Rapid differentiation of Streptococcus milleri from other beta-haemolytic group A, C, and G streptococci by simple screening tests.

Eighty five beta-haemolytic Lancefield group A(23), C(22), and G(40) streptococci were tested for sulphonamide sensitivity, ability to degrade beta-D glucuronide (groups C and G) and bacitracin sensitivity (group A). Identification of isolates was initially confirmed by the API 20 STREP system. Zones of beta-haemolysis were too variable for correct Streptococcus milleri identification in 10% of cases. However, only group C and G S. milleri were both sulphonamide-resistant and beta-D glucuronide-negative. Two group A S. milleri strains could have been mis-identified as Streptococcus pyogenes if combined sulphonamide and bacitracin resistance had not been noted. In a busy diagnostic laboratory, screening of beta-haemolytic group A, C, and G streptococci for sulphonamide sensitivity is recommended. Sulphonamide-resistant group A S. pyogenes can be distinguished from S. milleri by bacitracin sensitivity. S. milleri can be rapidly and cheaply differentiated from other sulphonamide-resistant group C and G streptococci by a simple 4 h commercial beta-D glucuronide assay.