Bactericidal activity of mammalian cathelicidin-derived peptides.

Endogenous antimicrobial peptides of the cathelicidin family contribute to innate immunity. The emergence of widespread antibiotic resistance in many commonly encountered bacteria requires the search for new bactericidal agents with therapeutic potential. Solid-phase synthesis was employed to prepare linear antimicrobial peptides found in cathelicidins of five mammals: human (FALL39/LL37), rabbit (CAP18), mouse (mCRAMP), rat (rCRAMP), and sheep (SMAP29 and SMAP34). These peptides were tested at ionic strengths of 25 and 175 mM against Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and methicillin-resistant Staphylococcus aureus. Each peptide manifested activity against P. aeruginosa irrespective of the NaCl concentration. CAP18 and SMAP29 were the most effective peptides of the group against all test organisms under both low- and high-salt conditions. Select peptides of 15 to 21 residues, modeled on CAP18 (37 residues), retained activity against the gram-negative bacteria and methicillin-sensitive S. aureus, although the bactericidal activity was reduced compared to that of the parent peptide. In accordance with the behavior of the parent molecule, the truncated peptides adopted an alpha-helical structure in the presence of trifluoroethanol or lipopolysaccharide. The relationship between the bactericidal activity and several physiochemical properties of the cathelicidins was examined. The activities of the full-length peptides correlated positively with a predicted gradient of hydrophobicity along the peptide backbone and with net positive charge; they correlated inversely with relative abundance of anionic residues. The salt-resistant, antimicrobial properties of CAP18 and SMAP29 suggest that these peptides or congeneric structures have potential for the treatment of bacterial infections in normal and immunocompromised persons and individuals with cystic fibrosis.

Alteration of the pilin adhesin of Pseudomonas aeruginosa PAO results in normal pilus biogenesis but a loss of adherence to human pneumocyte cells and decreased virulence in mice.

The disulfide loop domain of Pseudomonas aeruginosa PAO pilin was altered by insertion of a chloramphenicol acetyltransferase gene into the pilin gene so that the C-terminal nine amino acids were replaced with 11 new amino acids. The altered pilin gene was transferred into wild-type PAO by recombination, where it did not affect normal piliation as observed by transmission electron microscopy or change of sensitivity to f116, PO4, B9, and Pf1 pilus-specific bacteriophages. However, the binding to human pneumocyte A549 cells was markedly reduced when tested in an in vitro binding assay (2 to 6 bacteria bound per A549 cell for the mutant bacteria compared with 50 bacteria per A549 cell for the wild-type bacteria). Additionally, when susceptible A.BY/SnJ mice were challenged with wild-type P. aeruginosa PAO and with P. aeruginosa PAO-MP (altered pilin gene), a 50% lethal dose of 3 x 10(6) bacteria per mouse was observed for PAO-MP compared with 7 x 10(4) bacteria per mouse for PAO. Approximately 90 of the adherence capability of P. aeruginosa PAO is seemingly attributable to the C-terminal disulfide loop adherence domain of pili. The pilus adherence function contributes significantly to the virulence of P. aeruginosa PAO in the A.BY/SnJ mouse infection model.