The EO from P. sylvestris needles obtained by hydro-distillation exhibited excellent broad-spectrum in vitro antibacterial activities against all bacterial isolates tested with the value of Zi that ranged from 8 to 24 mm. The EO compared favourably with the standard antibiotics, streptomycin with the value of Zi that ranged from 10 to 25 mm, therefore making it a potential antibacterial agent of beneficial importance. Similarly, some studies have also reported broad-spectrum activities of EO of P. sylvestris needles [32–34] against multidrug-resistant bacterial strains of S. aureus, enterobacterial pathogens [35, 36], and antibiotic-resistant Pseudomonas spp. responsible for freshwater fish diseases (Sekiten-byo) and spoilage [27]. Hence, the EO of P. sylvestris needles can serve as an antibacterial agent for the control of multidrug-resistant pathogens and food spoilage bacteria.
The EO inhibited the growth of all the bacteria tested at an appreciable MIC ranging between 0.39 and 1.56 mg mL− 1 except for P. vulgaris, which also had the lowest Zi (8 mm). Vyas and Patil [36] obtained similar results with P. vulgaris (Zi of 4 mm, the lowest value among all the bacterial isolates tested) when the antibacterial activity of EO of P. sylvestris was examined against multidrug-resistant enterobacterial pathogens.
This study has validated the antimicrobial potential of EO derived from P. sylvestris. This might be linked to the biologically active constituents available in EO of P. sylvestris needles which enhanced the higher level of antibacterial properties reported against all the tested pathogens in this study. The result obtained from the GC-MS showed that seven antibacterial chemical constituents were present in the EO of P. sylvestris. This correlates with the result obtained by some previous researchers who identified seven chemical constituents in the EO of P. sylvestris [26–28].
Three oxygenated monoterpenes including α-terpineol (the most abundant chemical compound in the study P. sylvestris), borneol, and fenchol were identified as antibacterial compounds in the P. sylvestris. Antibacterial effects of oxygenated monoterpenes component of EO of P. sylvestris against a wide range of bacterial species, and their useful applications in the production of medicine and cosmetics have been reported [32]. For instance, borneol is one of the traditional Chinese medicine used to treat sore throat, ulcerations, and purulent ear discharge [35]. The presence of these oxygenated monoterpenes in P. sylvestris supports its potential as an antimicrobial agent against bacterial pathogens.
Also, hydrocarbon sesquiterpenes (caryophyllene and δ-cadinene) were observed as the antibacterial compound in the EO of P. sylvestris. Interestingly, some scientist in some part of Europe including Estonia, Lithuania and Romania has established the presence of hydrocarbon sesquiterpenes as the major component available in pine oil using GC/MS techniques. Furthermore, the presence of β- caryophyllene and δ-cadinene apart from hydrocarbon sesquiterpenes constitute the majority of the sesquiterpenes available in the EO of P. sylvestris [15, 36, 37]. Several scientists have established that hydrocarbon sesquiterpenes such as β- caryophyllene possess other biological properties like anti-inflammatory, anticancer, antioxidant, and antifungal properties in addition to their antibacterial activities [33, 38, 39]. It can also inhibit biofilm-forming bacteria implicated in tooth biofilm or plaque in dogs [38], suggesting its application in veterinary medicine. The principal active constituent and antibacterial agent in Schinus molle fruits have been affirmed to possess inhibitory effect against S. pneumoniae at MIC of 31.25 mg mL− 1. The author further utilized GC/MS to identify the presence of δ-cadinene as the active constitutes responsible for the antibacterial activity against S. pneumoniae.
During this study, the presence of oleic acid (monounsaturated fatty acid) and palmitic acid (saturated fatty acids) was also detected in the EO of P. sylvestris. It has been documented that oleic acid portends the capability to inhibits the FabI enzyme responsible for the fatty acid biosynthesis in bacteria [40], while it could also increase membrane permeability and its leakages, disruption of electron chain transport, and inhibition of the activity of bacterial enzymes [41, 42]. However, oleic acid has been established to exhibit antibacterial activities against Gram-positive clinical and foodborne bacterial pathogens such as Bacillus spp., Micrococcus kristinae, Streptococcus pyogenes, Staphylococcus aureus, and Methicillin-resistant S. aureus. Gram-negative bacteria are resistant to unsaturated fatty acids because their outer membrane is highly impermeable to hydrophobic substances [34, 40]. In another study, it was validated that Gram-negative bacteria were only susceptible to short-chain fatty acids (C6 and below) at high concentrations [43]. The study also reported that Gram positive-bacteria are more susceptible to fatty acid than Gram-negative bacteria, and unsaturated fatty acids like oleic acid have better antibacterial activities than saturated fatty acids like palmitic acid [43].
This study has been able to establish that the presence of oxygenated monoterpenes (α-terpineol, borneol, and fenchol), hydrocarbon sesquiterpenes (caryophyllene and δ-cadinene), and fatty acids (oleic acid and palmitic acids) might be contributing to the antibacterial activities of EO of P. sylvestris. However, future work is to understand the mechanism of action of each antibacterial components of EO of P. sylvestris and their synergistic effects on bacterial pathogens.