154 NUTRIENT MEDIA 



For example, "meat extract + 10, " indicates a 

 sample of meat extract which reacts acid to phenol- 

 phthalein, and would require the addition of 10 c.c. of 

 normal NaOH per litre, to neutralise it. 



NOTE. Such a solution would probably react alkaline to 

 litmus. 



Conversely, if as the result of our titration experi- 

 ments we find that 25 c.c. of meat extract require the 

 addition of 5 c.c. ~ NaOH to neutralise, then 1000 c.c. 

 of meat extract will require the addition of 200 c.c. ~ 

 NaOH = 20 c.c. -f NaOH. 



And this last figure, 20, preceded by the sign + 

 (i.e., + 20), to signify that it is acid, indicates the re- 

 action of the meat extract. 



NOTE. The standard soda solutions should be prepared by 

 accurate measuring operations, controlled by titrations, from a 

 stock solution of zoN NaOH, which should be very carefully 

 standardised. If a large supply is made or the consumption is 

 small this stock solution must be kept in an aspirator bottle to 

 which air can only gain access after it has been dried and rendered 

 free from CO 2 . This may be done by first leading it over H 2 SO 4 

 and soda lime, or soda lime alone, by some such arrangement as 

 is shown in figure 99, which also shows a constant burette arrange- 

 ment for the delivery of small measured quantities of the 

 dekanormal soda solution. 



STANDARDISATION OF MEDIA. 



Differences in the reaction of the medium in which 

 it is grown will provoke not only differences in the rate 

 of growth of any given bacterium, but also well-marked 

 differences in its cultural and morphological characters ; 

 and nearly every organism will be found to affect a 

 definite "optimum reaction" a point to be carefully 

 determined for each. For most bacteria, however, the 

 "optimum" usually approximates fairly closely to 

 H- 10 ; and as experiment has shown that this reaction is 

 the most generally useful for routine laboratory work, 

 it is the one which may be adopted as the standard for 

 all nutrient media derived from meat extract. 



