SUMMARY: ‘Indifferent’ strains of streptococci-i.e. strains which give no haemolysis or greening on blood-agar plates-frequently occur in cultures from dental root-canals. A serological study of over 200 strains of these streptococci showed that about half of them belonged to Lancefield groups F, G or C. It was shown by cross-absorption that the group-antigen of indifferent streptococci of group F is identical with the group-antigen of haemolytic strains of group F. Apart from the group-antigen, five independent carbohydrate type-antigens, localized in the cell wall, were demonstrated in group F strains. These type-antigens were found in groups other than F, e.g. the type-antigen I was observed in haemolytic and indifferent strains of group G; the type-antigen III occurred in indifferent strains of group C. Several strains with a type-antigen but without group-antigens were observed. The presence of carbohydrate type-antigens in formamide extracts can cause confusing cross-reactions in the grouping procedure, unless strains without type-antigen are used for the preparation of sera.


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