Screening of a Cell-Penetrating Peptide Library in Escherichia coli: Relationship between Cell Penetration Efficiency and Cytotoxicity

Cell-penetrating peptides (CPPs) have been used for the intracellular delivery of bio-active cargo, such as drugs, genes, and proteins. Similarly, CPPs have been successfully used against bacteria, viruses, and yeasts over the past few decades. To broaden the possible applications of CPPs, controlling the balance between their cell-penetrating efficiency and their cytotoxicity to target cells is essential. Here, we aim to identify a CPP for bacteria that has a high penetration efficiency and low cytotoxicity. We assayed a library of 55 CPPs for Escherichia coli (E. coli) DH5 alpha using a combination of fluorescence spectroscopy (FS) and confocal laser scanning microscopy (CLSM). We found that several cationic CPPs, such as R9-TAT, (KH)9, and Rev (34-50), and amphipathic CPPs, such as ppTG1 and pAntpHD (Pro50), have the highest penetration efficiency in E. coli DH5 alpha and low cytotoxicity. The analysis of the cellular penetration of E. coli DH5 alpha by TAMRA-labeled CPPs using two independent techniques, namely, FS and CLSM, and double staining with FM4-64, which is a cell membrane-staining dye, yielded quantitative and qualitative results, confirming that TAMRA-labeled CPPs accumulate in the E. coli cytoplasm. This study provides peptide library-based information of CPPs with high bacterial cell penetration efficiency and low cytotoxicity.