boydii 6 with Shigella sonnei in phase II, Shigella dispar and with fraction A of Shigella alkalescens (Wheeler and Ewing, 1946). In addition, Shigella boydii 6 has an antigen identical with that of Shigella sonnei in phase II and less important rela- tionships with Shigella flexneri 1 and 4. Several types of Shigella boydii have anti- genic fractions identical with those of Escherichia coli 053, and Shigella boydii 5 has fractions identical with those of Escherichia coli 079 (Ewing, Hucks and Taylor, Jour. Bad., 63, 1952, 319). Source: Isolated from feces in cases of dysentery. Habitat: Found only in the feces of the sick; occurs only in a low proportion of cases of bacillary dysentery. 5. Shigella flexneri Castellani and Chalmers, 1919. (Bacillus dysenteriae Flex- ner, Phil. Med. Jour., 6, 1900, 414; not Bacillus dysenteriae Shiga, Cent. f. Bakt., I Abt., ^4, 1898, 817; Bacillus paradysenteriae Collins, Jour. Inf. Dis., 2, 1905, 620; Castel- lani and Chalmers, Man. Trop. Med., 3rd ed., 1919, 937; Shigella paradysenteriae Weldin, Iowa State Coll. Jour. Sci.,1, 1927, 178.) flex'ne.ri. M.L. gen. noun flexneri of Fle.x- ner; named for Simon Flexner, the bacteriol- ogist who first isolated this species. Rods, 0.5 by 1.0 to 1.5 microns, occurring singly, often filamentous and irregularly shaped in old cultures. Non-motile. Gram- negative. Culturally identical with the other mem- bers of the genus except that growth in broth is more abundant. Gelatin: No liquefaction. Indole is produced (except by serotype 6) . Hydrogen sulfide not produced. Acid but no gas from glucose and ara- binose; irregularly from rhamnose, sucrose and maltose. Mannitol is fermented (except by certain strains of serotypes 4 and 6). Lactose, dulcitol, xylose, sorbitol, salicin and adonitol are not attacked. Nitrites produced from nitrates. Trimethylamine not produced from tri- methylamine oxide. Aerobic, facultatively anaerobic. Optimum temperature, 37° C. No growth at 45.5° C. Antigenic structure: The somatic antigen, extracted by diethyleneglycol, consists of a protein together with a polyoside and a phospholipid. The protein, dissociated with difficulty from the polyosides, is obtained in a pure state by a pancreatic digestion. In an acid medium, the toxicity of the antigen is related to the protein fraction and in an alkaline medium to the polyoside. The polyoside heptene is responsible for the serological characters (Tal and Goebel, 1950). The structure of the O antigen of Shigella flexneri is much more complex than those of the other shigellas. These organisms have a major type antigen and several minor group antigens (Murray, Jour. Roy. Army Med. Corps, SI, 1918, 257; Andrewes and Inman, Med. Res. Council, Special Rept. Ser. No. 42, London, 1919). Andrewes has identified five groups of major antigens: V, W, X, Y and Z; type Y is composed of variable pro- portions of the other four antigens. The mosaic of minor antigens determines con- stant cross agglutinations among the various types of Shigella flexneri, although there is not any serological relationship with the other groups of shigellas. Boyd has ascertained that the instability of the O antigen is due to a modification of the anti- genic structure of the mutants which are developed in artificial media. In each cell the specific type antigen diminishes or dis- appears, and thus the organism retrogresses toward a type common to every strain. The group diagnosis of Shigella flexneri is determined by agglutination with poly- valent serum; the diagnosis of the type is then determined by a monospecific serum obtained after absorption of the group agglutinins. Certain varieties of Shigella flexneri 6 are 0-inagglutinable because they have a K antigen which is similar to the surface antigen B of Escherichia coli (Mad- sen). On boiling the emulsion for an hour to inhibit the B antigen, the organism becomes agglutinable. Shigella flexneri has no impor- tant serological relationships with the other shigellas but has antigenic fractions identi-
