Physicochemical Characterization and Antibacterial Potential of Naturally-Occurring Antimicrobial Polypeptide-Bearing Phospholipid Liposomes Against the Foodborne Pathogens Listeria monocytogenes and Escherichia coli O157:H7

Liposomes are spherical bilayer vesicles formed from dispersion of polar lipids in aqueous solvents and have been studied for their ability to act as drug delivery vehicles by shielding reactive or sensitive compounds prior to release. In the food industry, liposomes have been used to deliver food flavors and nutrients; more recently they have been investigated for their ability to incorporate food antimicrobials.

Thermal stability of liposomes, in the presence and absence of nisin, was evaluated using differential scanning calorimetry (DSC) and low-intensity ultrasonic spectroscopy (US). Observed phase transition temperatures for liposomes measured via DSC and US were in close agreement, suggesting the utility of US for the non-destructive characterization of liposome vesicles.

Ability of liposomes to maintain integrity was tested by encapsulation efficiency (EE), zeta potential, and vesicle size. PC, PC/PG 8/2, and PC/PG 6/4 (mol fraction) liposomes retained between ~70-90% EE despite exposure to elevated temperature or extreme pH. Liposome size averaged between 100-240 nm. Zeta potential depended primarily on phospholipid composition and changed little with inclusion of nisin.

Nisin was entrapped in phospholipid liposomes at varying concentrations. Nisin was entrapped at 5 and 10 µg/ml and then added to media seeded with L. monocytogenes Scott A or 310. L. monocytogenes inhibition was observed to be only slightly dependent upon dose, but was heavily dependent upon phospholipid constituents of liposomes. Near complete inhibition of E. coli O157:H7 with liposomal antimicrobial and chelator at concentrations below those reported necessary for naked antimicrobial and chelator suggest that liposome encapsulation of antimicrobials may allow for control of foodborne pathogens and increase the spectrum of activity for some antimicrobials.

Liposomal nisin and chelator were added to milk samples of varying fat levels containing the pathogens L. monocytogenes and E. coli O157:H7 and incubated at 25°C for 48 hr and survivors enumerated. Liposomal nisin was inhibitory to L. monocytogenes strains, and effects on strains were equivalent, regardless of milk fat level. Encapsulation of nisin and chelator in liposomes abolished antimicrobial effects against both Gram- positive and Gram-negative strains.

Data indicate liposomes are useful candidates for the long-term control of foodborne pathogens in food.