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Pseudomonas aeruginosa is a major opportunistic human pathogen which employs a myriad of virulence factors. In people with cystic fibrosis (CF) P. aeruginosa frequently colonises the lungs and becomes a chronic infection that evolves to become less virulent over time, but often adapts to favour persistence in the host with alginate-producing mucoid, slow-growing, and antibiotic resistant phenotypes emerging. Cysteamine is an endogenous aminothiol which has been shown to prevent biofilm formation, reduce phenazine production, and potentiate antibiotic activity against P. aeruginosa , and has been investigated in clinical trials as an adjunct therapy for pulmonary exacerbations of CF. Here we demonstrate (for the first time in a prokaryote) that cysteamine prevents glycine utilisation by P. aeruginosa in common with previously reported activity blocking the glycine cleavage system in human cells. Despite the clear inhibition of glycine metabolism, cysteamine also inhibits hydrogen cyanide (HCN) production by P. aeruginosa , suggesting a direct interference in the regulation of virulence factor synthesis. Cysteamine impaired chemotaxis, lowered pyocyanin, pyoverdine and exopolysaccharide production, and reduced the toxicity of P. aeruginosa secreted factors in a Galleria mellonella infection model. Thus, cysteamine has additional potent anti-virulence properties targeting P. aeruginosa , further supporting its therapeutic potential in CF and other infections., Competing Interests: DF-P, DT-A, JH, DS, and DM are all employees of NovaBiotics Ltd. DAO is the CEO, CSO and a shareholder of NovaBiotics Ltd. DF-P and DO are authors of patent; WO2016198842A1, IL256162D0 - An amino thiol for use in the treatment of an infection caused by the bacterium mycobacterium spp. DO is also author of the following patents; US9364491B2 (and other territories) - Antimicrobial compositions with cysteamine/A composition comprising an antibiotic and a dispersant; WO2016046524A1 - Use of cysteamine in treating infections caused by yeasts/moulds; US9782423B2 (and other territories) - Antibiotic compositions comprising an antibiotic agent and cysteamine; US9339525B2 (and other territories) - Inhibition of biofilm organisms; and US20190175501A1 pending (and other territories) - Microparticles comprising a sulphur-containing compound. This does not alter our adherence to Frontiers policies on sharing data and materials. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Fraser-Pitt, Dolan, Toledo-Aparicio, Hunt, Smith, Lacy-Roberts, Nupe Hewage, Stoyanova, Manson, McClean, Inglis, Mercer and O’Neil.)

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., (Copyright © 2020 Mercer, Torres, Duay, Lovie, Simpson, von Köckritz-Blickwede, de la Fuente-Nunez, O'Neil and Angeles-Boza.)