THE PARATYPHOID GROUP 345 



like the typhoid bacillus; and B. paratyphosus beta, which produced 

 an initial acidity in litmus milk followed by a progressively alkaline 

 reaction. These observations, both clinical and bacteriological, have 

 been confirmed by later investigations. 



Morphology. The members of the intermediate group are indistin- 

 guishable morphologically. They are rod-shaped bacilli with rounded 

 ends, measuring from 0.8 to 1 micron in diameter, and 1.5 to 3.5 

 microns in length, occurring singly or in pairs, seldom in chains. In 

 actively-growing cultures the organisms may be short, almost ovoid. 

 In old cultures the organisms may be elongated; filamentous forms 

 are more commonly seen in old gelatin cultures. The members of the 

 group are actively motile and possess from four to twelve peritrichic 

 flagella. Motility is greater in dextrose broth than in plain broth; 

 this is particularly the case in young cultures. The organisms form 

 no spores and appear to possess no capsules. They stain readily with 

 ordinary anilin dyes; occasionally organisms from cultures several 

 days old exhibit a tendency toward bipolar staining. They are Gram- 

 negative. 



Isolation and Culture. The organisms of the paratyphoid group grow 

 readily upon ordinary artificial media, B. paratyphosus alpha somewhat 

 less luxuriantly than the remaining members. The colonies produced 

 on agar after eighteen hours' incubation at 37 C. resemble those 

 of the typhoid-dysentery group small, round, and transparent 

 measuring from 1 to 3 mm. in diameter. On Endo medium the colonies, 

 like those of B. typhosus and the dysentery bacilli, are clear and 

 colorless and somewhat smaller than those developing upon plain 

 agar. They usually measure from 0.75 to 2 mm. in diameter. The 

 organisms grow well in gelatin, but do not cause liquefaction. They 

 produce acid and gas in dextrose and mannite; lactose and saccharose 

 are not fermented. 



Milk. Plain milk is not coagulated. All the members of the group 

 except B. paratyphosus alpha cause a slow change in this medium, 

 which becomes thin, brownish, and almost opalescent after two or 

 more weeks' incubation. In litmus milk the cream ring is colored 

 a deep blue-green, which is so constant as to be suggestive diagnos- 

 tically. B. paratyphosus alpha produces a slight acidity which is 

 permanent; the milk assumes a lilac color. B. paratyphosus beta 

 and other members of the group produce a transient acidity 1 which 



1 For an explanation of the phenomenon, see page 222. 



