BackgroundStatin treatment has been associated with a beneficial outcome on respiratory tract infections. In addition, previous in vitro and in vivo experiments have indicated favorable effects of statins in bacterial infections.AimThe aim of the present study was to elucidate possible antibacterial effects of statins against primary pathogens of the respiratory tract.MethodsMIC-values for simvastatin, fluvastatin and pravastatin against S. pneumoniae, M. catarrhalis and H. influenzae were determined by traditional antibacterial assays. A BioScreen instrument was used to monitor effects of statins on bacterial growth and to assess possible synergistic effects with penicillin. Bacterial growth in whole blood and serum from healthy volunteers before and after a single dose of simvastatin, fluvastatin and penicillin (positive control) was determined using a blood culture system (BactAlert).FindingsThe MIC-value for simvastatin against S pneumoniae and M catarrhalis was 15 µg/mL (36 mmol/L). Fluvastatin and Pravastatin showed no antibacterial effect in concentrations up to 100 µg/mL (230 µmol/L). Statins did not affect growth or viability of H influenzae. Single doses of statins given to healthy volunteers did not affect growth of pneumococci, whereas penicillin efficiently killed all bacteria.ConclusionsSimvastatin at high concentrations 15 µg/mL (36 µmol/L) rapidly kills S pneumoniae and M catarrhalis. However, these concentrations by far exceed the concentrations detected in human blood during simvastatin therapy (1–15 nmol/L) and single doses of statins given to healthy volunteers did not improve antibacterial effects of whole blood. Thus, a direct bactericidal effect of statins in vivo is probably not the mechanism behind the observed beneficial effect of statins against various infections.

NK-lysin and granulysin are homologous cationic anti-bacterial peptides produced by pig and human cytolytic lymphocytes, respectively. The solution structure of NK-lysin comprises five amphipathic alpha-helices. To investigate the properties of a helix-loop-helix region postulated to be a membrane-docking part of NK-lysin, we synthesized 22- and 29-residue peptides reproducing this region for both NK-lysin and granulysin. CD spectroscopy of the synthetic peptides in a liposomal solution showed spectra typical of alpha-helical peptides. The peptides were active against Gram-positive and Gram-negative bacteria, with the two NK-lysin peptides showing higher anti-bacterial activities than the two from granulysin. One NK-lysin peptide was active against Pseudomonas aeruginosa and Staphylococcus aureus, two organisms against which NK-lysin is inactive. Granulysin peptides were inactive against these bacteria, in contrast with granulysin, which is known to be active against them. Both NK-lysin and all synthetic analogues killed Mycobacterium tuberculosis and K562 tumour cells, but did not display haemolytic activity. These results identify a potent anti-mycobacterial domain in NK-lysin and granulysin consisting of a 22-residue (helix 3) sequence plus a disulphide-constrained loop.

We have investigated the in vitro activity of antimicrobial peptides against Mycobacterium tuberculosis using a radiometric method and cfu determinations. PR-39, a proline-arginine-rich antibacterial peptide from porcine leucocytes, was found to be active against drug-susceptible as well as multi-drug-resistant (MDR) clinical isolates of M. tuberculosis. The activity of PR-39 was concentration dependent, with 80% growth inhibition of M. tuberculosis H37Rv at 50 mg/L. The MDR M. tuberculosis strains E1380/94 and P34/95 were less susceptible to PR-39, with 39 and 49% growth inhibition at 50 mg/L peptide, respectively, suggesting a lower susceptibility than strain H37Rv and drug-susceptible clinical isolates. Reduction of counts of M. tuberculosis H37Rv and the MDR M. tuberculosis strain E1380/94 by PR-39 indicated that the growth inhibition seen in the radiometric assay is due to a mycobactericidal effect of the peptide. These observations suggest that antimicrobial peptides may play an important role in host defence against MDR M. tuberculosis.

Granulysin and NK-lysin are homologous bactericidal proteins with a moderate residue identity (35%), both of which have antimycobacterial activity. Short loop peptides derived from the antimycobacterial domains of granulysin, NK-lysin, and a putative chicken NK-lysin were examined and shown to have comparable antimycobacterial but variable Escherichia coli activities. The known structure of the NK-lysin loop peptide was used to predict the structure of the equivalent peptides of granulysin and chicken NK-lysin by homology modeling. The last two adopted a secondary structure almost identical to that of NK-lysin. All three peptides form very similar three-dimensional (3-D) architectures in which the important basic residues assume the same positions in space. The basic residues in granulysin are arginine, while those in NK-lysin and chicken NK-lysin are a mixture of arginine and lysine. We altered the ratio of arginine to lysine in the granulysin fragment to examine the importance of basic residues for antimycobacterial activity. The alteration of the amino acids reduced the activity against E. coli to a larger extent than that against Mycobacterium smegmatis. In granulysin, the arginines in the loop structure are not crucial for antimycobacterial activity but are important for cytotoxicity. We suggest that the antibacterial domains of the related proteins granulysin, NK-lysin, and chicken NK-lysin have conserved their 3-D structure and their function against mycobacteria.

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