Abstract: The mechanisms of action of the antimicrobial peptides SP-AP-1 and iturin A derived from the fermentation of Spirulina residue by Bacillus subtili were studied by investigating their effects on the cell membrane permeability of Staphylococcus aureus using ultraviolet spectrophotometry, the O-nitrophenyl-β-D-galactopyranoside method and atomic absorption spectrometry, as well as on intracellular protein synthesis and genomic DNA binding using Coomassie brilliant blue, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and agarose gel electrophoresis. Moreover, changes in the secondary structures of the antimicrobial peptides after they exerted their antibacterial effect were investigated by simulating the hydrophobic environment of the cell membrane. The results showed that both SP-AP-1 and iturin A increased the cell membrane permeability of Staphylococcus aureus, thereby resulting in increased intracellular potassium and protein leakage and cell membrane hydrophobicity, with the former being significantly more effective than the latter (P < 0.05). In addition, both antimicrobial peptides could inhibit the synthesis of bacterial proteins, especially those with molecular masses of 17, 20 and 25–35 kDa. Although the two antimicrobial peptides had no significant blocking effect on Staphylococcus aureus genomic DNA, SP-AP-1 darkened the color of genomic DNA bands and competitively bound to DNA with ethidium bromide at increased concentrations, thereby affecting protein expression. This study lays a theoretical foundation for studies on the mechanisms of action of antimicrobial peptides derived from Spirulina residue against Staphylococcus aureus.

Key words: antimicrobial peptide SP-AP-1; iturin A; Staphylococcus aureus; antibacterial mechanism