194 Culture Media and Bacteria Cultivation 



determined. For the purpose of neutralization a normal solution of 

 sodium hydroxid or of hydric chlorid is used, and after being thor- 

 oughly stirred the fluid thus neutralized is again tested in the same 

 manner as at first, to insure the proper reaction of the medium being 

 attained. When neutralization is to be effected by the addition of 

 an alkali, it not infrequently happens that after the calculated amount 

 of normal solution of sodium hydroxid has been added, the second 

 test will show that the medium is acid to phenolphthalein, to the 

 extent sometimes of 0.5 to 1 per cent. This discrepancy is perhaps 

 due to side reactions which are not understood. The reaction of the 

 medium, however, must be brought to the desired point by the further 

 addition of sodium hydroxid, and the titrations and additions of 

 alkali must be repeated until the medium has the desired reaction 

 (*'. e., 0.0 per cent, to 0.005 per cent.; see below). 



"After the prescribed period of heating, it is frequently found that 

 the medium is again slightly acid, usually about 0.5 per cent. Without 

 correcting this, the fluid is to be filtered and the calculated amount of 

 acid or alkali is to be added to change the reaction to the one desired. 

 A still further change in reaction is not infrequently to be observed 

 after sterilization, the degree of acidity varying apparently with the 

 composition of the media and the degree and continuance of the heat." 



"Manner of expressing the reaction: Since at the time the reaction 

 is first determined culture media are more often acid than alkaline, it is 

 proposed that acid media be designated by the plus sign and alkaline 

 media by the minus sign, and that the degree of acidity or alkalinity 

 be noted in parts per hundred. Thus, a medium marked +1.5 

 would indicate that the medium was acid, and that 1.5 per cent, of 



sodium hydroxid is required to make it neutral to phenolphthalein ; 

 while 1.5 would indicate that the medium was alkaline and that 

 1.5 per cent, of acid must be added to make it neutral to the indicator." 



"Standard reaction of media (provisional): 



"Experience seems to vary somewhat as to the optimum degree 

 of reaction which shall be uniformly adopted in the preparation of 

 standard culture media. To what extent this is due to variation in 

 natural conditions as compared with variations of laboratory procedure 

 it seems impossible to state. Somewhat different degrees of reaction 

 for optimum growth are required, not only in or upon the media of 

 different composition and by bacteria of different species, but also 

 by bacteria of the same species when in different stages of vitality. 

 The bulk of available evidence from both Europe and America points 

 to a reaction of -j-1.5 as the optimum degree of reaction for bacterial 

 development in inoculated culture media. While this experience is 

 at variance with that in several of our own laboratories, it has been 

 deemed wisest to adopt -f 1.5 as the provisional standard reaction of 

 media, but with the recommendation that the optimum growth reaction 

 be always recorded with the species." 



Many bacteriologists regard a reaction of + 1.0 as a 

 more desirable standard and use it exclusively. 



BOUILLON. 



This is one of the most useful and most simple media. 

 It can be prepared from meat or from meat extract, and is 



