Fox, EUGENE N. (University of Chicago, Chicago, Ill.). Intracellular M protein of group A Streptococcus. J. Bacteriol. 85:536-540. 1963.-A heat-labile M protein antigen in protoplasts of a type 14 strain of group A Streptococcus has been demonstrated in a soluble form in the cytoplasm, and also bound to the protoplasmic membrane. When trypsinized whole cells (from which the M protein on the cell wall had been digested) or protoplasts were extracted with hot HCl, no M protein antigen was

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... , although the antigen was routinely extracted from the cell walls of normal streptococci by the hot acid procedure. Various serological techniques, including the use of fluorescein-labeled typespecific antisera, were employed to demonstrate the M antigen in association with the membrane from osmotically lysed protoplasts.