The present study aimed to evaluate the antibacterial effects of six EOs at the various concentrations on standard and clinical species of two common nosocomial pathogens (S. aureus and P. aeruginosa). In addition, we selected the most efficient antibacterial EO to investigate its antibacterial effects on nosocomial surfaces.
Generally, S. aureus is found approximately all over the environment, especially on the skin and nasal cavity. On the other hand, S. aureus is one the most opportunistic pathogens that would result in several nosocomial infections 14. The results showed that ASEO had the best antibacterial effects on SSA among examined EOs. In the following, CCEO and CIEO represented acceptable effects, but the other EOs had the lower inhibitory activity on SSA.
P. aeruginosa is another opportunistic pathogen that belongs to gram-negative strains. The genus Pseudomonas is available to survive in various difficult environments and causes life-threatening nosocomial infections. Totally, the experimented EO in the present study showed fewer antibacterial effects against P. aeruginosa. The best findings were observed in the case of CIEO, but the other EOs showed significant inhibitory effects, except CCEO 15.
The genus Artemisia is a well-known medicinal plant that grows worldwide 16. Artemisia sieberi is a special species of this large genus usually found in the northern regions of Iran and Saudi Arabia. Chemical analysis revealed that ASEO is mostly made up of Artemisia ketone and 1,8-cineole. Moreover, Vahdani et al. reported that the main compounds of ASEO were piperitone, camphor, and 1, 8-cineole 17. On the other hand, a recent GC-MS indicated that Camphor, 1,8-cineole, and β-Thujone were the first major components of ASEO 18. Therefore, we can conclude that 1,8-cineole is an improved component in the chemical composition of ASEO and would be one of the major antibacterial agents of this EO. Different species of genus Artemisia have encouraging antibacterial activities 19. Also, A. sieberi products indicated great inhibitory effects on both Gram-negative and Gram-positive strains 20. In 2013, Hedayati rad et al. conducted a novel Pullulan film incorporated with ASEO to evaluate its protective effects against different bacterial pathogens. This study revealed that ASEO at low concentrations (5%) has no inhibitory effects on bacterial growth, but with increasing the concentration (at least to 10%) the antibacterial effects would develop. The highest inhibitory zone (18.52 ± 0.73) was observed at 35% 21. Pure ASEO made large inhibitory zones against S. aureus and P. aeruginosa in Muller Hinton agar plates. Besides, incorporating basal seed gum films with ASEO resulted in a big inhibitory zone (23.2 ± 0.7 mm) on plates infected with S. aureus and a smaller inhibitory zone (10.2 ± 0.6 mm) in the case of P. aeruginosa 22. Supporting the results of the present study, others also reported that ASEO represented valuable MIC (10 µL) and minimal bactericidal concentration (MBC) (20 µL) against S. aureus 17. Moreover, recent studies on synergic antibacterial effects of ASEO combined with different routine antibiotics demonstrated that ASEO would increase the inhibitory effects of ampicillin, oxacillin, cephalothin, piperacillin, and vancomycin on S. aureus. On the other hand, the combination of various antibiotics, such as piperacillin, gentamicin, ampicillin, and erythromycin with ASEO indicated significant synergic effects on the growth of P. aeruginosa 23. Therefore, using pure ASEO and its combination with different products would provide novel antibacterial materials that represent higher inhibitory effects and decrease the difficulties of previous routine antibiotics.
Chemical composition analysis indicated that CCEO mostly contains 1-phenylpropanol and gamma-Terpinene 24. Besides, Hajlaoui et al. reported that CCEO mainly comprised cumin aldehyde and gamma-terpinene. On the other hand, the first evaluations revealed that Isobutyl isobutyrate, a-Thujene, and a-Pinene are the most common compositions in CCEO. CCEO is believed to have a protective role in food production and storage industries 25. Further studies demonstrated that the most protective CCEO concentrations will appear at 0.03% that prevented the S. aureus growth in feta cheese and infected hamburgers 24. CCEO demonstrated promising MBC and MIC on SSA (2.5 to 5 µg/mL, and 1.25 to 5 µg/mL). Moreover, this study showed that CCEO had the same antibacterial effect on SSA as vancomycin, a highly used antibiotic with several adverse effects 26. On the other hand, some other studies supported our results. Saee et al. reported that the extract and EO from C. cyminum significantly diminished the CPA with higher efficiency than several antibiotics such as gentamicin, amoxicillin, and clotrimazole 27. Therefore, utilizing CCEO in preventing and treating procedures would decrease the limitations of routine antibiotics in the future, but the controversies of its antibacterial effects on gram-negative strains remained for further studies.
C. citratus has a valuable protecting usage in food industries to restrict the activity of common food-borne pathogens. Utilizing CIEO in the preservation of cooked beef patties would save its quality and prevent the growth of pathogenic bacterial strains 28. The evaluations showed that neral, geranial, limonene, and geranyl acetate are the major components of CIEO. C. citratus products mainly prevent the growth of fungal (Candida strains) and Gram-positive bacterial species, such as S. aureus, biofilms. Based on the last in vitro trials, the CIEO provides the same antibacterial effects on S. aureus as vancomycin and would be a favorable product to prevent the growth of nosocomial pathogens with fewer adverse effects in the future 29. In addition, a recent study on EO from the leaves of C. citratus indicated a bigger zone of inhibition in the case of S. aureus (32.0 ± 0.75 mm) and no antibacterial effects on P. aeruginosa. Nevertheless, the other studies showed that C. citratus extracts would represent more antibacterial effects on gram-negative strains such as P. aeruginosa. Subramanian et al. reported that the leaf and root extracts resulted in a very large zone of inhibitions (> 30 mm), and would be encouraging herbal products against these pathogenic gram-negative strains 30. Moreover, the results of our study represented that CIEO mainly decreased the growth of SPA more than the other targeted strains. Therefore, further studies are required to evaluate these controversial results.
