Growth of Bacillus megaterium NCIB 7581 in a simple chemically defined medium was inhibited by D-glutamate above 0.01 mg ml-l ; equimolar L-glutamate prevented this inhibition. When DL-glutamate (2 mg ml-l) was present in the medium (with glycerol as the main carbon source), organisms grew exponentially until all the L-isomer had disappeared; growth then stopped for about 24 h during which there was a transient appearance of Dglutamine in the medium. Throughout the first stationary phase the concentration of Dglutamate in the medium fell continuously and when it was less than 0.01 mg ml-l there was a second phase of growth.Exponential phase organisms growing without glutamate contained only 4 mwglutamate in the free amino acid pool. During the first stationary phase with DL-glutamate added to the medium, the concentration of glutamate (all D-isomer) was 47 mM in the pool.Of four other strains of B. megaterium tested, only one was sensitive to D-glutamate. From strain 758 1 a D-glutamate-resistant substrain was easily developed. Among other amino acids added singly to the defined medium, only D-(and L-) serine was inhibitory to all five strains examined. Inhibition of B. megateriurn 7581 by D-glutamate was prevented by single addition of several amino acids, each of which could act as a sole source of nitrogen for growth.
I N T R O D U C T I O NBacillus megateriilm NCIB 7581 showed two distinct phases of growth in a simple chemically defined medium that contained only glycerol and DL-glutamate as sources of carbon (White, 1972). The cause of this diauxie has now been studied.
METHODSOrganisms. Bacillus megaterium NCIB 7581 and other strains of B. megaterium and their maintenance were described by White (1972). Lactobacillus arabinosus NCIB 6376 was grown overnight (and subcultured monthly) at 30 "C in stabs containing (g I-l): Difco tryptone (lo), Difco yeast extract (lo), glucose (1.0), KH,P04 (2.0), CaCO, (3.0) and agar (15). Stock cultures were kept at 2 "C.Media. Medium A1 (glycerol as carbon source) and medium A2 (glucose as carbon source) both supplemented with biotin (0.2 ,ug 1-l) were used for B. megaterium 7581 (White, 1972); biotin was omitted for the other strains. Growth from small inocula in these liquid media was more consistent when trisodium citrate (dihydrate; 20 mg 1-l) was added.Conditions of growth. Bacteria were grown in shaken 250 ml or 2 1 conical flasks containing 50 ml or 500 ml medium at 37 "C (or 30 "C for strain 13632) and growth was measured turbidimetrically in an EEL colorimeter (White, 1972). The inoculum (1 ml for 500 ml medium) was a suspension in water of organisms from a fresh slope culture on nutrient agar. The suspension was adjusted to a colorimeter reading of 1.0 (0.35 mg dry wt ml-l with strain 7581) and used undiluted or diluted 1 : 10 or 1 : 100.
