Among genera in the family Enterobacteriaceae, genus Escherichia and genus Shigella are described as H2S-negative on triple sugar-iron (TSI) agar. The existence of Escherichia coli strains which produce H2S on TSI agar has been reported (4). Recently we isolated two groups of Shigella sonnei which produced H2S on TSI agar. S. sonnei strains. Two groups of S. sonnei strains were found to be H2S-positive. The first one was isolated from a stool specimen of a patient with diarrhea in Sapporo City in 1964. Several semi-transparent colonies on deoxycholate-citrate-lactosesucrose (DCLS) agar were picked up and transferred to TSI agar and sulfide-indolemotility (SIM) medium. On TSI agar, lactose and sucrose were not fermented and glucose was fermented without gas production. The medium was blackened at the bottom of the slant (Fig. 1). In SIM medium, H2S was positive, indole production was negative and the indole-pyruvic acid (IPA) reaction was negative. They were non-motile. Moreover, lysine-decarboxylase was negative and acetyl-methylcarbinol was not produced. The isolated organism was found to be agglutinated by antiserum against S. sonnei, form I, and it was (strain Sapporo) identified as S. sonnei by additional biochemical tests. The second group of S. sonnei strains was isolated from stool specimens of one
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