Effects of cations and pH on antimicrobial activity of thanatin and s-thanatin against Escherichia coli ATCC25922 and B. subtilis ATCC 21332.

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 CaCl(2) and MgCl(2) 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(+)/Ca(2+)/Mg(2+). Both peptides lost their activities at 500 mM Na(+)/K(+) but retained them at 20 mM Ca(2+)/Mg(2+). 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 beta-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. Further investigation of the antimicrobial properties of these peptides is warranted.