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 V体育官网.
