THE PREPARATION OF AGAR AND GELATIN ii 

 EXERCISE IV 



THE PREPARATION OF AGAR AND GELATIN 



14. Of the solid media employed in cultivating bacteria, 

 agar and gelatin are most commonly used. They depend for 

 their nutritive properties largely upon the bouillon from which 

 they are made, the agar and gelatin forming simply the solidi- 

 fying elements. The striking difference between the two is 

 that the gelatin melts at the body temperature, whereas the 

 agar is not quickly liquefied below the boiling point. For this 

 reason gelatin is not used as a solid medium for cultivating 

 bacteria at a high (body) temperature. There are several pro- 

 cesses for preparing these media, but the addition of the dry 

 agar and gelatin to bouillon (12) either immediately after it is 

 filtered or later after it has been sterilized and stored in flasks 

 seems to be the most convenient procedure. The agar itself 

 is usually neutral in reaction, but the gelatin often has a 

 decidedly acid reaction. This necessitates the careful testing 

 of the reaction of the two media although the bouillon is 

 neutral or slightly alkaline. 



References. — Chapters on culture media in text-books on 

 Bacteriology, p. xi., The preparation of nutritive agar. Am. Mic. 

 Jour., May, 1890. A rapid method of making agar-agar, Johns Hop- 

 kins Hospital Bulletin, No. 24, July-Aug., 1892. Jour, of the Am. 

 Public Health Asso., Jan., i8g8.' 



15. Work for this Exercise. — Prepare 300 c.c. of agar and 

 300 c.c. of gelatin, i.e. start with 300 c.c. of bouillon for each. 

 There will be considerable shrinkage owing to the amount lost 

 on the dishes, filter, etc., so that the quantities of media will be 

 appreciably less than this amount. Distribute each medium as 

 follows : — 



Put 7 c.c. in each of lo small sterile test tubes. 

 Put 1 2 c.c. in each of 1 2 large sterile test tubes. 

 •Put the remainder in a small sterile flask. 



