Synergism and efficacy of some naturally occurring D-amino acids against clinically relevant bacterial and fungal species

Amino acids are predominantly synthesized and used in their L-enantiomeric form in all three kingdoms of life. However, bacteria produce diverse D-amino acids that are involved in the synthesis and cross-linking of peptidoglycan. Several studies reported possible antimicrobial activities of selected D-amino acids against Escherichia coli. The present study was undertaken to investigate the antibacterial and antifungal susceptibility patterns and growth inhibitory effects of certain D-amino acids, including D-alanine, D-lysine, D-serine, and D-proline. Our findings indicate that D-lysine is the most potent antibacterial and antifungal, among the examined D-amino acids, followed by D-alanine, whereas D-serine and D-proline had insignificant antimicrobial activities. Gram positive bacteria were more susceptible to the antibacterial effects of D-amino acids than Gram negative bacteria. Growth kinetic studies revealed that D-lysine and D-alanine resulted in extended lag phases, suggesting that the D-amino acids successfully influenced the microorganisms' ability to use nutrients efficiently and disrupted their normal biological functions. Additionally, synergism was evident between D-alanine and D-lysine when combined with either Ampicillin or Amphotericin B. These results suggest a new avenue for D-amino acids' potential as naturally occurring antimicrobial reagents for the treatment and prevention of microbial growth in food and agriculture applications