398 PATHOGENIC BACTERIA. Characteristics for TYPHOID BACILLUS. Bacilli usually slender. Flagella numerous (10-20), long, and wavy. Growth not very rapid, not particularly luxuriant. Upon Eisner's culture-medium de- velops slowly, the colonies remain- ing small. Upon fresh acid potato the so-called " invisible growth " formerly thought to be differential. Acid-production in whey not exceed- ing 3 per cent. Sometimes slight in ordinary media, and sometimes succeeded by alkaline production. Grows in media" containing sugars without producing any gases. Produces no indol. Growth in milk unaccompanied by coagulation. In Maassen's asparagin-glycerin solu- tion the bacillus does not grow. Gives the Widal reaction with the serum of typhoid blood. Differentiation. COLON BACILLUS. Bacilli inclined to be a little thicker. Flagella fewer (8-1 o). Growth rapid and luxuriant. This character is by no means constant. Upon Eisner's medium develops more rapidly, the colonies being larger. (Sometimes the colonies are small and remain so.) Upon potato a brownish-yellow, dis- tinct pellicle. Acid-production well marked. Gas-production well marked. Indol-production marked. Milk coagulated. Grows in Maassen's solution. Does not react with typhoid blood.