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One of the best producers is strain Cowan I (ATCC-12598; NCTC-8530) which contains about 1.7% protein A by weight and its cell wall contains 6.7% protein A. Protein A is mainly found covalent linked to the peptidoglycan of the cell wall and a small portion of it (-8%) is secreted into the growth medium. Lysostaphin digestion of the bacteria followed by ion-exchange, gel filtration and affinity chromatography on IgG-Sepharose yields relatively pure protein A. However, certain methicillinresistant swains of S. aureus synthesize protein A quite normally but do not incorporate it into the cell wall. Although there is a good correlation between protein A production and deoxyribonuclease and coagulase activity, there is no correlation between the absence or presence of protein A and any pathogenic property.