Differential and selective media for the faecal streptococci

Both sodium azide and thallous acetate have been incorporated separately in media for the selective isolation of the faecal streptococci (Lancefield group D). It has been found that the various methods developed for increasing the selectivity of the sodium azide media often eliminate Str. bovis. the importance of this in relation to these organisms as indicators of faecal pollution in water and foods is discussed. By the addition of tetrazolium to the thallous acetate agar at pH 6.0, it is possible to distinguish colonies of Str. faecalis and variants zymogenes and liquefaciens from Str. faecium and Str. durans. This method has been used for determining the numbers and types of faecal streptococci in the intestines of the various meat animals and poultry and to show changes in species following antibiotic feeding.