A novel cathelicidin from Bufo bufo gargarizans Cantor showed specific activity to its habitat bacteria

Toad Bufo bufo gargarizans Cantor is still used in China as traditional Chinese medicine. However, present investigations on its skin secretions were mainly focused on the bufadienolides, the proteins/peptides contained in the secretions are largely unknown. A cDNA encoding a novel cathelicidin termed BG-CATH was identified by analysis of the toad skin transcriptome. The BG-CATH precursor was predicted to have 2 possible cleavage sites following dibasic cleavage signals at its C-terminal, which will generate two mature peptides, BG-CATH37 and BG-CATH(5-37). Phylogenetic analysis suggests that amphibian cathelicidins might evolve from common ancestors. The two predicted mature cathelicidins from B. bufo gargarizans were synthesized and both of them showed weak antimicrobial activities against human pathogenic bacteria Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus (MIC ≥ 200 μg/mL). However, BG-CATH37 and BG-CATH(5-37) had strong antimicrobial activities against aquatic bacteria of Vibrio splendidus, Streptococcus iniae and Aeromorus hydrophila, which were common microorganisms in the habitat of B. bufo gargarizans (MIC 3.125-40 μg/mL). BG-CATH37 and BG-CATH(5-37) showed no hemolytic activity even at high concentrations (400 μg/mL). CD spectra analysis suggested that structure rigidity of BG-CATH37 and BG-CATH(5-37) might play an important role to regulate their biological activities. Selective antimicrobial activity against habitat microorganisms might reflect the adaptation of amphibians to their living environments.