An innate immune defense of frogs against microbial pathogens is secretion of antimicrobial peptides from skin granular glands. This study aimed to characterize the activities of skin antimicrobial peptides against seven different opportunistic bacterial pathogens associated with bacterial dermatosepticemia, a fatal infectious disease of frogs, including Aeromonas hydrophila, Chryseobacterium meningosepticum, Citrobacter freundii, Klebsiella pneumoniae, Pseudomonas aeruginosa, Proteus mirabilis, and Serratia liquefaciens and the bacterial saprophyte Lactococcus lactis. Natural peptide mixtures from African Clawed Frogs (Xenopus laevis), Green and Golden Bell Frogs (Litoria aurea), Southern Bell Frogs (Litoria raniformis), and Brown Tree Frogs (Litoria ewingii) were assayed for bacterial growth inhibition. Peptide mixtures from X. laevis, L. aurea, and L. raniformis inhibited the growth of C. freundii, K. pneumoniae, P. aerugionsa, and L. lactis with the peptide mixture of X. laevis having the strongest inhibitory effects on these pathogens and also inhibiting the growth of C. meningosepticum. Similarly, X. laevis produced more active peptides per unit surface area than did L. aurea or L. raniformis. The peptide mixture of L. ewingii was not active against any pathogen. Three pathogens abundant in skin microbiota including A. hydrophila, P. mirabilis, and S. liquefaciens were resistant to peptide mixtures of all tested species. Therefore, although the activities of skin peptides could control several different opportunistic pathogens including C. freundii, K. pneumoniae, P. aerugionsa, and C. meningosepticum and the saprophyte L. lactis, it is suggested that some opportunistic pathogens such as A. hydrophila, P. mirabilis, and S. liquefaciens may have coevolved to resist skin peptides.
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