Abstract
During the development of antimicrobial peptides (AMP) as potential therapeutics, antimicrobial susceptibility testing (AST) stands as an essential part of the process in identification and optimisation of candidate AMP. Standard methods for AST, developed almost 60 years ago for testing conventional antibiotics, are not necessarily fit for purpose when it comes to determining the susceptibility of microorganisms to AMP. Without careful consideration of the parameters comprising AST there is a risk of failing to identify novel antimicrobials at a time when antimicrobial resistance (AMR) is leading the planet toward a post-antibiotic era. More physiologically/clinically relevant AST will allow better determination of the preclinical activity of drug candidates and allow the identification of lead compounds. An important consideration is the efficacy of AMP in biological matrices replicating sites of infection, e.g., blood/plasma/serum, lung bronchiolar lavage fluid/sputum, urine, biofilms, etc., as this will likely be more predictive of clinical efficacy. Additionally, specific AST for different target microorganisms may help to better predict efficacy of AMP in specific infections. In this manuscript, we describe what we believe are the key considerations for AST of AMP and hope that this information can better guide the preclinical development of AMP toward becoming a new generation of urgently needed antimicrobials.
Highlights
Most antimicrobial susceptibility testing (AST), and its interpretation, is conducted using internationally recognised standards developed by bodies including the International Organization for Standardization (ISO), Clinical and Laboratory Standards Institute (CLSI), the European Committee on Antimicrobial Susceptibility Testing (EUCAST), The United States Committee on Antimicrobial Susceptibility Testing (USCAST) and the US Food and Drug Administration (FDA) Center for Drug Evaluation and Research (CDER) (Table 1) (Magiorakos et al, 2012; Kahlmeter, 2015; Humphries et al, 2019)
Most efficacy end-points are 100% growth inhibition, there may be a lesser burden for some pathogen/antimicrobial combinations (e.g., ≥50% growth inhibition for fluconazole, flucytosine and ketoconazole for non-dermatophyte moulds CLSI, 2017c) or the determination of minimum effective concentrations (MEC), rather than MIC
In the method described by the Hancock laboratory for AST of antimicrobial peptides (AMP), they recommend the use of a diluent of 0.01% acetic acid containing 0.2% bovine serum albumin (BSA) to reduce peptide binding to plastic surfaces (Wiegand et al, 2008), it is difficult to ascertain whether this recommendation has been broadly adopted
Summary
Antimicrobial resistance (AMR) is a global health crisis. The over-use and inappropriate prescribing of antibiotics has set us on a path toward a post-antibiotic era where our existing armory of antibiotics will no longer be effective. Increasing numbers of microorganisms are already becoming widely resistant to existing antibiotic classes (O’Neill, 2016; Barlow, 2018; Chatterjee et al, 2018; Van Puyvelde et al, 2018; Roope et al, 2019). The need for new antimicrobial agents is more important than it has ever been. Too is the need for better antimicrobial stewardship; prudent and appropriate use of antimicrobials
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