Abstract
The amount of tetrazolium (2,3,5-triphenyltetrazolium chloride) reduced by strains of Lancefield group D streptococci was estimated by measuring colorimetrically the formazan produced. S. faecalis caused 80% reduction, S. faecium only 45%, and S. bovis and S. durans little or none. Sixty-eight fresh strains were readily differentiated by this method. Glucose nutrient agar containing tetrazolium was used to distinguish colonies of S. faecalis (deep red) from those of S. faecium (colorless).