Branched-chain amino acid fermentation by a marine spirochete: strategy for starvation survival.

An anaerobic marine spirochete (strain MA-2) fermented glucose and formed ethanol, acetic acid, CO(2), and H(2) as end products. The organism required carbohydrates as growth substrates. Amino acids did not support the growth of strain MA-2. However, when the spirochete was grown in media containing branched-chain amino acids and glucose, significant quantities of 4- and 5-carbon branched-chain volatile fatty acids were formed in addition to products of glucose fermentation. Smaller quantities of branched-chain alcohols were also formed under these conditions. The spirochete converted l-valine, l-isoleucine, and l-leucine to isobutyric, 2-methylbutyric, and isovaleric acids, respectively. CO(2) formation accompanied each of these conversions. Spirochete MA-2 did not require branched-chain amino acids for growth, but these compounds could serve as sole sources of nitrogen for the organism. In addition, the survival of starving cells (no growth substrate available) of spirochete MA-2 was prolonged significantly when l-valine, l-isoleucine, and l-leucine were present in starvation media. Starving cells fermented these amino acids, forming adenosine 5'-triphosphate and branched-chain fatty acids. Our findings indicate that energy derived from amino acid fermentation allows the spirochete to survive periods of growth substrate starvation. Apparently, dissimilation of branched-chain amino acids can provide this bacterium with maintenance energy for cell functions not related to growth. In its natural environment spirochete MA-2 may catabolize branched-chain amino acids as a strategy for survival when growth substrates are not available.