Brevibacterium ammoniagenes responds to increased osmotic stress by accumulating pipecolic acid, a cyclic amino acid, in addition to taking up exogenous glycine betaine. When grown on minimal medium with elevated salt concentrations (up to 2 M-NaCl), the bacterium sustained slow growth despite severe biomass reduction. While glycine betaine was readily transported and accumulated intracellularly at high osmolality, it did not enhance growth rates. Notably, glycine betaine was neither metabolized nor required for survival. Using radioactive labeling and spectroscopic analysis (NMR and mass spectrometry), researchers identified pipecolic acid as the major accumulated organic solute, with concentrations increasing from 40 to 131 nmol per mg dry weight as osmolality increased from 0 to 1 M-NaCl. The compound's accumulation was osmotically regulated and appeared to derive from lysine metabolism, though its maximum accumulation was lower than typical osmoprotectants in other halotolerant bacteria. When glycine betaine was externally supplied, pipecolic acid synthesis was suppressed, suggesting the bacterium preferentially uses external osmoprotectants over energy-expensive endogenous synthesis.

Key findings

• Brevibacterium ammoniagenes accumulates pipecolic acid in response to osmotic stress, the first report of this phenomenon in bacteria
• Glycine betaine is transported and accumulated but does not stimulate growth or undergo metabolism, yet still functions as an osmoprotectant by suppressing pipecolic acid synthesis
• Pipecolic acid accumulation is osmolality-dependent and derived from lysine metabolism, with synthesis strongly regulated by external osmotic pressure
• The non-permeant solutes NaCl, KCl, sucrose, and sorbitol trigger pipecolic acid accumulation, while the permeant solute glycerol does not, indicating osmotic turgor loss as the stimulus