%0 Journal Article %A Maxted, W. R. %A Widdowson, Jean P. %A Fraser, Cherry A. M. %A Ball, Lyn C. %A Bassett, D. C. J. %T The Use of the Serum Opacity Reaction in the Typing of Group-A Streptococci %D 1973 %J Journal of Medical Microbiology, %V 6 %N 1 %P 83-90 %@ 1473-5644 %R https://doi.org/10.1099/00222615-6-1-83 %I Microbiology Society, %X SUMMARY A large collection of group-A streptococci was examined for the production of opacity in horse serum and for the presence of M and T antigens. Opacity production is a constant character of certain M types and is consistently absent from the rest. Testing for opacity production is a valuable addition to the serological typing procedure especially if it is carried out first. It has proved useful in (1) reducing the number of tests with M antisera that have to be set up and (2) drawing attention to errors in M typing due to the presence of cross-reacting precipitins in typing sera. The serological specificity of the opacity factor corresponds to that of the M antigen. Inhibition of opacity production therefore provides a useful alternative method of typing strains that form the opacity factor. M antisera for opacity-producing serotypes are often of poor quality and difficult to prepare. In these circumstances, the opacity inhibition test has proved useful for preliminary identification of new M types of streptococci. %U https://www.microbiologyresearch.org/content/journal/jmm/10.1099/00222615-6-1-83