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A 30-residue N-terminally acetylated peptide derived from the N-terminal part of histone H1 was identified as the dominant antimicrobial peptide in skin mucus from Atlantic salmon (Salmo salar). The peptide (termed salmon antimicrobial peptide [SAMP H1]) was purified to homogeneity by a combination of reversed-phase and cation-exchange chromatographies. By Edman degradation of the deacetylated peptide and by sequencing of the PCR-amplified DNA that encodes the peptide, the complete amino acid sequence was determined to be AEVAPAPAAAAPAKAPKKKAAAKPKKAGPS. The theoretical molecular weight of N-terminally acetylated SAMP H1 was calculated to be 2,836, which is the same as that determined by matrix-assisted laser desorption ionization mass spectrometry. The peptide was active against both gram-negative and -positive bacteria. The N-terminal acetyl group was not necessary for activity since deacetylation did not reduce the activity. A synthetic peptide whose sequence was identical to that of the isolated fragment was initially inactive but could be activated by binding it to a cation-exchange column. Treatment of the synthetic peptide when it was bound to the exchange column with peptidylproline cis-trans-isomerase increased the amount of active peptide, indicating that isomerization of the proline peptide bond(s) was necessary for activation of the synthetic peptide. Comparison of the active and inactive forms by circular dichroism and chromatographic analyses suggests that the active form, both the natural and the synthetic forms, is more structured, condensed, and rigid than the inactive form, which has a more nonstructured conformation. This work shows for the first time the importance of proline isomers in the activity of an antimicrobial peptide.

The antimicrobial effect obtained upon combining the prokaryotic antimicrobial peptides (AMPs; more commonly referred to as bacteriocins) pediocin PA-1, sakacin P, and curvacin A (all produced by lactic acid bacteria [LAB]) with the eukaryotic AMP pleurocidin (from fish) has been investigated. The three LAB AMPs alone were active against gram-positive Listeria ivanovii bacteria at nanomolar concentrations, whereas they were inactive against gram-negative Escherichia coli bacteria. Pleurocidin alone was active against both of these types of bacteria at micromolar concentrations. Little if any synergy between the LAB AMPs and pleurocidin against the gram-positive L. ivanovii strain was obtained. In contrast, the LAB AMPs and pleurocidin acted highly synergistically against the gram-negative E. coli strain. Nanomolar concentrations of LAB AMPs increased the growth inhibitory potency of pleurocidin by about fourfold. When micromolar concentrations of LAB AMPs were combined with 2 g of pleurocidin/ml, 100% growth inhibition was attained, whereas pleurocidin alone at a concentration of 2 g/ml gave no growth inhibition. Most noteworthy, when high concentrations (128 g/ml) of pleurocidin in the absence of LAB AMPs were used over a long period of incubation (1 week), some growth of E. coli was observed, whereas 16 g of pleurocidin/ml completely abolished growth in the presence of 64 to 128 ng of LAB AMPs/ml over the same period of time. The results clearly demonstrate that combining eukaryotic and prokaryotic AMPs can greatly increase the specific activity and broaden the target-cell range of these peptides.

Aims:  To isolate and characterize peptides from bovine colostrum with antimicrobial activity. Methods and Results:  Three peptides were purified from fresh colostrum by a range of chromatographic methods using antimicrobial activity against Escherichia coli DH5α to screen for the most active fractions. Two peptides, with antimicrobial activity, casecidin 17 and casecidin 15, were identical to sequences in the C‐terminal of bovine β‐casein (YQEPVLGPVRGPFPIIV and YQEPVLGPVRGPFPI) and had corresponding molecular masses of 1881·00 and 1669·06 Da, respectively. The third peptide was the known peptide isracidin which has a mass of 2763·80 Da and sequence of RPKHPIKHQGLPQEVLNENLLRF. Casecidin 17 and casecidin 15 had identical minimal inhibition concentrations (MICs) against E. coli DPC6053 of 0·4 mg ml−1. Structural modelling suggested amphiphilic structures having identical inhibitory and structural properties. The MIC value of isracidin against E. coli DPC6053 was 0·2 mg ml−1. Conclusions:  This study shows the presence of three antimicrobial peptides in colostrum which may contribute to a bioprotective role to limit pathogen contamination. Furthermore, the discovery of casecidin 17 and 15 may provide the basis for novel antimicrobial peptide design. Significance and Impact of the Study:  This is the first study to characterize peptides with antimicrobial activity present in fresh bovine colostrum.

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