704 SALMONELLA 



and carnivores are not uncommonly infected. Except for one or two members, 

 like the typhoid bacillus, that are non-pathogenic to animals, man does not seem 

 to act as an important reservoir of infection ; the invading organisms are quickly 

 thrown off, and the chronic carrier state is unusual. 



Morphology. — The shape, size, structure, and arrangement of the bacterial 

 cells do not differ materially from those in the Bacterium group. The usual length 

 is 2-3 ft and the usual width 0-Q fi ; but considerable deviation from these modal 

 values is found under different environmental conditions and in different cultural 

 variants. With the exception of Salm. gallinarum and its variant pullormn, all 

 species are motile ; though individual non-motile strains of the typhoid bacillus, 

 for example, may be encountered occasionally in the body, and non-motile variants 

 may be thrown off under cultural conditions in the laboratory. Motility, however, 

 is such a general characteristic that failure of a strain to exhibit it on primary 

 isolation from the body must be regarded as almost sufficient in itself to exclude 

 it from the Salmonella group. Whether the flagella are peritrichate, which has 



been the conclusion drawn by most 

 workers from a study of fixed and stained 

 bacilli, or whether, as appears possible 

 from Pijper's (1938, 1940) studies of liv- 

 ing typhoid bacilli by sunlight darkground 

 photomicrography, they are disposed one 

 on each side of the bacillus, is disputable, 

 and is for our purpose more of academic 

 than of practical importance. 



Capsules are not ordinarily formed, 

 but many species, notably Salm. paratyphi 

 B, may give rise to mucoid colonies in 

 which the individual organisms are sur- 

 rounded by a polysaccharide-containing 

 capsular material (Fletcher 1918). The 

 Fig. \4n.—Salm. enteritidis. formation of a capsule by the typhoid 



Colony on agar plate after 24 hours (X 8). bacillus has been described by Kuhne- 



mann (1911), Carpano (1913), Marrassini 

 (1913), Shimidsu (1913), and Gay and Claypole (1913). 



Cultural Characters.— The members of this group grow readily on ordinary 

 nutrient media and cannot be distinguished from coliform bacilli. A few species, 

 however, such as paratypJd A, abortus-ovis, typhi-suis, sendai, and pullorum, grow 

 less abundantly and form but a thin layer of growth on an agar slope. On brilliant 

 green agar plates the difference is particularly noticeable, the growth being both 

 slower and less abundant than that of other members of the group. Salm. typhi 

 and Salm. rostock likewise grow poorly on brilliant green agar, though developing 

 fairly well on ordinary nutrient agar. In broth, smooth strains give rise to a 

 uniform turbidity, increasing rapidly during the first 12 to 18 hours of growth, 

 and then more slowly up to 48 to 72 hours. Pellicle formation is rare, and when 

 present is slight. A deposit forms as growth increases ; this disperses readily 

 on shaking, leading to an increase in the turbidity of the culture. 



On agar, the colonies are relatively large, with an average diameter of 2-3 mm., 

 but they vary considerably in size. They may be circular and low convex with 

 a smooth surface and entire edge ; they may be flatter with a less regular surface 



