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.