Gliotoxin, a new candidate against methicillin-resistant Staphylococcus aureus showing synergistic effect with classical antimicrobial drugs in Caenorhabditis elegans infection model

Abstract

[Background] Multidrug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) is among the major causes of hospital-acquired and community infections, and pose a challenge to the human health care system. Therefore, it is important to find new drugs that show activity against these bacteria, both in monotherapy and in combination with other antimicrobial drugs. Gliotoxin is a mycotoxin produced by Aspergillus fumigatus and other fungi of the Aspergillus genus. Some evidence suggests that gliotoxin shows antimicrobial activity, especially against S. aureus. This work aims to evaluate the antibiotic efficacy of gliotoxin as monotherapy or in combination with other therapeutic against MRSA in vitro and in vivo using a C. elegans infection model.

[Materials/methods] MRSA ATCC700699 sensitivity against gliotoxin was analysed by different methods: disc diffusion, time-mortality curves, microdilution assays. The synergistic effects of gliotoxin with others anti-staphylococcal drugs (vancomycin, fusidic acid and linezolid) were tested by a checkerboard assay. Transformed synchronized C. elegans L4 worms were infected with MRSA. The culture was supplemented separately with gliotoxin, vancomycin or its combination to different concentrations. The survival rate of C. elegans was scored manually every 12h for 4 days.

[Results] Sensitivity analysis of MRSA against gliotoxin showed that gliotoxin has a potent antimicrobial activity (MIC = 2 μg/ml). In addition, synergistic activity was observed, when the gliotoxin was combined with vancomycin and fusidic acid, using checkerboard and time-kill assays. This synergistic effect was confirmed by CalcuSyn software (CI = 0.15 for 1 μg/ml of gliotoxin combined with 2 μg/ml of vancomycin). The in vivo study of S. aureus infection in a C. elegans model showed that combined treatment of vancomycin with gliotoxin increased 70% in survival rate of the MRSA-infected worms.

[Conclusions] Our results suggest that gliotoxin is effective against MRSA strains indicating their potential use as a new drug against MRSA infections.