Human antimicrobial peptide, LL-37, induces non-inheritable reduced susceptibility to vancomycin in Staphylococcus aureus

Cathrine Friberg,Veronique Perrot,Bruce R Levin,Martin Vestergaard,Anaëlle Fait,Hanne Ingmer,Jakob Krause Haaber

doi:10.1038/s41598-020-69962-4

Abstract

Antimicrobial peptides (AMPs) are central components of the innate immune system providing protection against pathogens. Yet, serum and tissue concentrations vary between individuals and with disease conditions. We demonstrate that the human AMP LL-37 lowers the susceptibility to vancomycin in the community-associated methicillin-resistant S. aureus (CA-MRSA) strain FPR3757 (USA300). Vancomycin is used to treat serious MRSA infections, but treatment failures occur despite MRSA strains being tested susceptible according to standard susceptibility methods. Exposure to physiologically relevant concentrations of LL-37 increased the minimum inhibitory concentration (MIC) of S. aureus towards vancomycin by 75%, and resulted in shortened lag-phase and increased colony formation at sub-inhibitory concentrations of vancomycin. Computer simulations using a mathematical antibiotic treatment model indicated that a small increase in MIC might decrease the efficacy of vancomycin in clearing a S. aureus infection. This prediction was supported in a Galleria mellonella infection model, where exposure of S. aureus to LL-37 abolished the antimicrobial effect of vancomycin. Thus, physiological relevant concentrations of LL-37 reduce susceptibility to vancomycin, indicating that tissue and host specific variations in LL-37 concentrations may influence vancomycin susceptibility in vivo.

Highlights

Bacterial resistance to antibiotics is generally associated with genetic changes, such as point mutations in the core genome or acquisition of resistance genes present on mobile genetic e lements[1], or intrinsic resistance to antimicrobials, such as impermeability of the outer membrane of Gram-negative bacteria or the active efflux of antimicrobials provided by chromosomally encoded efflux p umps[2]
Reduced vancomycin susceptibility state will remain undetectable by routine clinical testing and the minimum inhibitory concentration (MIC) of vancomycin upon colistin exposure in vivo may be higher than expected from such testing
We have previously shown that pre-exposure of S. aureus to the antimicrobial peptide, colistin, reduces vancomycin susceptibility[3]

Summary

Introduction

Bacterial resistance to antibiotics is generally associated with genetic changes, such as point mutations in the core genome or acquisition of resistance genes present on mobile genetic e lements[1], or intrinsic resistance to antimicrobials, such as impermeability of the outer membrane of Gram-negative bacteria or the active efflux of antimicrobials provided by chromosomally encoded efflux p umps[2]. We showed that exposure to the synthetic antimicrobial peptide (AMP) colistin reduces susceptibility to vancomycin in the clinically important, methicillin resistant Staphylococcus aureus (MRSA) strain USA300 via induction of the GraRS cell wall r egulon[3]. In the last decade vancomycin-intermediate S. aureus strains (VISA) have emerged with reduced vancomycin susceptibility (MIC = 4–8 μg/ml)[6]. These strains are characterized by chromosomal mutations that reduce negative cell wall c harge[7], increase cell wall thickness[8] or reduce Triton-X mediated a utolysis[9]. Reduced vancomycin susceptibility state will remain undetectable by routine clinical testing and the MIC of vancomycin upon colistin exposure in vivo may be higher than expected from such testing. We speculate that natural variation in LL-37 concentrations may lead to variation in susceptibility to vancomycin therapy between tissues and individuals

Methods

Results

Conclusion