The 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 (O 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.o.r.) is positive both possess the O factor, but there is a difference in binding of the factor to other cellular components in the two variants. The O 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 1% 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 O factor is poorly antigenic, but when it is possible to obtain a good antiserum the inhibitory action is type specific. The O factor produced by several M types appears to be inhibited by normal rabbit serum.


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