Journal of Applied Microscopy. 235 



LABORATORY METHODS IN BACTERIOLOGY. 



Dr. F. G. Now. 

 University of Michigan, Ann Arbor, Michigan. 



V. — Preparation of Culture Media. 



A number of nutrient media are employed in the laboratory for the purpose 

 of cultivating bacteria. Three of these are especially important, inasmuch 

 as they are used almost daily. They are gelatin, bouillon, and agar. These 

 standard culture media may be readily modified for special purposes by the addi- 

 tion of glucose, glycerin, or litmus. Although the details for the preparation of 

 these common culture media are not lacking, either in text-books or in current 

 journals, it may not be amiss to describe the methods as followed in the 

 Hygienic Laboratory of the University of Michigan. 



NUTRIENT GELATIN. 



Place five hundred grams of chopped lean beef in a beaker or flask, or better 

 in an enamelled jar such as is shown in Fig. 1. 



Now add 1000 cc. of tap-water and stir thoroughly. Immerse 

 the jar in a water-bath and warm gently till the temperature of the 

 meat suspension reaches 5.5 to 60 degrees C. Maintain this tem- 

 perature for three-quarters to one hour, stirring frequently. The 

 soluble constituents are thus brought into solution. Special care 

 should be taken not to allow the temperature to rise above 60 

 degrees, inasmuch as the albuminous substances would then coagu- 

 late. By keeping these in solution at this stage, they will subse- 

 quently assist in clarifying the final product, and hence the addition Fig. i7 

 of the white of an egg will be unnecessary. 



When the digestion is completed strain the liquid through muslin and 

 thoroughly squeeze the residue. The filtrate is dark red in appearance and 

 should measure 1<M»0 cc. To this amount of the filtrate returned to the jar, add 

 luO grams of gelatin, 10 grams of Witte's pepton, 5 grams of common salt, and 

 warm at 60 degrees in the water-bath with constant stirring, till the gelatin has 

 completely dissolved. The next step is to render the liquid slightly but dis- 

 tinctly alkaline. 



The neutralization is usually accomplished by the cautious addition of a satur- 

 ated solution of sodium carbonate. After each addition of one to two cubic 

 centimeters of alkali, the liquid is well stirred and a drop taken out by means of 

 a glass rod, and touched to a blue litmus paper. If the reaction is acid this 

 paper will turn red. The addition of alkali is continued until the paper remains 

 blue and a red one turns slightly blue. 



In the hands of the beginner this procedure not infrequently fails because of 

 the difficulty of judging of the end reaction. Moreover, even the practiced eye 

 cannot establish the same degree of alkalinity in two separate preparations. For 

 these reasons some workers prefer to titrate the solution with an alkali of known 

 strength, using phenol-phthalein as an indicator. The latter is a most delicate 



