A single medium for the isolation of acetylene-reducing (dinitrogen-fixing) bacteria from soils.

A single medium, containing standard basal salts and three common carbon sources (sucrose, mannitol, and sodium lactate) is proposed to replace nitrogen-free media in common use for isolating dinitrogen-fixing bacteria. Eight commonly isolated genera of dinitrogen-fixing bacterial exhibited growth on this combined carbon medium that equalled or bettered growth on other carbon-containing media. Combined carbon medium also yielded the highest counts of putative dinitrogen-fixing bacteria from three southern Alberta soils. A survey of the bacteria isolated aerobically from the Burdett soil on combined carbon agar indicated that, at higher dilutions, 75% of the isolates exhibited acetylene reduction. These bacteria were identified as Azospirillum spp., Bacillus polymyxa, B. macerans, Klebsiella pneumoniae, Erwinia herbicola, and Enterobacter cloacae. The inclusion of yeast extract in combined carbon medium is considered essential to supply organic growth factors and may supply "starter" nitrogen that promotes growth without inhibiting acetylene reduction.