Ocimum sanctum as a Source of Quorum Sensing Inhibitors to Combat Antibiotic Resistance of Human and Aquaculture Pathogens

Abstract

Biofilms play a decisive role in the infectious process and the development of antibiotic resistance. The establishment of bacterial biofilms is regulated by a signal-mediated cell-cell communication process called "quorum sensing" (QS). The identification of quorum sensing inhibitors (QSI) to mitigate the QS process may facilitate the development of novel treatment strategies for biofilm-based infections. In this study, the traditional medicinal plant Ocimum sanctum was screened for QS inhibitory potential. Sub-MICs of the extract significantly affected the secretion of EPS in Gram-negative human pathogens such as Escherichia coli, Pseudomonas aeruginosa PAO1, Proteus mirabilis, and Serratia marcescens, as well as aquaculture pathogens Vibrio harveyi, V. parahaemolyticus, and V. vulnificus, which render the bacteria more sensitive, leading to a loss of bacterial biomass from the substratum. The observed inhibitory activity of the O. sanctum extract might be attributed to the presence of eugenol, as evidenced through ultraviolet (UV)-visible, gas chromatography-mass spectroscopy (GC-MS), Fourier transformer infrared (FTIR) spectroscopy analyses, and computational studies. Additionally, the QSI potential of eugenol was corroborated through in vitro studies using the marker strain Chromobacterium violaceum.

Keywords: Ocimum sanctum; antibiotic resistance; biofilms; eugenol; quorum sensing inhibition.

Figure 1

Inhibition of violacein production by O. sanctum extract. The images show violacein production by C. violaceum 12472 cultivated in the absence (a) and presence (b) of O. santum (12 mg/mL).

Figure 2

Inhibition of biofilm biomass by O. sanctum extract in Gram-negative human bacterial pathogens (a) and aquatic bacterial pathogens (b).

Figure 3

Inhibition of EPS production by O. sanctum extract in Gram-negative human bacterial pathogens (a) and aquatic bacterial pathogens (b).

Figure 4

Effect of O. sanctum at test concentrations on the growth of bacterial pathogens by WDAA.

Figure 5

Biochemical analysis of O. sanctum extract through GC-MS analysis.

Figure 6

(A) 2D interaction plot profile of (a) acetyl eugenol and (b) eugenol with RhlR. (B) Root Mean Square deviation for the protein–ligand complexes calculated during the simulations. (C) Post-MD simulation interaction profile of the protein–ligand complexes of: (a) acetyl eugenol and (b) eugenol.

Figure 7

Comparison of the FTIR spectrua of active fraction F34 and eugenol. (a) FTIR profile of fraction F34. (b) FTIR profile of eugenol.

Figure 8

Determination of QSI compounds through UV spectrum analysis: (a) UV spectrum of fraction F34. (b) UV spectrum of the commercial eugenol.

Figure 9

Confirmation of QSI activity in the identified compound eugenol using the biomarker C. violaceum 12472. (a) Untreated culture (b) Eugenol treated culture.