Lippia grata Essential Oil Acts Synergistically with Ampicillin Against Staphylococcus aureus and its Biofilm.

Antimicrobial resistance (AMR) presents a global challenge as microorganisms evolve to withstand the effects of antibiotics. In addition, the improper use of antibiotics significantly contributes to the AMR acceleration. Essential oils have garnered attention for their antimicrobial potential. Indeed, essential oils extracted from plants contain compounds that exhibit antibacterial activity, including against resistant microorganisms. Hence, this study aimed to evaluate the antimicrobial and antibiofilm activity of the essential oil (EO) extracted from Lippia grata and its combination with ampicillin against Staphylococcus aureus strains (ATCC 25923, ATCC 700698, and JKD6008). The plant material (leaves) was gathered in Mossoro, RN, and the EO was obtained using the hydrodistillation method with the Clevenger apparatus. The antimicrobial activity of the EO was assessed through minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assays. Antibiofilm activity was evaluated by measuring biomass using crystal violet (CV) staining, viable cell counting, and analysis of preformed biofilms. In addition, the synergistic effects of the EO in combination with ampicillin were examined by scanning electron and confocal microscopy. The EO displayed a MIC value of 2.5 mg/mL against all tested S. aureus strains and an MBC only against S. aureus JKD6008 at 2.5 mg/mL. L. grata EO caused complete biofilm inhibition at concentrations ranging from 10 to 0.312 mg/mL against S. aureus ATCC 25923 and 10 to 1.25 mg/mL against S. aureus ATCC 700698 and S. aureus JKD6008. In the viable cell quantification assay, there was a reduction in CFU ranging from 1.0 to 8.0 logs. The combination of EO with ampicillin exhibited a synergistic effect against all strains. Moreover, the combination showed a significantly inhibiting biofilm formation and eradicating preformed biofilms. Furthermore, the EO and ampicillin (individually and in combination) altered the cellular morphology of S. aureus cells. Regarding the mechanism, the results revealed that L. grata EO increased membrane permeability and caused significant membrane damage. Concerning the synergy mechanism, the results revealed that the combination of EO and ampicillin increases membrane permeability and causes considerable membrane damage, further inhibiting bacteria synergistically. The findings obtained here suggest that L. grata EO in combination with ampicillin could be a viable treatment option against S. aureus infections, including MRSA strain.

Ecophysiological response of Lippia gracilis (Verbanaceae) to duration of salt stress.

The objective of this work was to investigate the effects of salinized water on Lippia gracilis by determining the ion accumulation capacity and morphophysiological changes as a function of salinity and time of exposure to stress. L. gracilis was irrigated with water containing different concentrations of salts and electrical conductivities. The analyses were conducted 25 and 50 days after stress (DAS), in leaf, stem/branches, and roots. Plant height (Hplants), basal stem diameter (Φstem), and mean unitary leaf area (ULA) were determined. After that, the plant material was dried, and the total dry biomass (TDB) (in g plant-1), leaf dry biomass (LDB), stem dry biomass (SDB), and the roots dry biomass (RDB) were determined. Based on these data, the following variables were calculated: total leaf area (TLA) in cm2 plant-1; biomass allocation fraction (BAF) obtained by dividing the biomass of the stem, leaves, and roots, separately, with the total plant biomass; relative growth rate (RGR); net assimilation rate (NAR); leaf area ratio (LAR); specific leaf area (SLA); and sclerophylly index (SI). The concentrations of potassium and sodium ions were quantified by atomic absorption spectrophotometry and the chloride ion concentration was determined by argentometric titration. L. gracilis showed a variable response to salt stress as a function of time, having a considerable capacity to withstand an increasing load of toxic ions, while maintaining a positive growth rate, highlighting the potential use of the species in moderately saline environments.