Summary. A modification of Bergersen's synthetic medium gave improved growth of Rhizobium. When potassium was included in the medium the biotin concentration could be decreased from the recommended 150–250 to 0·5 μg/l. Growth of 15 strains from 6 Rhizobium spp. was compared in the original and modified media and the responses of those strains to biotin, thiamine and calcium pantothenate noted.

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SUMMARYSeveral yeast water preparations and commercial yeast extracts varied in their toxicity to two strains of Rhizobium trifolii. Strains of R. japonicum and R. leguminosarum were also susceptible to yeast extract, while strains of R. meliloti were resistant.An interaction between glycine and monovalent cations was the main cause of yeast extract toxicity. Divalent cations, notably Ca2+, added to inhibitory media prevented the toxicity and enhanced growth with increased concentrations of yeast extract. I N T R O D U C T I O NYeast extract mannitol agar (YEMA) is the most commonly used medium for the culture of rhizobia, and is made from either yeast water preparations, or commercial yeast extract powders and pastes. Attempts to recover Rhizobium trifolii quantitatively by plate counts, gave anomalous results which were traced to inhibition by commercial yeast extracts.Toxicity of yeast extracts to other bacterial species has been reported occasionally but there have been very few references to inhibition of rhizobia. Jordan (1965) reported poor growth, with apparent bacteroid formation, of Rhizobium leguminosarum in mannitol broth containing 0.35% Difco yeast extract. On the other hand, many workers have described experiments with rhizobia in media containing 0.5 to I 76 (w/v) commercial yeast extracts, without comment on the growth yield. This paper reports an examination of the effects of a number of yeast extracts on the growth of Rhizobium trfolii. METHODSMedia. Unless otherwise stated, YEMA contained (g/l) : mannitol, 10; K2HP04, 0.5 ; MgS04.7H20, 0.2; NaCl, 0.1; Oxoid no. 3 agar, 15; CaCO,, 3 (Fred, Baldwin & McCoy, 1932). Commercial yeasts extracts or yeast water preparations were added at specified concentrations w/v for extracts and v/v for yeast water. Standard YEMA contained 0.3% Oxoid yeast extract ('L 20' Batch 118). All media were adjusted to pH 6.8; additives were sterilized separately. Chemicals were analytical reagent grade where possible.Yeast extract mannitol broth (YEMB) was YEMA without agar. The medium was boiled, cooled, while the CaCO, settled, then the supernatant liquid was decanted.Yeast extracts'. Six commercial yeast extract preparations were used : two Oxoid pastes (L 20), batches 118 and 123; three Oxoid powders (L 21), batches 2233, 2719 and 2878; one Difco powder, batch 493534.

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