GLOSSARY OF TERMS 369 



Solution B : 



Sulphanilic acid . . . 0-5 gm. 



Dilute acetic acid . . . 150 ml. 



Add 1 ml. of Solution A, followed by 1 ml. of Solution B. 

 Pink, red, or maroon colour = positive. 

 No coloration = negative. 



A negative reaction may sometimes be due to the reduction of the nitrite to 

 gaseous nitrogen almost as rapidly as it is formed, or to the production of hydroxyla- 

 mine. The first possibility may be examined by growth in a gas fermentation tube, 

 or by chemical estimation of the nitrate, the second by testing for nitrite in the way 

 just described, after preliminary oxidation of the hydroxylamine with iodine (see 

 Lindsey and Rhines 1932, Conn 1936, Reed 1942). A control tube should always 

 be tested. 



Ammonia. — Tested on a peptone water culture, grown for 5 days at 37° C, by 

 adding Nessler's reagent. 



Brown colour = positive. 



Faint yellow colour = negative. 

 Hydrogen Sulphide. — Tested on lead acetate medium (heart extract broth con- 

 taining 4 per cent, peptone and 2-5 per cent. agar. Sterilize, and add an 

 equal quantity of a sterile 0-1 per cent, solution of basic lead acetate.) 

 Brown or black coloration = positive. 

 No coloration = negative. 



The lead acetate may be replaced by 0-05 per cent, ferric ammonium citrate or 

 0-03 per cent, ferrous acetate (Zobell and Feltham 1934). A higher proportion of 

 positive reactions is obtained with some organisms by incubating at 30° C. instead 

 of 37° C. (Tittsler 1931). The most delicate method is to grow the organisms in 

 a slope tube of liver extract agar, and to include between the cotton-wool plug 

 and the tube a slip of filter paper soaked in 10 per cent, lead acetate solution and 

 subsequently dried. The amount of browning or blackening of the paper is 

 measured in millimetres. A fresh slip may be inserted daily. 

 Methylene Blue Reduction. — Tested on a 24-hours' broth culture at 37° C. Add 

 1 drop of 1 per cent, aqueous methylene blue, and incubate at 37° C. 

 Complete decolorization = strong positive. 

 Green coloration = weak positive. 



No decolorization = negative. 



Catalase. — Tested on a 24-hours' agar slope culture at 37° C. One nil. of HjO^ 

 (10 vols.) is poured over the growth, and the tube is set in an inclined 

 position. 



Gas bubbles produced = positive. 

 No gas produced = negative. 



For an account of the methods of examining the antigenic structure and the 

 pathogenicity of bacteria, reference must be made to the chapters dealing with 

 the particular organism under consideration. 



Glossary of Descriptive Terms 



Aerobic : growing in the presence of free oxygen ; strictly aerobic, growing only 

 in the presence of free oxygen. 



