This is a Provisional monograph.

The original method for counting faecal streptococci in water was a most probable number method based on the selectivity achieved by using a sodium azide broth (Hannay and Norton, 1947). Later, Slantez and Bartley (1957) described a solid agar based on sodium azide as the selective agent together with tetrazolium chloride, which is reduced to the insoluble red pigment formazan during microbial growth. This is now recognised internationally as M-Enterococcus agar (q.v.). The present medium is a further modification by the APHA (2005), which is rendered more selective by the addition of nalidixic acid and cycloheximide and also provides an indicator of aesculin hydrolysis.