Effects of Cations and PH on Antimicrobial Activity of Thanatin and s-Thanatin Against Escherichia coli ATCC25922 and B. subtilis ATCC 21332

Abstract

This study analyzes the in vitro effects of cations and pH on antimicrobial activity of thanatin and s-thanatin against Escherichia coli ATCC25922 and B. subtilis ATCC21332. Thanatin and s-thanatin were synthesized by the solid-phase method using a model 432A synthesizer. The bacterial strains tested included two antibiotic-susceptible strains of Escherichia coli ATCC25922 and B. subtilis ATCC21332. Susceptibility determinations were carried out either in a variety of cation concentrations or in pH conditions from pH 5 to pH 8. NaCl or KCl was added to the media to final concentrations of 0, 10, 50, 100, 200, and 500 mM, whereas CaCl₂ and MgCl₂ were added to the media to final concentrations of 0, 1, 2, 5, 10, and 20 mM. The antimicrobial activity of thanatin and s-thanatin against Escherichia coli ATCC25922 and B. subtilis ATCC21332 decreased, as indicated by the increasing minimal inhibitory concentrations (MICs) of both peptides with increasing concentrations of Na⁺/K⁺/Ca²⁺/Mg²⁺. Both peptides lost their activities at 500 mM Na⁺/K⁺ but retained them at 20 mM Ca²⁺/Mg²⁺. Both peptides have MICs that are not significantly different at a variety of pH levels, with the antimicrobial activity slightly higher in neutral or slightly basic media than under acidic conditions. The antimicrobial peptides thanatin and s-thanatin, which have an anti-parallel β-sheet constrained by disulfide bonds, were salt sensitive against both Gram-positive and Gram-negative pathogens in vitro. Determining the reason why the thanatins are salt sensitive would be useful to provide an understanding of how thanatin and s-thanatin kill bacteria. Futher investigation of the antimicrobial properties of these peptides is warranted.