30 BACTERIOLOGY FOR NURSES 



a product derived from the stems of seaweed growing in the 

 Chinese seas, has been substituted. Agar is not liquefied by the 

 action of bacteria, does not melt below 98 C., and on cooling 

 solidifies at about 39 C. 



Certain technical procedures such as adjusting the reaction, 

 clearing and filtering of media, should be Understood before the 

 preparation is attempted. 



Titration and Adjustment. A moderately alkaline reaction 

 to litmus is suitable for the growth of most pathogenic bacteria. 

 Generally the medium mixture is at first too acid; it may be 

 reduced by the addition of 4 per cent sodium hydrate solution until 

 red litmus paper is turned slightly blue and blue litmus retains 

 its color. The solution must be well stirred into the media and the 

 test paper immersed in the liquid ; on no account should the test- 

 ing be done by dropping media on to the paper by means of a glass 

 rod. 



Litmus is not a delicate indicator and if a more accurate adjust- 

 ment is required a more complicated method is adopted, with 

 phenolphthalein as an indicator. The neutral points are different ; 

 media alkaline to litmus may be still acid to phenolphthalein. 



The standard method for titration is as follows: 5 c.c. of 

 medium and 45 c.c. of distilled water are mixed in a casserole 

 and boiled for one minute, then 1 c.c. of the phenolpthalein solution 

 (0.5 per cent in 50 per cent alcohol) is added. If no color appears 

 the medium is acid, and while hot twentieth normal sodium hydrox- 



ide solution f NaOH J is run into the casserole from a burette 

 \ ZO/^ 



until a faint but distinct color is seen. This color must remain 

 on stirring, otherwise more alkali is needed. From the amount 

 added the acidity of the medium is determined and an estimate 



( N\ 



made of the amount of normal solution f NaOH 1 to give the 



required reaction to the bulk of the medium necessary. 



N 

 For example, if 5 c.c. of medium required 2.4 c.c. of ^ NaOH 



Z\J 



to neutralize it, 100 c.c. (twenty times as much) would require 



