THE TYPHOID BACILLUS 415 



malachite-green, agar. These are incubated at 37 C. 

 for twenty-four to forty-eight hours, and the colonies 

 which develop are tested by microscopical examination 

 and by agglutination and cultural methods. The Bacillus 

 typhosus has the following characters : 



Morphology. A bacillus averaging 3 ^ in length, and 

 0-6 p, broad. Under cultivation it is a markedly pleo- 

 morphic organism, and very short rods, long rods, and 

 thick filaments 10 to 30 p, in length occur ; the latter 

 are known as involution forms (Plate XII, a). It does 

 not form spores, but granulation and vacuolation may 

 be observed in the protoplasm, particularly in old 

 cultures. 



It is actively motile, and possesses a number of flagella, 

 arranged peritrichically (Plate XII, c). The flagella are 

 long and wavy, and average eight to twelve in number on 

 the ordinary bacillus, a point of differentiation from the 

 Bacillus coli, which usually has only three or four. It 

 stains with the ordinary anilin dyes, but is Gram-negative. 



Cultural characters. The B. typhosus is aerobic and 

 facultatively anaerobic, and grows well on the ordinary 

 culture media. On agar it forms a thick, moist, greyish 

 layer. On gelatin it grows slowly without liquefaction, 

 and the growth, which is usually scanty and confined to 

 the needle-track, is white and shining, and somewhat 

 irregular (Plate XII, b). The colonies in gelatin are 

 visible in about forty-eight hours, and form small roundish- 

 white points, which are granular and brownish in colour 

 by transmitted light. In broth it produces a general 

 turbidity, without film formation. On potato acid in 

 reaction it forms a thin, moist, grey layer, which is almost 

 invisible. If, however, the reaction of the potato is 

 neutral or alkaline, the growth may be yellowish. 



A lead acetate medium is darkened. On agar on which 

 it has already grown, the growth being scraped off, it fails 



