Technetium-99m labelled antimicrobial peptides discriminate between bacterial infections and sterile inflammations

Human lactoferrin and peptides derived from its N terminus are highly effective against infections with antibiotic-resistant bacteria

Since human lactoferrin (hLF) binds to bacterial products through its highly positively charged N terminus, we investigated which of the two cationic domains is involved in its bactericidal activity. The results revealed that hLF lacking the first three residues (hLF(-3N)) was less efficient than hLF in killing of antibiotic-resistant Staphylococcus aureus, Listeria monocytogenes, and Klebsiella pneumoniae. Both hLF preparations failed to kill Escherichia coli O54. In addition, hLF(-3N) was less effective than hLF in reducing the number of viable bacteria in mice infected with antibiotic-resistant S. aureus and K. pneumoniae. Studies with synthetic peptides corresponding to the first 11 N-terminal amino acids, designated hLF(1-11), and fragments thereof demonstrated that peptides lacking the first three N-terminal residues are less effective than hLF(1-11) in killing of bacteria. Furthermore, a peptide corresponding to residues 21 to 31, which comprises the second cationic domain, was less effective than hLF(1-11) in killing of bacteria in vitro and in mice having an infection with antibiotic-resistant S. aureus or K. pneumoniae. Using fluorescent probes, we found that bactericidal hLF peptides, but not nonbactericidal peptides, caused an increase of the membrane permeability. In addition, hLF killed the various bacteria, most probably by inducing intracellular changes in these bacteria without affecting the membrane permeability. Together, hLF and peptides derived from its N terminus are highly effective against infections with antibiotic-resistant S. aureus and K. pneumoniae, and the first two arginines play an essential role in this activity.

Candidacidal activities of human lactoferrin peptides derived from the N terminus

In light of the need for new antifungal agents, the candidacidal activities of human lactoferrin (hLF) and synthetic peptides representing the first, hLF(1-11), and second, hLF(21-31), cationic domains of its N terminus were compared. The results revealed that hLF(1-11) was more effective in killing fluconazole-resistant Candida albicans than hLF(21-31) and much more effective than lactoferrin, as determined microbiologically and by propidium iodide (PI) staining. By using hLF(1-11) and various derivatives, it was found that the second and third residues of the N terminus of hLF(1-11) were critical for its candidacidal activity. Detailed investigation to elucidate the mechanism of action of hLF(1-11) revealed a dose-dependent release of ATP by Candida upon exposure to hLF(1-11). Our observations that sodium azide reduced the PI uptake and candidacidal activity of hLF(1-11) and that, upon exposure to hLF(1-11), the fluorescent dye rhodamine 123 first accumulated inside the mitochondria and later was released into the cytoplasm indicate that the peptide triggers the energized mitochondrion. Furthermore, oxidized ATP, which interferes with the interaction of ATP with its extracellular receptors, blocked the candidacidal action of hLF(1-11), as measured microbiologically and by PI staining. Addition of ATP (or analogues) was not a sufficient stimulus to kill C. albicans or to act synergistically with suboptimal concentrations of the peptide. The main conclusions are that the first two arginines at the N terminus of hLF are critical in the candidacidal activity of hLF(1-11) and that extracellular ATP is essential but not sufficient for the peptide to exert its candidacidal activity.