SUMMARYThe ability to produce opacity in horse serum is a characteristic of certain M types of group A streptococci. The types that produce the opacity factor (0 factor) are generally those for which it is difficult to produce good anti-M sera. M-positive (M+) and true M-negative (M-) variants of strains that belong to serotypes in which the serum opacity reaction (s.0.r.) is positive both possess the 0 factor, but there is a difference in binding of the factor to other cellular components in the two variants. The 0 factor is closely bound to the wall-membrane fraction of M-cells, whereas in M + cells it is easily extracted by Lancefield's method or I yo sodium deoxycholate.It is detectable in broth culture supernatants, in the cytoplasm and in the areas surrounding colonies in pour plates of M+ but not of M-cultures. The 0 factor is poorly antigenic, but when it is possible to obtain a good antiserum the inhibitory action is type specific. The 0 factor produced by several M types appears to be inhibited by normal rabbit serum.
I N T R O D U C T I O NThe production of opacity in horse serum by group A streptococci is believed to be mediated by a lipoproteinase which acts upon the a,-lipoprotein fraction of the serum (Krumwiede, 1954;Rowen & Martin, 1963). Streptococci of groups other than A apparently do not produce opacity in serum, and within group A the serum opacity reaction (s.0.r.) is positive only in certain serotypes. Gooder (1961) and Kohler (1963) grew streptococci in a liquid medium consisting of 3 parts horse serum and I part Hartley digest broth, and recorded the presence of opacity in the culture supernatant as a positive s.0.r. Consistently s.0.r.-positive serotypes were those in which no M antigen was detectable, or for which satisfactory anti-M sera were difficult to prepare; on the other hand, members of a number of easily recognizable M serotypes were almost invariably s.0.r.-negative. Both workers suggested that there was an inverse relationship between a positive s.0.r. and the production of M antigen. Top & Wannamaker (1968a) incubated killed cells or deoxycholate extracts of fractions consisting predominantly of cell walls and cell membranes (' wall-membrane fractions') in horse serum. They confirmed that opacity was frequently produced by members of 'difficult' M types, but could find no true inverse relationship between M antigen and opacity production, except among type 1 2 strains.Most of the evidence concerning the supposed inverse relationship was obtained by examining collections of M-typable and M-untypable strains with common T-antigen