Identification of factors involved in the growth of Symbiobacterium thermophilum

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Symbiobacterium thermophilum was originally isolated as a symbiotic bacterium requiring support of the associating Bacillus strain S for growth. Although full growth of S. thermophilum needs mixed culture with the bacillus, limited growth of less than 1 x 10^7 cells/ml was found to occur in the absence of the bacillus in nutrient broth containing yeast extract and tryptone-peptone. Two types of the activity to support the independent growth were detected in charcoal-treated yeast extract and tryptone-peptone, respectively. The activity in charcoal-treated yeast extract was replaced by monovalent cations such as Rb^+, and NH_4^+, and several compounds such as CO_2, bicarbonate, cyanate and oxaloacetate, which are related to the reactions catalyzed by carboxylase and carbonic anhydrase. The activity in tryptone-peptone was replaced by the tryptic digest of casein or myoglobin, in which the activity distributed in peptides of various molecular sizes. In addition to these factors as well as anaereobic condition required for the limited independent growth, dialysis culture against a large volume of fresh medium released the limitation to achieve the full growth exceeding 1 x 10^9 cells/ml in the axenic culture. Thus hyper-sensitivity to the own metabolites was indicated as another key of the symbiotic characteristics of S. thermophilum. Branched iso-C17 and anteiso-C17 fatty acids, components of cellular fatty acids of S. thermophilum, caused potent growth inhibition, suggesting their possible involvement in the self-limited growth. We conclude that symbiotic growth of S. thermophilum is supported by the dual functions offered by Bacillus, one providing the complex growth factors and the other to remove the inhibitory metabolites from the culture.