The ethanol, methanol and acetone extracts of Aloe vera gel were studied for their antimicrobial activity against four Gram-positive and Gram-negative bacteria using agar well diffusion method. The extracts showed varied levels of antimicrobial activity against the tested pathogens. The ethanol and methanol extracts showed higher activity while acetone extract, showed least or no activity against most of the tested pathogens. Fractions obtained from the extracts by Thin Layer and Column Chromatography were studied for their antagonistic properties using Spot Assay Technique. Compounds with maximum antibacterial activity isolated from the ethanol and methanol extracts were identified as p -coumaric acid (Mol. wt.165), ascorbic acid (Mol. wt.177 ), pyrocatechol (Mol. wt.110 ) and cinnamic acid (Mol.wt.148), on the basis of Gas Chromatography Mass Spectrometry. The study suggests the antimicrobial activity of the A. vera gel extract to be dependant on the synergistic effect of different compounds. With the broad spectral antimicrobial effect of A. vera gel, it could be further recommended in the treatment of various bacterial diseases.

Injudicious consumption of antibiotics in the past few decades has arisen the problem of resistance in pathogenic organisms against most antibiotics and antimicrobial agents. Scenarios of treatment failure are becoming more common in hospitals. This situation demands the frequent need for new antimicrobial compounds which may have other mechanisms of action from those which are in current use. Limonene can be utilized as one of the solutions to the problem of antimicrobial resistance. Limonene is a naturally occurring monoterpene with a lemon-like odor, which mainly present in the peels of citrus plants like lemon, orange, grapefruit, etc. The study aimed to enlighten the antimicrobial properties of limonene as per previous literature. Advantageous contributions have been made by various research groups in the study of the antimicrobial properties of limonene. Previous studies have shown that limonene not only inhibits disease-causing pathogenic microbes, however, it also protects various food products from potential contaminants. This review article contains information about the effectiveness of limonene as an antimicrobial agent. Apart from antimicrobial property, some other uses of limonene are also discussed such as its role as fragrance and flavor additive, as in the formation of nonalcoholic beverages, as solvent and cleaner in the petroleum industry, and as a pesticide. Antibacterial, antifungal, antiviral, and anti-biofilm properties of limonene may help it to be used in the future as a potential antimicrobial agent with minimal adverse effects. Some of the recent studies also showed the action of limonene against COVID-19 (Coronavirus). However, additional studies are requisite to scrutinize the possible mechanism of antimicrobial action of limonene.

Antibacterial potential of Limonene against Multi Drug Resistant (MDR) pathogens was studied and mechanism explored. Microscopic techniques viz. Fluorescent Microscopy (FM), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) indicated membrane disruption, cellular leakage and cell death of Escherichia coli (E. coli) cells when treated with limonene. Leakage of intracellular proteins, lipids and nucleic acid confirmed membrane damage and disruption of cell permeability barrier. Further, release of intracellular ATP, also suggested disruption of membrane barrier. Interaction of limonene with DNA revealed its capability in unwinding of plasmid, which could eventually inhibit DNA transcription and translation. Differential expression of various proteins and enzymes involved in transport, respiration, metabolism, chemotaxis, protein synthesis confirmed the mechanistic role of limonene on their functions. Limonene thus can be a potential candidate in drug development.

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This paper reports the mixed ligand–metal complexes of CuSO 4 ·5H 2 O and ZnSO 4 ·7H 2 O with salicylaldehyde thiosemicarbazone (2-hydroxybenzaldehyde thiosemicarbazone) as primary ligand and imidazole (im), pyridine (py) and triphenylphosphine (PPh 3 ) as secondary ligands through a general preparatory route. The ligand and complexes were characterized by FTIR, UV, 1 H-NMR and molar conductance techniques. Computational studies to know the physicochemical parameters, bioactivity scores, absorption, distribution, metabolism, excretion and toxicity (ADMET) properties were carried out through Molinspiration, SwissADME and admetSAR softwares. Molecular docking was perfomed with M pro of SARS-CoV-2 (PDB i.d.6LU7), Aspartate Kinase (PDB i.d.5YEI) and Transforming Growth Factor β (PDB i.d. 3KFD) using PyRx automated docking software. The antibacterial activity was tested using Agar well method. Computational findings revealed that almost all the complexes had clogP values less than 5 indicating their bioavailability. The bioactivity scores of the complexes were between moderate to good. The mixed ligand complexes having imidazole as secondary ligand displayed relatively high FCsp 3 , indicating their potential as lead candidates. [Zn(C 8 H 9 N 3 OS)(PPh 3 ) 2 (SO 4 )] and [Cu(C 8 H 9 N 3 OS)(im) 2 (SO 4 )] exhibited appreciable binding affinity against the selected proteins. Furthermore, the molecular simulation findings with the ligated [Cu(C 8 H 9 N 3 OS)(im) 2 (SO 4 )] and aspartate kinase showed compact folding, less deviations and significant stability. The stability of the ligand was further confirmed by the frontier molecular orbitals (FMOs) gap. The energy gap (− 0.423 eV) indicated molecular stability. The ligand was active against L. monocytogenes, S. aureus and E.coli having zone of inhibition of 11, 11 and 10 mm respectively. Among the complexes, [Cu(C 8 H 9 N 3 OS)(im) 2 (SO 4 )] had the minimum inhibitory concentrations (MIC) ranging between 32 and 128 µg/mL against the selecetd bacterial strains. Graphical Abstract Supplementary Information The online version contains supplementary material available at 10.1007/s42250-023-00640-4.

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