Evaluation of Statens Serum Institut enteric medium for detection of enteric pathogens

Abstract

The efficacy of the Statens Serum Institut (SSI) enteric medium for isolation and direct identification of enteric pathogens was evaluated. Six different biochemical reactions can be read by using the SSI enteric medium, allowing direct identification of a range of enteric pathogens. All 248 gram-negative bacterial species that were tested grew on the SSI enteric medium. Only 10 of 248 bacteria (4%) showed discrepant results in the biochemical reactions, and none of these were enteric pathogens. Forty-three of 47 enteric pathogens (92%) produced identical rates of semiquantitative growth on the SSI enteric medium and 5% blood agar, whereas three Vibrio spp. and one Aeromonas spp. showed reduced growth. Gram-positive bacteria did not grow on the SSI enteric medium. Most enteric pathogens had a detection limit of 50 bacteria per ml of feces, but higher numbers of Vibrio spp. and some Shigella spp. were required for detection. The growth rates of 125 enteric pathogens and 12 Yersinia spp. on the SSI enteric medium, xylose lysine deoxycholate (XLD), Hektoen enteric (HE), Salmonella-Shigella (SS), and cefsulodin-irgasan-novobiocin (CIN) agar were compared. Detection rates after application of 200 CFU were 99% for SSI enteric medium, 92% for XLD, 88% for HE, and 82% for SS agar. The 12 Yersinia spp. grew excellently on both the SSI enteric medium and CIN agar. We conclude that the performance of the SSI enteric medium compares favorably to those of other media tested. Its ability to detect Yersinia spp. may limit the number of media needed in the typical laboratory. The direct identification of enteric pathogens on the medium may also provide a more rapid diagnosis.

FIG. 1

Colony appearance of four different genera of the Enterobacteriaceae on SSI enteric medium. SSI enteric medium yields six differentiating biochemical reactions that allow direct identification of a range of enteric pathogens (see the text). In this figure, only the four major reactions are illustrated, with different bacteria. (A) Shigella sonnei, with the characteristic rough transformations, causes the surfaces and the edges of the colonies to become serrated. (B) E. coli and Shigella sonnei. E. coli organisms appear as red colonies due to lactose production, and there is no rough transformation. (C) E. coli and Y. enterocolitica. Yersinia spp. appear as tiny round translucent colonies which resemble pearls on a string (indicated by the arrow). They are easily recognized compared to the much larger, red colonies of E. coli. (D) H₂S-positive Salmonella spp. H₂-positive organisms appear with a black precipitate located centrally and deeply in the anaerobic parts of the colony.