Activities of the frog skin peptide, ascaphin-8 and its lysine-substituted analogs against clinical isolates of extended-spectrum β-lactamase (ESBL) producing bacteria

Extended-spectrum β-lactamase (ESBL)-producing Gram-negative bacteria are becoming increasingly prevalent and their antibiotic resistance necessitates novel therapeutic intervention. Ascaphin-8 is a cationic α-helical peptide that shows broad-spectrum antibacterial activity but is also toxic to human erythrocytes (LC₅₀ = 55 μM). This study assesses the activity of ascaphin-8, and a series of l-lysine-substituted analogs, against a range of clinical isolates of ESBL-producing bacteria. All ESBL-producing Escherichia coli (MIC = 1.5-6 μM) and Klebsiella pneumoniae (MIC = 12.5-25 μM) strains tested were susceptible to ascaphin-8, as well as a group of miscellaneous ESBL-producing strains (Citrobacter, Salmonella, Serratia, Shigella spp.) (MIC ≤ 25 μM). The Lys⁴- and Lys⁸-substituted analogs were generally the most potent against bacteria but showed the highest hemolytic activity. However, the Lys¹⁰, Lys¹⁴, and Lys¹⁸ analogs also displayed potent antibacterial activity while showing very low hemolytic activity (LC₅₀ > 500 μM). An unexpected finding was the susceptibility of ESBL-producing Proteus mirabilis strains to ascaphin-8 (MIC = 12.5-25 μM) and its Lys⁴-substituted analog (MIC = 6 μM), although non-ESBL isolates of this organism were resistant to these peptides (MIC > 100 μM).