D-enantiomeric antimicrobial peptides as antibacterial agent against drug-resistant Mycobacterium tuberculosis

Abstract

BACKGROUND: Tuberculosis (TB) control efforts have taken on increased urgency due to the emergence of multidrug-resistance (MDR) and extensive drug-resistant (XDR) TB. Effective antibacterial therapeutics are desperately needed to combat the drug resistant TB. Antimicrobial peptides (AMPs) have the ability to target microbial pathogens within eukaryotic cells. In the present study, we investigated the activity of a family of structurally related D-enantiomeric AMPs (D-LAK peptides) against clinical isolates of MDR-TB and XDR-TB. Methods: The antibacterial activity of six D-LAK peptides (with different hydrophobicity and structural conformation) were tested against clinical isolates of drug susceptible, MDR and XDR Mycobacterium tuberculosis (Mtb) using broth micro-dilution assay in 96 well plates. The cytotoxicity of the D-LAK peptides on human macrophage-like cells (THP-1) was examined by lactate dehydrogenase (LDH) and 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide) (MTT) assay. In addition, the effect of combining D-LAK peptides with isoniazid was also assessed against MDR-TB. RESULTS: All the D-LAK peptides tested could successfully inhibit the growth of Mtb in vitro to a certain extent and this was similarly effective against MDR and XDR strains. D-LAK peptides effectively broke down the heavy clumping of mycobacteria, consistent with a ‘detergent-like effect’ that could reduce the hydrophobic interactions between the highly lipidic cell walls of the mycobacteria, preventing bacteria cell aggregation. D-LAK peptides could not eradicate Mtb at non-toxic concentration, however they were effective as adjunct agent at low concentration to potentiate the efficacy of isoniazid against drug-resistant Mtb without inducing cytotoxicity to mammalian cells, possibly by facilitating the access of isoniazid into the mycobacteria by increasing the surface permeability of the pathogen. CONCLUSIONS: We have identified that out of the six tested D-LAK peptides, D-LAK120-A is the optimal peptide within the family based on the balance between anti-TB activity and the low cytotoxicity towards mammalian cells. Although D-LAK peptides alone may not be sufficiently potent at their nontoxic concentrations as anti-TB agent, they could be used as adjunct agent to improve the efficacy of the existing antibiotics against drug-resistant TB.