Surfactant Mediated Dispersion and stabilization of nano-emulsion droplets enhance antimicrobial activity against multidrug resistant bacteria

• Different herbal oils were screened against MRSA , S. aureus and E.coli. • Thyme oil was most effective among different herbal oils. • TNE1 from rhamnolipid (0.5%) shows higher antimicrobial activity over other NEs. • TNE1 remained stable at 4 °C for 12 months. • TNE1 retaining antimicrobial activity against three pathogens. In the present study, the efficacy of chemical as well as biological surfactants-based oil-in-water nano-emulsion has been evaluated with optimum antibacterial activity against multidrug-resistant bacteria (MDR). The antimicrobial activity of different essential oils like bergamot, spearmint, orange, turmeric, thyme, dill seed, and neroli was evaluated against methicillin-resistant Staphylococcus aureus (NCIM-5718), Staphylococcus aureus (ATCC 33591) and Escherichia coli . Among all these discrete herbal oils, thyme oil has shown an optimum zone of inhibition of 5.27 ± 0.08, 4.7 ± 0.14, and 4.2 ± 0.11 cm for MRSA, S. aureus, and E. coli respectively. Henceforth, thyme oil has been selected for the preparation of nano-emulsion. A comparative study among four different surfactants namely, tween 80, lecithin, guar gum, and rhamnolipid was carried out in varying ratios with thyme oil for the formation of stable nano-emulsion with optimum antibacterial activity. Pseudo-ternary diagram has been constructed with thyme oil and surfactant in multiple ratios for the development of a kinetically stable and effective nano-emulsion. Dynamic light scattering (DLS) was further used to characterize the thyme oil-based nano-emulsions. Among the different nano-emulsions, the selected nano-emulsion (TNE1), exhibited a negatively charged zeta potential of −58.03 ± 0.50 mV with a droplet size of 49.71 ± 4.70 nm and a polydispersity index (PDI) value of 0.28 ± 0.07. Comparative antibacterial activity of thyme oil alone as well as the different nano-emulsions has been assessed. Thyme oil alone has a zone of inhibition of 3.5 ± 0.00, 4.45 ± 0.02 and 3.45 ± 0.02 cm, whereas, the selected formulation TNE1 has a comparatively higher zone of inhibition of 4.12 ± 0.0, 4.1 ± 0.00 and 3.85 ± 0.14 cm against MRSA, S. aureus and E. coli respectively. Furthermore, the shelf life and stability study of TNE revealed that TNE 1 kept at 4 °C showed consistent antimicrobial activity against all three bacterial strains even after 12 months of storage. Thus, biosurfactant-based oil in water nano-emulsion TNE1 was found to be potentially stable and effective against multidrug resistant (MDR) bacteria. The knowledge gained from the study might provide the basic guideline for the fabrication of stable nano-emulsion from the biosurfactant with antimicrobial activity against MDR bacteria for the pharmaceutical field.