244 OUTLINES OF BACTERIOLOGY. 



Proteus vulgaris is the best known. The rods are 0'9-1'2/x in 

 length, 0-4-0-6 /x in breadth, and almost always occur in couples. In 

 addition, elongated forms occur frequently, attaining a length of 3 -7 /* 

 and more. In Fig. 130 a ciliated long and a similar short rod are 

 shown. As is evident from the large number of cilia possessed by this 

 species, the motility is very pronounced. Indeed, motility is a charac- 

 teristic possessed by all the members of this group. As already 



FIG. 130. Proteus vulgaris. Showing a ciliated long and ciliated short rod. 



mentioned, the term "Proteus" refers to the variety of the forms 

 assumed by the individuals of the same species ; and in Proteus 

 vulgaris not only do we have the long and short individuals, but also 

 spirilla with two to four convolutions, threads which are 100/x and 

 more in length, threads bent in the form of a bow, with ends twisted 

 into a queue. These are normal structures. When involution forms 

 are exhibited, we see more than the usual variety in shape ; thus, there 

 are pear-shaped and dumb-bell shaped individuals, amongst many other 

 departures from the rod form. 



Proteus mirabilis also shows many departures in the shape of its 

 individuals, a common form being globular or pear-shaped structures 

 measuring from 3 ^ to 7 //. across. This species is very similar to P. 

 vulgaris in most respects, but one distinctive feature is that in this 

 species some threads may attain a length of 200 ft, i.e. double the 

 maximum size of the threads of P. vulgaris. 



Proteus Zenkeri differs from the two preceding species in being unable 

 to liquefy gelatine and having normally smaller and shorter rods (about 

 0'8 fJL long). In addition, there are numerous small globular individuals, 

 about 0*4 /A in diameter. 



Another group which needs a short description is the one of which 

 Bac. lactis aerogenes forms the central type. The members of this 

 group are non-motile, do not liquefy gelatine, but are able to curdle and 

 acidulate milk, and can ferment sugars other than glucose. 



The rods of Bac. lactis aerogenes are 0'5 /A to 0-9 /A broad, and 1 /x 

 to 2/x long, are non-motile, and do not form spores. In milk the 



