Inhibition of cathelicidin activity by bacterial exopolysaccharides

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The interaction of bacterial exopolysaccharides, produced by opportunistic lung pathogens, with antimicrobial peptides of the innate primate immune system was investigated. The exopolysaccharides were produced byPseudomonas aeruginosa, Inquilinus limosusand clinical isolates of theBurkholderia cepaciacomplex, bacteria that are all involved in lung infections of cystic fibrosis patients. The effects of the biological activities of three orthologous cathelicidins fromHomo sapiens sapiens, Pongo pygmaeus(orangutan) andPresbitys obscurus(dusky leaf monkey) were examined. Inhibition of the antimicrobial activity of peptides was assessed using minimum inhibitory concentration assays on a referenceEscherichia colistrain in the presence and absence of exopolysaccharides, whereas complex formation between peptides and exopolysaccharides was investigated by means of circular dichroism, fluorescence spectroscopy and atomic force microscopy. Biological assays revealed that the higher the negative charge of exopolysaccharides the stronger was their inhibiting effect. Spectroscopic studies indicated the formation of molecular complexes of varying stability between peptides and exopolysaccharides, explaining the inhibition. Atomic force microscopy provided a direct visualization of the molecular complexes. A model is proposed where peptides with an α-helical conformation interact with exopolysaccharides through electrostatic and other non-covalent interactions.

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