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IN a recent paper, Fleming [1929] showed that a mould which appeared as an air contaminant in laboratory bacterial cultures, and which resembled Penicillium rubrum Grasberger-Stoll, produced when grown on nutrient broth a very powerful antibacterial substance. Filtrates of the broth cultures were named "penicillin" and it was found that the active substance affected different bacteria in different degrees. The action was very marked on the pyogenic cocci and the diphtheria group of bacilli, whilst the coli-typhoid and the influenza bacillus groups were quite insensitive to it. Staphylococci, Streptococcus pyogenes and the pneumococcus were completely inhibited in a 1 in 800 dilution of the filtrate, which did not affect the growth of H. influenzae (Pfeiffer's bacillus), and a method was described for the isolation of H. influenzae by incorporating penicillin in a suitable solid medium.Examination of Fleming's organism by Dr Charles Thom at our request showed that it was not Penicillium rubrum but a strain closely related to P. notatum Westling in the P. chrysogenum Thom series. Dr Thom writes as follows: "I have cultivated Fleming's organism under several different conditions and cannot agree with the nomenclature as P. rubrum either in the sense of my 1910 paper or in the sense of Biourge's monograph. In fact, I believe this culture, although showing some divergencies in culture reactions, to be closer to P. notatum of p. 264 in my book than to the groups discovered on pp. 249-50 as indicated by the nomenclature used."In his book [1930, pp. 261-2] Dr Thom says that the type strain of P. chrysogenum (No. 26) rapidly digests milk and liquefies gelatin giving a yellow colour. "Studied on a series of culture media, it changes colour with the composition of the substratum and with the age or stage of metabolism Biochem. 1932 xxvi 121

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