Several forms of penicillin have been identified in cultures of Penicillium notatum. Three of these, known as penicillin F, G and X in this country and as penicillin I, II and III respectively in Great Britain, have been isolated in crystalline form.1 Other active substances have also been described which differ from these three in that they are probably proteins, nondiffusible, active only in the presence of glucose and are effective against gram negative as well as gram positive bacteria. The latter have been variously labeled notatin, penatin, penicillin B and "second factor" but are all closely related or identical.2
Commercial penicilins prepared from deep vat cultures consist almost entirely of penicillin G, but those prepared from shallow surface cultures in flasks may contain appreciable- amounts of penicillin X varying up to 20 or 25 per cent.3
The only published paper in which more than mention is
ORY EM, MEADS M, FINLAND M. PENICILLIN X: COMPARISON WITH PENICILLIN G WITH RESPECT TO SENSITIVITY OF PATHOGENIC ORGANISMS AND SERUM LEVELS. JAMA. 1945;129(4):257–261. doi:10.1001/jama.1945.02860380015004
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