The Effects of Antimicrobial Substances on Rhizobium spp. and their Use in Selective Media

S ummary . Fortyseven strains of rhizobia were tested for sensitivity to 6 antibiotics (chloramphenicol, erythromycin, penicillin, streptomycin, sulphafurazole and tetra‐cycline) using Oxoid Multodisks on yeast extract‐mannitol agar (YMA); penicillin was the least inhibitory. Growth of selected strains on YMA was also compared with that on YMA containing one or more antimicrobial substances. Penicillin (1 i.u./ml of YMA) only slightly inhibited growth of some strains that were sensitive to it by the Multodisk test but higher concentrations were too inhibitory to be used in a selective medium. YMA containing pentachloronitrobenzene (5 p/m, suspended in a 5 p/m solution of Triton X‐100), brilliant green (0.5 p/m) and sodium azide (0.5 p/m) did not inhibit 15 of 18 strains of rhizobia but was more inhibitory to the general microflora of 4 soils than was YMA. This medium (ABPA) was very inhibitory to several Rhizobium strains when supplemented with 1 i.u./ml of penicillin.