Characterization of a novel LysM domain from Lactobacillus fermentum bacteriophage endolysin and its use as an anchor to display heterologous proteins on the surfaces of lactic acid bacteria.

The endolysin Lyb5, from Lactobacillus fermentum temperate bacteriophage phiPYB5, showed a broad lytic spectrum against Gram-positive as well as Gram-negative bacteria. Sequence analysis revealed that the C terminus of the endolysin Lyb5 (Ly5C) contained three putative lysin motif (LysM) repeat regions, implying that Ly5C was involved in bacterial cell wall binding. To investigate the potential of Ly5C for surface display, green fluorescent protein (GFP) was fused to Ly5C at its N or C terminus and the resulting fusion proteins were expressed in Escherichia coli. After being mixed with various cells in vitro, GFP was successfully displayed on the surfaces of Lactococcus lactis, Lactobacillus casei, Lb. brevis, Lb. plantarum, Lb. fermentum, Lb. delbrueckii, Lb. helveticus, and Streptococcus thermophilus cells. Increases in the fluorescence intensities of chemically pretreated L. lactis and Lb. casei cells compared to those of nonpretreated cells suggested that the peptidoglycan was the binding ligand for Ly5C. Moreover, the pH and concentration of sodium chloride were optimized to enhance the binding capacity of GFP-Ly5C, and high-intensity fluorescence of cells was observed under optimal conditions. All results suggested that Ly5C was a novel anchor for constructing a surface display system for lactic acid bacteria (LAB). To demonstrate the applicability of the Ly5C-mediated surface display system, beta-galactosidase (beta-Gal) from Paenibacillus sp. strain K1, replacing GFP, was functionally displayed on the surfaces of LAB cells via Ly5C. The success in surface display of GFP and beta-Gal opened up the feasibility of employing the cell wall anchor of bacteriophage endolysin for surface display in LAB.