The designer proline-rich antibacterial peptide A3-APO prevents Bacillus anthracis mortality by deactivating bacterial toxins.

Proline-rich antibacterial peptides protect experimental animals from bacterial challenge even if they are unable to kill the microorganisms in vitro. Their major in vivo modes of action are inhibition of bacterial protein folding and immunostimulation. Here we investigated whether the proline-rich antibacterial peptide dimer A3-APO was able to inhibit Bacillus cereus enterotoxin production in vitro and restrict the proliferation of lethal toxin-induced Bacillus anthracis replication in mouse macrophages. After 24 h incubation, peptide A3-APO and its single chain metabolite reduced the amount of properly folded B. cereus diarrhoeal enterotoxin production in a concentration-dependent manner leading to only 10-25% of the original amount of toxin detectable by a conformation-sensitive immunoassay. Likewise, after 4 h incubation, A3-APO restricted the proliferation of B. anthracis in infected macrophages by 40-45% compared to untreated cells both intracellularly and in the extracellular cell culture milieu. Although the peptide had a minimal inhibitory concentration of >512 mg/L against B. anthracis in vitro, in systemic mouse challenge models it improved survival by 20- 37%, exhibiting statistically significant cumulative efficacy when administered at 3x5 mg/kg intraperitoneally or intramuscularly. We hypothesize that the activity in isolated murine macrophages and in vivo is due to deactivation of bacterial toxins. Bacterial protein folding inhibition in synergy with other types of antimicrobial modes offers a remarkable novel strategy in combating resistant or life-threatening infections.