Critical Concentration of Lecithin Enhances the Antimicrobial Activity of Eugenol against Escherichia coli

Abstract

Lecithin is a natural emulsifier used in a wide range of food and nonfood applications to improve physical stability, with no known bioactive effects. In this study, the effect of lecithin on the antimicrobial performance of a constant eugenol concentration was tested against three Escherichia coli strains (C600, 0.1229, and O157:H7 strain ATCC 700728). This is the first study, to our knowledge, focusing on lecithin at concentrations below those commonly used in foods to improve the stability of oil in water emulsions (Յ10 mg/100 ml). For all three cultures, significant synergistic antimicrobial effects were observed when E. coli cultures were exposed to a constant eugenol concentration (ranging from 0.043 to 0.050% [wt/wt]) together with critical lecithin concentrations ranging from 0.5 to 1 mg/100 ml. Increasing the concentration of lecithin above 1 mg/100 ml (up to 10 mg/100 ml lecithin) diminished the antibacterial effect to values similar to those with eugenol-only treatments. The formation of aggregates (Ͻ100 nm) at the critical lecithin concentration was observed using cryo-transmission electron microscopy (cryo-TEM), together with a reduction in light absorbance at 284 nm. At critically low concentrations of lecithin, the formation of nanoscale aggregates is responsible for improving eugenol antimicrobial effects.IMPORTANCE Essential oils (EOs) are effective natural antimicrobials. However, their hydrophobicity and strong aromatic character limit the use of essential oils in food systems. Emulsifiers (e.g., lecithin) increase the stability of EOs in water-based systems but fail to consistently improve antimicrobial effects. We demonstrate that lecithin, within a narrow critical concentration window, can enhance the antimicrobial properties of eugenol. This study highlights the potential bioactivity of lecithin when utilized to effectively control foodborne pathogens.

Critical Synergistic Concentration of Lecithin Phospholipids Improves the Antimicrobial Activity of Eugenol against Escherichia coli

“…When the critical synergistic concentrations were above the CMC, the enhanced antimicrobial property of eugenol may have been due to the encapsulation of eugenol into phospholipid micelles. However, with concentrations below the CMC, the phospholipid may form phospholipid-rich "patches" on the eugenol droplets as discussed in our previous study (11). The partition of phospholipids into the E. coli cell membrane (and therefore the modification of the membrane fluidity and permeability for hydrophilic compounds) is another mechanism that we plan to investigate (32).…”

“…However, previous reports showed that the antimicrobial properties of EO against E. coli were not enhanced when EOs were combined with lecithin (8,9) or observed that waterinsoluble surfactants (including lecithin) in nanoemulsions diminished the antimicrobial efficacy of EOs (10). In contrast, we previously reported that the antimicrobial properties of eugenol were significantly improved when lecithin was added at a critical low concentration (11). Nanoscale (Ͻ50-nm) aggregates were observed when eugenol was mixed with critical synergistic concentration of lecithin, and it was suggested that electrostatic interactions between charged phospholipids and the negatively charged bacterial cell membrane were responsible for the improved antimicrobial properties.…”

“…Eugenol-only control samples showed a lower overall level of reduction (around 0.2 log CFU/ml) than was seen in our previous study (ca. 2 log CFU/ml [11]), suggesting lower eugenol antimicrobial power. However, the synergistic effect of lecithin was consistent with previous results; i.e., the synergistic effect was observed regardless of the antimicrobial power of the eugenol for the results shown in Fig.…”

“…E. coli C600 (36) was obtained from the Yale E. coli Genetic Stock Center. It is a prophage-cured derivative of E. coli K-12 (37) that showed a positive response to eugenol-lecithin buffered systems in a previous study (11). After growth in tryptic soy broth (TSB; Difco, Sparks, MD) under aerobic conditions at 37°C for 24 h, a glycerol stock was made by mixing 1-ml culture aliquots with 0.3 ml 50% (vol/vol) glycerol-water.…”

Nutritional and physiological responses of broiler chickens to dietary supplementation with de-oiled soyabean lecithin at different metabolisable energy levels and various fat sources

“…Emulsifiers promote fatty acids' incorporation into micelles and increase the digestibility of dietary lipids in the duodenum of chickens (13,14) . De-oiled soyabean lecithin (DSL) is a mixture of amphiphilic phospholipids, which has good emulsification property (15) .…”