60 ACTIVITIES OF BACTERIA. 



only with an acid reaction), but to a trypsin. The individ- 

 ual bacterial trypsins differ very much as regards resist- 

 ance to heat (they withstand 55° to 70° moist heat for one 

 hour), susceptibility to injury by various acids, etc. 

 Some are active with a suitable acid reaction, but never 

 more so than with an alkaline reaction. 



Much more feeble than the action upon glue, is that upon fibrin. 1 

 Fermi has employed the following method as the easiest and surest 

 way of proving the presence of even traces of proteolytic ferments : 

 Tubes of the same size are filled to an equal height with an unneutral- 

 ized 7% solution of gelatin in 1% aqueous solution of carbolic acid. 

 The solution to be tested for proteolytic ferment has 2% carbolic acid 

 added to it, and is placed in layers upon the solidified gelatin. The 

 tubes are kept at room temperature and observations are made by 

 means of a millimeter scale, as to how much the liquefaction of the 

 gelatin extends in the course of days and weeks. For qualitative ex- 

 amination, the upper layer may consist of 1 c.c. of a liquefied gelatin 

 culture, sterilized with carbolic acid. 2 This material also suffices if 

 one wishes to test the influence of the nutrient medium on ferment 

 formation. One may also, by this method, compare the action of 

 various concentrations of different purely prepared bacterio-trypsins. 

 The lower the percentage of gelatin and the nearer the temperature 

 approaches that of the incubator, the more certainly will one observe 

 effects from traces of ferment. In such critical cases the examination 

 is continued fourteen days, and it is determined whether the gelatin 

 remains fluid in the ice-box, while that in the control tube solidifies. 



To demonstrate the formation of true peptone from the albu- 

 minous bodies one proceeds as follows: 



The variety of bacterium to be tested is grown upon a fluid nutrient 

 medium, rich in albumin, but containing no peptone (blood-serum, 

 milk-serum, milk). After the culture is grown, all albuminous bodies 

 except peptone are precipitated by the addition of strong ammonium 

 sulphate (about 30 gm. to 20 c.c). Milk and milk-serum may be 

 warmed to 60° to 80° and blood-serum to about 40°. The precipitate 

 is filtered off, and the filtrate cooled. A sample is made strongly alka- 

 line with potassium hydroxid, and 1 % solution of copper sulphate added 

 drop by drop. A rose-red color indicates the presence of peptone. 3 

 Fermi has shown, by similar methods, that no variety of bacterium pro- 

 duces true peptone. 



1 Fermi found only a few bacterio-trypsins acting upon fibrin, and 

 none upon egg-albumin. 



2 Naturally, a control test with 2% carbolic- water (ferment-free) 

 must never be omitted. 



3 Through more recent investigations it is certainly known that some 

 albumoses besides peptone remain partly unprecipitated by ammonium 

 sulphate. 



