Antimicrobial effect and mechanism of bovine lactoferrin against the potato common scab pathogen Streptomyces scabiei

Abstract

Lactoferrin (LF) is a multifunctional protein with a broad spectrum of antimicrobial activities. In this study, we investigated the antimicrobial activity of LF against the potato common scab pathogenStreptomyces scabiei, which causes severe damage to potato tubers. LF derived from bovine (bLF) had much higher activity againstS.scabieithan human LF. The minimal inhibitory concentration of bLF was 3.9 μM. The effects of both apo-bLF (iron-free) and holo-bLF (iron-saturated) onS.scabieiwere not different. Bovine lactoferricin (LFcinB), a short peptide with a length of 25 amino acid residues located in the N-terminal region of bLF, showed antimicrobial activity againstS.scabiei, similar to that of bLF. These results indicated that the antimicrobial activity of bLF againstS.scabieicannot be attributed to its iron-chelating effect but to the bioactivity of its peptides. WhenS.scabieiwas treated with the fusion protein of mCherry-LFcinB (red fluorescent protein) expressed inEscherichia coli, the pseudohyphal cells instantly glowed, indicating that the peptide electrostatically binds to the surface ofS.scabiei. An assay of synthetic peptides, with modified number of arginine (Arg) and tryptophan (Trp) residues based on the antimicrobial center (RRWQWR) of LFcinB showed that Trp residues are implicated in the antimicrobial activity againstS.scabiei; however, Arg residues are also necessary to carry Trp residues to the cell surface to fully exert its activity. Although the single amino acid effect of Trp had low activity, Trp derivatives showed much higher activity againstS.scabiei, suggesting that the derivatives effectively bind to the cell surface (cell membrane) by themselves without a carrier. Thus, amino acid derivatives might be considered effective and alternative antimicrobial substances.