1915] C. H. Crabill and H. S. Reed 39 



Skitn-milk-agar. Some Separator skim milk is diluted with an 

 equal volume of water and 20 gm. o£ agar added per liter. This 

 mixture is heated for half an hour in an autoclave to coagulate the 

 casein. The medium is then boiled in an Arnold sterilizer for 4 

 hrs., when the casein is in a very finely divided condition. Plates 

 are prepared in the usual way. See Table 8. 



This medium is most excellent for the demonstration of erepsin 

 secretion as shown in Plate i, Figs. 2, 3, and 4. The most satis- 

 factory organisms to use for the demonstration of ereptic activities 

 on skim-milk-agar are: B. megatherium, M. citriciis, B. hutyricus, 

 B. fluorescefis liquifac'iens, B. prodigiosus and B. pyocyaneits in the 

 Order named. Among those tested which showed least activities 

 are : B. campestris, M. candicaiis, B. hartlehii, Bad. lactis acidi, and 

 B. lactis aerogcnes. 



Peptone-agar is also very useful in the demonstration of erepsin, 

 It is prepared by adding Witte peptone (i percent) to the stock 

 agar. Good halos are produced by many organisms. 



Trypsin. One of the most convenient methods of demonstrat- 

 ing the action of trypsin, produced by microorganisms, depends upon 

 the use of egg-albumen-agar. Many microorganisms grow readily 

 upon egg-albumen-agar and digest the coagulum. 



TABLE 9 



Data pertaining to the action of certain organisms on egg-albumen-agar 



Organism Growth Digestion 



B. campestris Good Good 



Sarcina lutea Good Very good 



B. pyocyaneus Great Very good 



Oidium lactis Great Slight 



B. subtilis Good Slight 



B. mycoides Good None 



B. prodigiosus Good Good 



B. m.egatherium Good Good 



Streptothrix sp Good Slight 



Bact. lactis acidi Fair Slight 



Bact. tumefaciens Small None 



B. amylovorus Small None 



B. vulgaris Slight None 



B. lactis (crogenes Slight None 



B. hutyricus Small Slight 



B. putidum Small Slight 



Egg-alhumen-agar is prepared by taking the " whites " of three 

 eggs and beating them in 75 c.c. of water with a Dover egg-beater. 

 This material is then added to 500 c.c. of melted stock agar, shaken, 



