149 



Reaction of Culture Media. 



H. A. NoYES, Purdue University. 



The reaction of culture media has worried every bacteriologist at 

 some time in his career. During the past two years there have ap- 

 peared several papers, in American publications, dealing with the reac- 

 tion of bacteriologic culture media. Among these may be mentioned 

 those by Clark (1), (2), (3), (4); Itano (5); Anthony and Ekroth 

 (6). Clark and Lubs have presented papers (3) and published a series 

 of articles entitled, "Colorimetric Determination of Hydrogen Ion Con- 

 centration and Its Applications in Bacteriology" (4). This work, as 

 well as all papers published to date, including those presented at the 

 1916 meeting of the American Society of Bacteriologists shows that 

 bacterial activities in general are greatest when the culture medium is 

 neutral or approximately so. A simple, practically neutral medium is 

 most desirable for general use. Anything which tends to produce or 

 make it necessary to adjust acidity should be avoided if possible. 



Evidence points to physical and chemical laws applying to culture 

 media just as well as they do to water solutions of pure salts, the only 

 difference being, media are more complicated and not as fully under- 

 stood. Bacteriological media are of two kinds, liquid and solid. This 

 paper is almost entirely confined to solid media. The bases of solid 

 media are usually agar agar, gelatin or silicate jelly. Chemicals are 

 added to these bases to furnish food for bacterial life and to make the 

 reaction of the media such, that bacteria may thrive. Moi'e attention 

 has been paid to the adding of chemicals for supposed food values than 

 to the ascertaining of the reactions that take place between the chemi- 

 cals themselves and the basis of the media. 



Acidity or alkalinity of culture media are due to the nature of the 

 basic substance used in making the media, and to the nature of the 

 chemicals added to this base. The resultant equilibrium, produced by 

 physio-chemical phenomena, notably ionization and hydrolysis, as in- 

 fluenced by mass action, temperature and pressure determines the reac- 

 tion of the culture media. 



