An evaluation of two composite media for preliminary identification of Shigella and Salmonella.

Because of the great demand for bacteriological examination of faecal specimens for Salmonella and Shigella organisms, there is need for a simplified biochemical method of preliminary differentiation of the Enterobacteriaceae to replace the widely used differential set of sugars involving inoculation of six or more tubes of separate media. The basic requirements of such a method are that it should be as accurate and reliable as the conventional method, and should effect economies in time, labour, and materials. An obvious way of effecting these economies is by using composite media, thus testing two or more biochemical or other characteristics in one culture tube. In 1911 Russell introduced a double-sugar medium for testing fermentation of glucose and lactose respectively in the depth and at the surface of the medium. Numerous modifications have since been introduced. Krumwiede, Pratt, and McWilliams (1916) substituted Andrade's indicator for the original litmus to obtain sharper colour changes and allow readings to be made in artificial light. Krumwiede and Kohn (1917) added 1 % of sucrose to exclude " many 'intermediates' which have a greater avidity for sucrose than for lactose when grown aerobically." Kligler (1918), by incorporating lead acetate in the medium, offered a method of differentiating S. typhi from the Shigellae, and, in association with Defandorfer (1918), substituted mannitol for lactose in Russell's original recipe, thus permitting differentiation between Shiga and Flexner types of dysentery bacilli. More recent modifications (Hajna, 1945; Bader and Hotz, 1951) indicate the limit of biochemical characteristics which can profitably be tested by a single-tube composite medium. Recently, Kohn (1954) described a technique employing two composite media, and this paper gives the results obtained by using his media in routine diagnosis in comparison with the conventional method normally used in the biochemical selection of pathogenic organisms before serological identification.