Antibacterial effects of thyme oil loaded solid lipid and chitosan nano-carriers against Salmonella Typhimurium and Escherichia coli as food preservatives

Abstract

Objectives: Escherichia coli and Salmonella Typhimurium are frequent causes of foodborne illness affecting many people annually. In order to develop natural antimicrobial agents against these microorganisms, thyme oil (TO) was considered as active antibacterial ingredient. TO contains various bioactive compounds that exhibit antimicrobial properties. To increase the antibacterial effects and stability of thyme oil, two promising carrier systems, solid lipid nanoparticles (SLN) and chitosan nanoparticles have been fabricated in this study.

Methods: Nanoparticles were made using natural-based lipids and polymers by a probe sonication method. They were characterized using infrared spectrometry (FTIR), transmission electron microscopy (TEM), particle size, cytotoxicity, etc. Antibacterial effects of TO, thyme oil loaded in SLN (TO-SLN) and thyme oil loaded in chitosan nanoparticle (TO-CH) was evaluated against E. coli and S. typhimurium using Minimum inhibitory/bactericidal concentrations (MIC/MBC) determination. Encapsulation efficiency (EE%) and drug release profile were also studied in vitro.

Results: TEM analysis revealed spherical/ovoid-shaped particles with clear edges. TO-SLN had an average size of 42.47nm, while TO-CH had an average size of 144.8nm. The Encapsulation efficiency of TO-CH and TO-SLN nanoparticles were about 81.6±1% and 73.4±1%, respectively. Results indicated 92% cumulative release in TO-CH in comparison with 88% in TO-SLN in 72 h. MIC against E. coli and S. typhimurium for TO-CH, TO-SLN, and pure TO were 4 and 1.5 μg/mL, 60 and 40 μg/mL, and 180-150 μg/mL, respectively.

Conclusion: Nanoencapsulation of thyme oil significantly potentiated its antimicrobial effects. TO-CH exhibited a significantly higher antibacterial effect compared to TO-SLN (6-fold) and pure thyme oil (more than 10-fold).

Fig 1. Data representative for physico-chemical characteristics of thyme oil nanoparticles.

a) hydrodynamic diameter measurement of TO-SLN by DLS. b) zeta potential of TO-SLN. c) hydrodynamic diameter measurement of TO-CH using DLS. d) zeta potential of TO-CH.

Fig 2. TEM two-dimensional structural qualification.

a) TO-SLN on the scale of 60 nm, b) TO-CH on the scale of 250 nm.

Fig 3. ATR-FTIR spectra of thyme oil (blue), TO-SLN (green) and TO-CH (red).

Fig 4. Data represents the cumulative release profile of thyme oil from TO-SLN and TO-CH in 72 h.

Fig 5. The cell viability% treated with thyme oil (mg mL^-1) compared with thyme oil loaded SLN and chitosan nanoparticles (5 mg mL^-1).

Fig 6. Variation of S. typhimurium viability in presence of TO, TO-SLN and TO-CH with the concentrations of MIC, ½ MIC and ¼ MIC in days 0,2,4 and 6.