to deal with several varieties. 



It is to be regretted that, as a result of these obser- 

 vations, one of the most convenient diagnostic aids, 

 viz., the liquefaction of gelatin, has lost consider- 

 ably in value. 



The causes of the increase and decrease of liquefac- 

 tion with prolonged culture are looked for in our 

 artificial nutrient media, or in the influence of the 



* Recent investigations have shown, however, that in addition 

 to peptone a few albumoses remain unprecipitated in part by 

 ammonium sulphate. 



