SUMMARYAs a preliminary to an investigation of the inheritance of penicillin productivity in Penicillium chrysogenum several strains were genetically labelled with nutritional deficiencies and mutant spore colours. Few auxotrophs of P. chrysogenum retained the potential for penicillin yield of their parents. An analysis suggests that when loss of penicillin production occurred in an auxotroph it was most likely due to pleiotropic effects of the single mutation to auxotrophy.

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SUMMARYThe nutritional requirements for growth on agar media a t 28" and a t 37" were investigated for strains representative of Pmteurella pseudotuberculosis, P . pestis and Pasteurella strain ' X ' ( Yersinia enterocolitica). At 28", strains of P. pseudotuberculosis either required no growth factors or were dependent on thiamine or pantothenate. Most strains of P. pestis required cystine, methionine and phenylalanine but some strains could dispense with methionine or phenylalanine while others required additional factors.Strains of Pasteurella ' X ' either were thiamine-dependent or required no factors. At 37" all strains showed additional requirements. Most strains of P. pseudotuberculosis could then grow with any 3 of the 4 factors glutamic acid, thiamine, cystine and pantothenate ; other strains specifically required all 4 factors and nicotinamide. Strains of P. pestis grew reliably on media supplemented with cystine, methionine, phenylalanine, glycine, valine, isoleucine, glutamic acid and thiamine when incubated in C0,-enriched air, All strains of Pasteurella 'X' required thiamine and either cystine or methionine ; some substrains showed additional requirements. Virulent and avirulent representative strains of P. pestis had similar calcium requirements for growth on defined media.

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SUMMARYHeterozygous diploids were produced between a wild-type strain of Penicillium chrysogenum and two strains derived from it. The derived strains had been isolated after several and latterly divergent selection steps, selection being made a t each step for higher penicillin yield. The diploids had penicillin titre values near that of the wild-type strain so that mutations for increased penicillin production which had been accumulated in the derived strains appeared in the main to be recessive. First-order segregants isolated from the heterozygous diploids were in each case predominantly of one phenotype.