Extent and Mode of Action of Cationic Legume Proteins against Listeria monocytogenes and Salmonella Enteritidis

The methylated soybean protein and methylated chickpea protein (MSP and MCP) with isoelectric points around pI 8 were prepared by esterifying 83 % of their free carboxyl groups and tested for their interactions with Listeria monocytogenes and Salmonella Enteritidis. The two substances exhibited a concentration-dependent inhibitory action against the two studied bacteria with a minimum inhibitory concentration of about 100 μg/mL. The IC_50 % of the two proteins against L. monocytogenes (17 μg/mL) was comparable to penicillin but comparatively much lower (15 μg/mL) than that of penicillin (85 μg/mL) against S. Enteritidis. The two proteins could inhibit the growth of L. monocytogenes and S. Enteritidis by about 97 and 91 %, respectively, after 6–12 h of incubation at 37 °C. The constituting subunits of MSP (methylated 11S and methylated 7S) were both responsible for its antimicrobial action. Transmission electron microscopy of the protein-treated bacteria showed various signs of cellular deformation. The cationic proteins can electrostatically and hydrophobically interact with cell wall and cell membrane, producing large pores, pore channels and cell wall and cell membrane disintegration, engendering higher cell permeability leading finally to cell emptiness, lysis and death.