INGREDIENTS OF CULTURE MEDIA 297 



two and darken to become black. Oxidase negative colonies do not change color. 



p-Aminodimethylaniline monohydrochloride is used in 1 per cent concentra- 

 tion in distilled water. The solution should be prepared on the same day it is to 

 be used. However, if kept longer, it should be refrigerated and then warmed be- 

 fore use. In no case should the solution be used if more than 5 days old or if a 

 precipitate is present. A complete discussion of the use of this reagent is given 

 under Bacto-Proteose No. 3 Agar and Bacto-G C Medium Base, pages 116 and 

 122, for use of the oxidase test in the cultural diagnosis of gonococcal infections. 



Tetramethyl p-phenylenediamine hydrochloride may also be used in exactly 

 the same way as the dimethyl reagent. The tetramethyl compound has the dis- 

 advantage of sometimes coloring the surrounding medium and is much more 

 expensive than the dimethyl. 



1 J. Path. Bact., 31:185:1928. 3 Diagnostic Procedures and Reagents, 3rd Edi- 



2 Seventh Annual Year Book (1936-37) p. 133. tion: 107: 1950. 

 Suppl., Am. J. Pub. Heahh, 27:No. 3:1937. 



/>-AMINODIMETHYLANILINE OXALATE, 



DIFGO (B329) 



p-Aminodimethylaniline Oxalate, Difco, is recommended for the detection of 

 oxidase production by microorganisms. It has the advantage over the mono- 

 hydrochloride salt in that it is more stable in the powdered form and also in 

 solution. The oxidase reagent is prepared by dissolving 1 gram of p-Aminodi- 

 methylaniline Oxalate, Difco in 100 ml. distilled water by heating gently. This 

 solution is used to flood plates in determining the oxidase reaction. Positive 

 colonies assume a pink color, finally turning to maroon and black. Carpenter^ 

 reported that the dry crystalline oxalate salt is more stable than the mono- 

 hydrochloride salt. No change was observed after six months storage. Aqueous 

 solutions were also more stable. Both salts showed about the same toxicity for 

 microorganisms. The oxalate salt is slightly less soluble in cold distilled water, 

 but solution is hastened by warming gently. Carpenter^ further stated the oxalate 

 salt possesses the additional advantages over the monohydrochloride in that it 

 does not form the marked black precipitate on Chocolate Agar sometimes ob- 

 served with the use of the monohydrochloride, especially when freshly prepared 

 solutions are not employed. 



1 Science, 105:649:1948. ^Diagnostic Procedures and Reagents, 3rd Edi- 



tion: 107: 1950. 



SODIUM THIOGLYGOLLATE, DIFGO (B233) 



Sodium thioglycollate or thioglycollic acid is recommended for use in liquid 

 culture media for testing the sterility of biological and other materials contain- 

 ing heavy metal compound preservatives, such as the mercurials. The active 

 sulfhydryl group annuls the toxicity of the metallic preservatives, thus per- 

 mitting the development of any viable organisms present and, further, behaves 

 similarly to glutathione, cysteine and the alkali sulfides in being able to lower 

 the oxidation-reduction potential of the medium. This principle was first de- 

 scribed by Trenkmann^ who showed that the presence of an alkali sulfide 

 induced "aerobic growth of anaerobic organisms". 



Quastel and Stephenson^ showed likewise that cysteine and thioglycollic acid 

 made possible the growth of anaerobes through the lowering of the oxidation- 

 reduction potential of the medium. Brewer^-* combined the principle of lowering 



