Novel synthetic anitmicrobial peptides against streptococcus pneumoniae

Abstract

Background: According to WHO, 1.6 million deaths are caused by pneumococcal infections every year with 0.7 to 1 million in children younger than 5 years mostly in Asia and Africa. Like other gram positive bacteria, Streptococcus pneumoniae is increasingly difficult to treat due to the irrational use of antibiotics. Antimicrobial peptides (AMPs) represent a possible alternative for current antibiotics against drug resistant pathogens. Methods & Materials: In this study, thirteen antimicrobial peptides were designed based on two natural peptides indolicidin and ranalexin. the in vitro activity of these peptides was investigated using broth microdilution assay, hemolytic activity assay, time killing assay, and toxicity assay against two cell lines WRL-68 and NL-20. Mechanisms of action of peptides were assessed using transmission electron microscopy (TEM), scanning electron microscopy (SEM), DNA binding assay, and in silico molecular docking against three virulent fators. Results: Our results revealed that four hybrid peptides RN7-IN10, RN7-IN9, RN7-IN8, and RN7-IN6 possess potent antibacterial activity against 30 pneumococcal clinical isolates (MIC 7.81-15.62g/ml). These four hybrid peptides showed broad spectrum antibacterial activity (7.81g/ml) against S. aureus, methicillin resistant S. aureus (MRSA), and E. coli. Furthermore, the time killing assay results indicated that the hybrid peptides were able to eliminate S. pneumoniae within less than one hour which is faster than the standard drugs erythromycin and ceftriaxone. The cytotoxicity was tested against human erythrocytes, WRL-68 normal liver cell line, and NL-20 normal lung cell line. The results revealed that none of the thirteen peptides have cytotoxic or hemolytic activities at their MICs. TEM and SEM results showed that these four peptides are killing the bacteria by destroying the integrity of thier membranes. DNA binding assay evealed that the hybrid peptides were able to bind to DNA at 62.5 g/ml preventing it from migration through the agarosre gel. The effect of hybrid peptides on the integrity of bacterial membranes