PROTEOLYTIC FERMENTS. 61 



The production of proteolytic ferments fluctuates 



with many, perhaps with all, species in a greater degree 

 than one would be led to suppose from the ordinary 

 descriptions. Beijerinck found that one of two photo- 

 genic vibriones at first liquefied slowly, but that after 

 longer culture gelatin was always liquefied more rapidly; 

 the other showed exactly the opposite. The same was ob- 

 served by Katz in the Australian photogenic bacterium. 

 Max Gruber and Firtsch (A. H. viii, 369) have studied 

 particularly closely liquefying cultures of Vibrio proteus, 

 but they have also reported similar experiences with the 

 cholera vibrio, Bact. vulgare, the Micrococcus pyogenes; 

 indeed, many observers have even seen liquefying Strepto- 

 cocci pyogenes. 



^^'e have observed, also, in many ^•;n•ieties, that u})on thin 

 plates single, distinctly visible, superficial colonies of the 

 same bacterium present such varying degrees of liquefac- 

 tion that a beginner could scarcely be convinced that sev- 

 eral varieties were not present. 



It is very unfortunate that, through these observations, one of 

 the readiest applied diuf/ii.i)sttc aids, the liquefaction of gela= 

 tin, has lost not a little in value. 



The causes of the decrease and increase of liquefaction 

 under prolonged cultivation we ascribe to our artificial 

 nutrient media or to the influence of the metabolic 

 products of the micro-organisms, but without being able to 

 give anything more decisive. 



Regarding the influence of nutrient media upon the for- 

 mation of trypsin in a culture and the liquefaction of gela- 

 tin, the following facts are known : 



1. Most circumstances which interfere with the growth 

 of a variety of bacterium on a nutrient medium also inter- 

 fere with the liquefaction of gelatin; for example, the addi- 

 tion of phenol and a large amount of glycerin. Wood has 

 observed that the lessened power of liquefying gelatin pro- 

 duced by phenol may be propagated for several genera- 

 tions upon favorable nutrient media (C. B. viii, 266). 



2. In hydrogen and nitrogen the liquefying facultative 

 anaerobes do not liquefy gelatin, ^ while in CO 2, if they 



1 A single exception is the B. prodigiosum; but if grape-sugar be 

 added to the gelatin, it also ceases to liquefy. 



