STANDARD MEDIA 75 



sign and acidity by the plus ( + ) sign. Thus, if the reaction 

 of a medium is given as + i.oo it indicates that it would be 

 necessary to add i per cent, of normal sodic hydrate solution to 

 the medium in order to bring it to the neutral point (to phenol- 

 phthalein). It will be observed that while the titrating is done 

 with the N/20 caustic soda solution, the normal solution is added 

 to bring the medium to the desired reaction, the stronger solution 

 being preferred because it reduces the amount of liquid intro- 

 duced. The Committee on Standard Methods specifies that the 

 reaction of all standard culture media shall be + i.o per cent, 

 and if it differs in reaction by more than 0.20 per cent, the medium 

 shall be readjusted and when a reaction other than the standard 

 is used it shall be indicated and the reasons for using a different 

 reaction shall be fully stated. 



Media are preferably made in large quantities as this will 

 reduce to a minimum the discrepancies due to variation in the 

 composition of the ingredients used. As soon as made and 

 titrated, the media should be put into tubes and in other culture 

 containers, after which media containers and all are to be sterilized 

 according to the methods already described. To guard against 

 the evaporation of moisture from the media, the tubes, flasks, etc., 

 should be sealed by dipping the plugged ends into melted parafhn, 

 or they may be capped with rubber coverings especially made 

 for that purpose. In case media are to be used within a few 

 days, sealing is not necessary but they should be kept in a moist 

 place, preferably in the ice-box. 



6. Preparation of Required Standard Culture Media 



Culture media used in bacteriological work may be divided 

 into those which are required for general purposes and those 

 which have special uses. The former should by all means be 

 prepared according to the standards suggested by the Committee 

 of the A. H. A. If special media are used, their exact composi- 

 tion and mode of preparation should be fully and explicitly given. 



