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An obligately symbiotic bacterium, strain T, associating with a specific strain S of a thermophilic Bacillus sp. was isolated during screening for micro-organisms that produce heat-stable tryptophanase and β-tyrosinase. A colony containing both strains was finally isolated by successive selection with bacitracin. In mixed culture with Bacillus strain S under low aeration conditions at 60 °C, growth of strain T was initiated following lysis of the Bacillus cells. No independent growth of strain T was observed in any of the media tested, even in the presence of other thermophilic Bacillus strains or cell-free extracts of strain S or its killed cells. No physical adhesion between cells of strains S and T was observed microscopically. Tryptophanase and β-tyrosinase production by strain T were induced by l-tryptophan and L-tyrosine, respectively. The name Symbiobacterium thermophilum gen. nov., sp. nov. is proposed for strain T which is a Gram-negative rod (0·25–0·35 µm diameter) with a multilayer surface structure containing meso-diaminopimelic acid. The G+C content of the DNA is 65·1 mol%; iso-C15:0, iso-c17:0 and anteiso-C17:0 acids are the major cellular fatty acids. The type strain is strain IAM 13621.
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