Medicinal plants contribute to the development of new chemo-preventive agents. It is therefore important to determine their bioactive compounds as well as antibacterial activities [22]. Plants produce a wide collection of phytochemicals that are related to stress, defence mechanisms and antimicrobial activities [23]. The ethyl acetate leaf extract of O. basilicum had a variety of phytochemical classes and exhibited antibacterial activities.
Flavonoids are free radical scavengers and water soluble antioxidants with the ability to prevent oxidative cell damage. These metabolites protect the body against cancer, inflammation, allergens, microbes, platelet aggregation, tumors and hepatotoxins [24]. The methylated flavonoid, 6,3'-Dimethoxyflavone exhibits antimicrobial activities. Its derivative has broad spectrum antibacterial activities on E. coli, P. aeruginosa and S. aureus [25, 26].
The two alkaloids 3-Quinolinecarboxylic acid, 6,8-difluoro-4-hydroxy-, ethyl ester and Benzo[h]quinoline, 2,4-dimethyl-. Benzo[h]quinolones have been used to obtain novel compounds that possess antibacterial activities on S. aureus, Bacillus subtilis, and Streptococcus pyogenes. These compounds are also strong antioxidants, show significant wound healing activities and protect oxidative DNA damage from harmful free radical reactions [27]. Synthetic derivatives of alkaloids are medicinally important for their antispasmodic, analgesic and bactericidal effects. Its physiological activities are apparent when this metabolite is administered in animal models [28].
Terpenoids represent the most diverse and largest class of chemicals among the many metabolites produced by plants. They are a group of compounds possessing an isoprene unit as their basic structure. They are classified based on the number of carbon atoms. Terpenoids provide protection against pathogenic microorganisms [29]. Nootkatone is a sesquiterpene that is synthesized by the oxidation of valencene [30]. Nootkatone has been shown to exhibit antibacterial activities on Gram-positive bacteria including S. aureus, Enterococcus faecalis, Corynebacterium diphtheriae, Listeria monocytogenes and Bacillus cereus [30]. This compound exhibits antibacterial effects by targeting metabolites or structures that are specific to Gram-positive bacteria such as the peptidoglycan component of cell walls. Synthetic retinoids contain an isoprene unit that is capable of killing MRSA by penetrating and disrupting the lipid bilayers [31]. There is a possibility that nootkatone inhibits bacterial proliferation by acting on the synthetic pathway of peptidoglycan.
β-caryophyllene is a sesquiterpene, component of dietary regimes that is consumed as a food preservative, flavour or additive and is often referred to as a ‘dietary cannabinoid’ [32]. β-caryophyllene has exhibited therapeutic potential due to multiple pharmacological properties such as antioxidant, anti-inflammatory [32] and antimicrobial [33]. Triterpenoids such as β-amyrin, a pentacyclic triterpene, are plant constituents with great pharmacological potentials [34]. Lupeol acetate, a pentacyclic triterpene, exhibits a wide spectrum of pharmacological effects including antimicrobial effects [35]. The triterpene 2,6,10,14,18-Pentamethyl-2,6,10,14,18-eicosapentaene was identified in the ethyl acetate leaf extract of Ocimum basilicum. Manool oxide, a diterpene, has been found to be in abundance in several essential oils, from several plants, which have been shown to have antibacterial activities [36, 37].
Fatty acids identified include Oxirane, 2-methyl-2-(1-methylethyl)-; 11-Eicosenoic acid, methyl ester and 1,3-Dimethyl-5-isobutylcyclohexane. 11-Eicosenoic acid, methyl ester has been associated with antibacterial activity and anti-inflammatory effects [38]. The exact mechanisms by which fatty acids impose their antibacterial effects remain unknown. It has, however, been hypothesized that these molecules induce peroxidative processes that inhibit bacterial fatty acid synthesis. Fatty acids may also interact with cellular membranes thereby causing leakage of molecules from the cells, reduction of nutrient uptake or inhibiting cellular respiration [42]. Amines eluted from the extract include 9-Thiabicyclo [3.3.1] non-6-en-2-amine, N-methyl-, endo-; 1,2,4-Triazol-3-amine, 5-(1,3,5-trimethyl-4-pyrazolyl) amino- and 2-Methoxyamphetamine. Bicyclic amines have been documented to have antimicrobial and antiparasitic effects [39, 40]. Aldehydes with long chain fatty alcohols such as E-11(13-Methyl) tetradecen-1-ol acetate, Tridecenol < 2E-> and tetradecanal have been shown to have antibacterial activities on S. aureus [41]. Tetradecanaal and tridecanol are long chain alcohols that exhibit their antibacterial activity by damaging cell membranes thereby leading to the leakage of K + ions together with subsequent reactions that lead to further leakage [41]. Compounds that possess an alkyl chain promote antibacterial activity and resensitize methicillin susceptible and resistant S. aureus to antibiotics [41].
Phenolic compounds with imidazole moieties such as 1H-Imidazole, 2-ethyl-4,5-dihydro-4-methyl-; 3,6,6-Trimethyl-cyclohex-2-enol and l-Alanine, N-(2-thienylacetyl)-, butyl ester possess antibacterial properties. The presence of the imidazole ring to the quinolone moiety increases its antibacterial activity [43]. The antibacterial effects exhibited by the extract were broad spectrum and could be attributed to the effects of these phytochemicals. The inhibitory effects were exhibited in a dose dependent manner with a better efficacy being exhibited on the gram positive bacteria compared to the gram negative. The difference in susceptibility indices could be attributed to the fact that gram negative bacteria have a stable peptidoglycan layer that allows bioactive compounds into the cytoplasm at a lower rate compared to the gram positive bacteria. The dose dependent active nature of the extract on the bacterial strains was due to the decreasing concentrations of active compounds. The ethyl acetate leaf extract of O. basilicum was found to have strong antibacterial properties [20].