MEMBRANE PERMEABILIZATION AND ANTIMYCOPLASMIC ACTIVITY OF THE 18-RESIDUE PEPTAIBOLS, TRICHORZINS PA

The membrane permeabilisation properties of six linear natural 18-resi due peptaibols, termed trichorzins PA, have been assessed on liposomes and on mollicutes (trivial name, mycoplasmas), a class of parasitic b acteria characterized by a small genome, the lack of a cell wall, a mi nute cell size, and the incorporation in their plasma membrane of exog enously supplied cholesterol. The trichorzins PA used in this study (P A II, PA TV-VI, PA VIII, and PA IX) differ between them by amino acid or amino alcohol substitutions at positions 4, 7, and 18, and form sli ghtly amphipathic cr-helices. They proved bactericidal for mollicutes belonging to the genera Acholeplasma, Mycoplasma, and Spiroplasma, wit h minimal inhibitory concentrations (3.12 less than or equal to MICs l ess than or equal to 50 mu M) generally 2 to 4 fold higher than those of alamethicin F50, a related 20-residue peptide (1.56 less than or eq ual to MTCs less than or equal to 12.5 mu M). Spiroplasma cells were a pparently not protected by the presence of spiralin on their surface. The activities of the six trichorzins PA were not influenced by their sequence variations and no synergistic effect was observed. Consistent with the marginal effect of cholesterol on the incorporation of the t richorzins PA into liposome bilayers, the antibiotic activity was inde pendent of the amount of cholesterol in the membranes of the different mollicutes. The trichorzins PA and alamethicin inhibited the motility of Spiroplasma melliferum, the helical cells being deformed and split into coccoid forms. Membrane potential measurements in Acholeplasma l aidlawii and S. melliferum showed that trichorzin PA V and alamethicin F50 very efficiently depolarized the plasma membrane of mollicutes. T his was consistent with fluorescence and Na-23 NMR measurements on lip osomes that revealed the permeabilisation of the lipid bilayer and the nonselective ionophoric activity of the trichorzins PA. These data su ggest that the bactericidal activity exhibited by the trichorzins PA o n mollicutes is due to the permeabilisation of the plasma membrane. (C ) 1998 Elsevier Science B.V. All rights reserved.