Abstract
Free D-amino acids (D-AAs) are one of the most striking features of the peptidoglycan composition in bacteria and play a key role in regulating and disassembling bacterial biofilms. Previous studies have indicated that the antimicrobial peptide nisin can inhibit the growth of the cariogenic bacteria Streptococcus mutans. The present study investigated the effect of free amino acids either alone or in combination with nisin on biofilm and on planktonic S. mutans bacteria. The results of the MIC and MBC analyses showed that D-cysteine (Cys), D- or L-aspartic acid (Asp), and D- or L-glutamic acid (Glu) significantly improve the antibacterial activity of nisin against S. mutans and that the mixture of D-Cys, D-Asp, and D-Glu (3D-AAs) and the mixture of L-Cys, L-Asp, and L-Glu (3L-AAs) at a concentration of 40 mM can prevent S. mutans growth. Crystal violet staining showed that the D- or L-enantiomers of Cys, Asp, and Glu at a concentration of 40 mM can inhibit the formation of S. mutans biofilms, and their mixture generated a stronger inhibition than the components alone. Furthermore, the mixture of the three D-AAs or L-AAs may improve the antibacterial activity of nisin against S. mutans biofilms. This study underscores the potential of free amino acids for the enhancement of the antibacterial activity of nisin and the inhibition of the cariogenic bacteria S. mutans and biofilms.
Highlights
The addition of D-Cys, D-Asp, and D-glutamic acid (Glu) can significantly improve the antibacterial activity of nisin, and the addition of D-Val, D-Phe, D-Leu, D-Ile, D-Thr, D-Pro, D-Tyr, and D-Ser showed slight improvement
The addition of D-Ala, D-Lys, D-Met, D-Trp, D-His, and D-Gly resulted in no improvement, and even the addition of D-Arg decreased the antibacterial activity of nisin
The combination of the three effective amino acid D-Cys, D-Asp, and D-Glu (3D-AAs) displayed inhibition for S. mutans even in the absence of nisin, and the same inhibition was observed after treatment with 3L-AAs
Summary
The microorganisms in biofilms are generally encapsulated by extracellular polymeric substances and tightly adhered to different surfaces [1,2]. Because biofilms are difficult to eradicate and are resistant to antimicrobial agents, biofilms have caused a range of problems and profoundly affect human health [3]. The inhibition of biofilms remains one of the crucial problems that current microbiologists aim to overcome. Brandenburg et al reported that tryptophan inhibits Pseudomonas aeruginosa biofilm formation on tissue culture plates and further inhibits the growth of existing biofilms [7]. Kolodkin-Gal et al reported that Dleucine, D-methionine, D-tyrosine, and D-tryptophan prevent Bacillus subtilis biofilm formation and can break down existing biofilms [5]
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