A bactericidal permeability-increasing protein (BPI) from manila clam Ruditapes philippinarum: Investigation on the antibacterial activities and antibacterial action mode.

Bactericidal permeability-increasing protein (BPI) is an antimicrobial protein with potent endotoxin-neutralising activity and plays a crucial role in innate immunity against bacterial infection. In the present study, a bpi (designed as rpbpi) was identified and characterized from manila clam Ruditapes philippinarum. Multiple alignments and phylogenetic analysis suggested that rpbpi was a new member of the bpis family. In non-stimulated clams, rpbpi transcripts were ubiquitously expressed in all tested tissues with the highest expression level in hemocytes. After Vibrio anguillarum challenge, the expression levels of rpbpi mRNA in hemocytes were up-regulated significantly at 3 h and 48 h compared with that in the control, which were 4.01- and 19.10-fold (P < 0.05), respectively. The recombinant RpBPI (rRpBPI) showed high antibacterial activities against Gram-negative bacteria Escherichia coli and V. anguillarum, but not Staphylococcus aureus. Moreover, membrane integrity analysis revealed that rRpBPI increased the membrane permeability of Gram-negative bacteria, and then resulted in cell death. Overall, our results suggested that RpBPI played an important role in the elimination of invaded bacteria through membrane-disruptive activity.