L. citriodora and hundreds of other species in the Lippia genus are well-known ancient plants utilized for several decades, especially in Western regions of the world. GC-MS analysis reported that neral and geranial are the major compounds of experimented LCEO 31. In addition, other studies added terpenic alcohols, monoterpenic hydrocarbons (dl-limonene), and α-curcumene to previously reported chemical components. Several studies reported various medicinal properties, such as sedative, diuretic, and antispasmodic use in recent years. Moreover, the recent papers introduced LCEO as a moderate antibacterial product 32. Although pure LCEO provided an acceptable zone of inhibition on sterile Whatman paper discs infected with gram-positive (S. aureus) and gram-negative (E. coli) strains, this EO mainly affects the gram-negative strains more than gram-positive species. On the other hand, some other studies represented controversial results. Evaluating the effects of LCEO on different pathogens showed that this EO has no significant inhibitory or bactericidal effects on P. aeruginosa and the MIC and MBC are higher in the case of E. coli than S. aureus 33. Therefore, the present study and all other studies introduced LCEO as an encouraging antibacterial product, especially for gram-positive strain, but further studies are required to put an end to the controversial results in the case of P. aeruginosa.
Laurus nobilis is another aromatic plant with antimicrobial and flavor properties in food industries. According to the high requirement of enough water and humidity, L. nobilis usually grows in warmer climate places such as the south of the Mediterranean region. Methyl eugenol, 1,8-cineole, and α-terpinyl acetate are the major compounds of LNEO. LNEO has significant inhibitory effects on all gram-positive and -negative strains, like P. aeruginosa, S. aureus, E. coli. The measurements of LNEO inhibition zone showed higher antibacterial effects than tetracycline (8 mg/mL), especially in the case of gram-positive strains. The evaluations revealed that LNEO has high antibacterial effects on gram-positive strains such as S. aureus (inhibition zone = 13 mm), S. intermedius, and K. pneumoniae. Also, LNEO significantly reduces the biofilm activity with acceptable MIC, ranged from 0.3 to 96.4 mg/mL 34. Further comparisons showed that gram-positive strains are more susceptible to LNEO than P. aeruginosa and E. coli. The recent evaluations demonstrated that minimal inhibitory activities of LNEO would be represented at 0.25 mg/mL. On the other hand, LNEO showed remarkable protective effects against common foodborne pathogens, such as E. coli 35.
The aromatic herbal families, such as Apiaceae, are common plants that grow in different regions of Iran. The genus of Ferula is a medicinal herb mostly found in EMERO and north Africa floras 36. Several studies revealed different protective effects of F. gummosa against infection, inflammatory and spasmodic processes. EO from F. gummosa seeds mostly showed strong antibacterial effects on gram-positive strains and slight inhibitory activity against gram-negative strains, such as P. aeruginosa. Abbaszadegan et al. reported that β-pinene made up more than 50% of the chemical composition of FGEO. Moreover, FGEO at 50 µg/mL resulted in a big inhibitory zone (22.5 ± 0.82 mm) on plates infected with S. aureus 37. Considering the antibacterial effects on both S. aureus and P. aeruginosa, our findings showed that FGEO is the most effective agent among the evaluated EOs in the present study. As a result, we selected FGEO to investigate its disinfection activity on the surface of emergency hospital.
In the last two decades, several studies have attempted to use EO-based disinfectants instead of conventional disinfectants. Sharma et al. indicated that a novel green disinfectant, named Neutral Biodegradable Disinfectant (BND) would appropriately clear the contaminated surfaces. Their findings confirmed that phenol-based disinfectants and NBD products disinfect environmental microorganisms equally 38. As a result, given that natural products provide much less resistance and reduce the population of microorganisms evenly, many of these products are expected to replace conventional phenol-based disinfectants in the coming years 39. Increasing bacterial resistance to conventional disinfectants resulted in a great challenge in different healthcare services, especially in dental procedures 40. In recent years, several observations indicated that green disinfectants would become one of the main agents to provide streel dental practices. In 2019, Nausheen et al. confirmed that aloe vera gel at high concentrations (90%) is a safe disinfectant and shows less poor outcomes than conventional chemical products 41. In addition, our findings supported the previous results. FGEO relatively could decrease the growth of both bacterial agents on the trolley surface. Although the increasing concentration of FGEO developed its antibacterial activity, a significant population of bacterial colonies grew on the evaluated samples of our study. Therefore, the application of FGEO-based disinfectants would not be able to completely clear nosocomial surfaces, similar to conventional and novel green disinfectants 42.
The present study has several strengths and limitations. The main strength of our study was the comparison of the antibacterial effects of above-mentioned EOs on clinical and standard samples of two major gram-positive and gram-negative strains. Based on our expanded literature review on previous studies, we mainly hypothesized that the gram-negative strains and clinical samples would represent more resistance against different antibacterial EOs. Generally, the findings supported these hypotheses with little controversy (especially in the case of CIEO and LNEO). Moreover, we faced some conflicts in comparison with previous studies. On the other hand, we just reported the inhibition activity of EOs on bacterial growth in percent. Therefore, further studies with more targeted pathogens and expand measurements would be required to collect more sufficient data and resolve observed controversies.