166 THE SOIL FLORA 



acid is produced from sugar or glycerin. Some cultures have pro- 

 duced growth and have acidified one or even all of the sugars. This 

 suggests that the irregularity may be due to the poor growth in 

 liquids, tested on "slants" of litmus agar, in fact, B. megatherium 

 has been found to produce acid from dextrose and sucrose quite 

 regularly. Similarly, its poor growth in broth raises a doubt as to 

 whether the second figure of its group number (denoting it to be a 

 strict aerobe) may be correct; for it grows so poorly even in the open 

 arm of a fermentation tube that its failure to grow in the closed arm 



i- 0000000 



FIG. 24. B. megatherium. X 1000 diameters. (After Conn) 



does not necessarily prove its inability to grow in the absence of 

 oxygen. It is also possible that its failure to reduce nitrates may 

 be due merely to the fact that it grows poorly in nitrate broth. 



"B. mycoides Fliigge, 1886. This is the most easily recognized of 

 all the soil bacteria. It can readily be distinguished by its rhizoid 

 growth on agar. 



"Morphology. Young cultures consist of rods about 0.8 to 1.3 by 

 2 to 6 /I. They occur in long chains which often lie parallel and 

 show false branching, an arrangement which gives the colonies 



ofl o 



FIG. 25. B. mycoides. X 1000 diameters. (After Conn) 



their rhizoid structure. The very young rods are apparently 

 slightly motile, but no success has been obtained in staining flagella. 

 Gottheil describes several peritrichic flagella. In older cultures, 

 highly refractive globules that do not take ordinary stains (probably 

 fat drops) appear within the rods, particularly if growing on dex- 

 trose agar, sometimes causing the rods to swell to extremely large 

 size and to lose all resemblance to their original form. The lightly 

 stained ovoid bodies that characterize B. megatherium have never 

 been observed. Spores are borne centrally and the remnants of the 



