THE PREPARATION OF GELATIN AND AGAR II 



EXERCISE IV 



THE PREPARATION OF GELATIN AND AGAR 



15. 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 solidifying 

 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 processes 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 usu- 

 ally 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. The preparation of nutritive agar, American Micro- 

 scopic Journal, May, 1890. A rapid method of making agar-agar, 

 Johns Hopkins Hospital Bulletin, No. 24, July-August, 1892. Jour, 

 of the Am. Public Health Asso., January, 1898. 



16. Work for this exercise. See that bouillon made in 

 Exercise III is properly sterilized. Prepare 300 cc. of gelatin 

 and 300 cc. of agar, i.e. start with 300 cc. 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 : 



