Expression, purification, and in vitro comparative characterization of avian beta-defensin-2, -6, and -12

The mature peptides of avian β-defensin-2 (AvBD-2), AvBD-6, and AvBD-12 were expressed as 6xHis-tagged recombinant proteins using the Escherichia coli BL21(DE3)pLysS system. The yields of rAvBD-2, rAvBD-6, and rAvBD-12 were approximately 0.92 mg/L, 1.24 mg/L, and 1.52 mg/L, respectively, of bacterial culture. The antimicrobial activities of rAvBDs were characterized under different salt, nutrient, and pH conditions. At concentrations of 8 μg/ml, 16 μg/ml, and 32 μg/ml, rAvBDs inhibited the growth of Staphylococcus aureus, E. coli, and Salmonella enterica serovar Typhimurium. While no synergistic inhibitory activity was found, a significant antagonistic effect was detected between rAvBD-2 and rAvBD-12. Treatment of E. coli and Salmonella Typhimurium with rAvBDs diminished their natural resistance to bile salts. Under the nonreplicating low-nutrient condition, rAvBDs at a concentration of 16 μg/ml were able to kill E. coli and S. aureus within 30 min of contact. The antimicrobial activities of rAvBDs were enhanced by lowering salt concentration and pH from 7 to 6. The antimicrobial potency against S. aureus and E. coli could be characterized as rAvBD-6 > rAvBD-2 > rAvBD-12, which coincided with the net positive charges of these peptides. In conclusion, data from the current study warrant the investigation of the potential use of rAvBD-2, -6, and -12 as therapeutic and prophylactic antimicrobial agents against common bacterial pathogens.