BACTEROIDS 303 



in this stage, and sometimes a bacteroid is seen to oscillate as though 

 swung about by some propelling force in one end. Division of the 

 bacteroids into bacilli, as represented by Dawson, may also occur. 



" When first plated out, the young colonies consist of small rods 

 which show considerable variation in length. No bacteriods are 

 present, though the rods are sometimes slightly club-shaped and 

 sometimes show vacuolation. However, they never attain the size 

 of bacteroids. With frequent transfers the rods become quite 

 uniform in size and stain deeply and evenly, especially with anilin- 

 gentian violet. 



" In very old cultures (three months on ash agar, without transfer) 

 the small, oval swarmers and the normal rods predominate, though 

 a few club-shaped and a few branched bacteroids are found. The 

 bacteroids produced upon artificial media are never so large nor so 

 numerous as those seen in mounts direct from a young nodule. 



"Staining. The organisms do not stain well with ordinary 

 aniline stains. Carbol-fuchsin and aniline-gentian-violet (used 

 steaming) are the most satisfactory stains. Though carbol- 

 fuchsin was preferred, anilin-gentian-violet stains were always 

 used as checks, because the former stain accents the vacuolated 

 appearance, particularly in bacteroids. Carbol-fuchsin is especially 

 useful in staining bacteroids, direct from the nodule and also old 

 agar cultures. Kiskalt's amyl-Gram stain, described by Harrison 

 and Barlow, is useful since the amyl alcohol clears up the field, 

 leaving the bacteria stained, though not so intensely. This stain, 

 however, should not be considered a means of identifying Ps. 

 radicicola. 



FIG. 41. Bacteroids, showing shape and occurrence of vacuoles. (After Whiting). 



"Bacteroids. While Ps. radicicola produces.no spores, it produces 

 bacteroids which are very evidently more resistant than the normal 

 rods. Unfavorable conditions, such as unsuitable media, infrequent 

 transfer, or addition of caffein to the medium, cause their appear- 

 ance. This is in accord with what takes place in the nodule. In the 

 growing nodule, when development is most rapid, the bacteroids are 

 at their maximum; they enable the organisms to multiply rapidly 

 in spite of the resistance offered by the plant cells. Transferred to 

 favorable media from this stage the normal uniform bacilli are 

 produced. The bacteroid then must be regarded as a normal and 



