Ocimum Species: A Review on Chemical Constituents and Antibacterial Activity

Infection by bacteria is one of the main problems in health. The use of commercial antibiotics is still one of the treatments to overcome these problems. However, high levels of consumption lead to antibiotic resistance. Several types of antibiotics have been reported to experience resistance. One solution that can be given is the use of natural antibacterial products. There have been many studies reporting the potential antibacterial activity of the Ocimum plant. Ocimum is known to be one of the medicinal plants that have been used traditionally by local people. This plant contains components of secondary metabolites such as phenolics, flavonoids, steroids, terpenoids, and alkaloids. Therefore, in this paper, we will discuss five types of Ocimum species, namely O. americanum, O. basilicum, O. gratissimum, O. campechianum, and O. sanctum. The five species are known to contain many chemical constituents and have good antibacterial activity against several pathogenic bacteria.


Introduction
Infection by microbes is one of the main problems that causes several diseases. One of the causes of infection is bacteria that can have an impact on public health. Based on the collection of data from 52 sentinel hospitals in North America for 7 years (1998-2004) on contemporary strains of 12,737 strains of pediatric patients under 18 years of age, E. coli as a pediatric pathogen ranks in the top six [1]. Mapanguy et al. [2] also reported that oral antibiotics such as cefixime, amoxicillin, and ciprofloxacin were resistant to E. coli infection about 50-60%. In addition, microorganisms can also cause wound infections and inhibit healing. Some of the bacteria associated with wound infections are Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Streptococcus pyogenes, Proteus spp., Streptococcus spp., and Enterococcus spp. [3,4]. Bacteria is also can be pathogenic in the skin, such as Staphylacoccus, Micrococcus, and Corynebacterium sp. [5]. In relation to dental and oral health, Streptococcus sanguinis and Streptococcus mutans can cause dental caries [6].
Some of the infections mentioned can be treated with antibiotics. However, several studies have proven that antibiotics can cause resistance [7]. Antibiotic resistance occurs due to the use of drugs in large quantities, causing selection pressures on human and natural microbial systems. Microbes can undergo mutations to survive, thereby reducing antibiotic sensitivity [8,9]. Infections due to drug resistance have caused the death of up to 700,000 people every year worldwide [10]. There are several antibiotic resistances, namely vancomycin and teicolanin against S. aureus [11], gentamicin aminoglycoside

Ocimum basilicum
Ocimum basilicum, commonly called sweet basil, is one of the species of genus Ocimum from Asia, Africa, and South America regions [36,37]. O. basilicum can live in different climates and ecology, grows in cool humid areas to tropical areas with the temperatures between 6 and 24 • C, and also favors warm conditions [38]. This plant is the species of Ocimum which is commercially available in the market [39]. It has six different morphologies, namely true basil with green leaves, small-leaf basil, which belongs to green cultivars with short narrow leaves and grows rounded, lettuce-leaf with broad leaves, purple basil A, which has green leaves with purple flowers and stems, purple basil B, where the leaves, flower, and stems are purple, and purple basil C, which has a similarity to purple basil B and has broad listered leaves [40,41]. O. basilicum is 20 to 80 cm tall with glabrous and woody stems, large green leaves, and is broadly elliptical, measuring 2.5 to 5 cm × 1 to 2.5 cm. The flowers are red, pink, or white, with a size of 3 mm, and are arranged in terminal spikes [42].
Traditionally, the fruit of O. basilicum was used as folk medicine against inflammation, diarrhea, worm infestation, and eye-related disease [43]. Leaves and flowers of O. basilicum were used as tonic and vermifuge, and can also be used as a tea to treat nausea, flatulence, and dysentery. O. basilicum contains essential oils that are commonly used to treat colds, seizures, and treatment of wasp stings and snakebites [44]. The polysaccharide component of O. basilicum was traditionally used as cancer treatment in China [45,46]. In South Europe, they used O. basilicum as Mediterranean food, such as the cuisines of Italian and Greek [47].

Ocimum gratissimum
O. gratissimum, with the common name of clove basil, is a species of family Labiate which grows in tropical region, namely India and West Africa [71,72]. It is 1-3 m tall and has leaves that are 3-4 cm × 1-2 cm [73]. The flowers have several colors, such as yellowish white, greenish purple, hairy, calyx greenish purple, brown seeds, and not slimy [74]. In Africa, eastern, central, and western Kenya, O. gratissimum is commonly found in scrub and disturbed highland forests at elevations of 600 to 2400 m above sea level [75].

Ocimum campechianum
O. campechianum is a plant of the Lamiaceae family group which originates from the tropics of South and Central America. This plant is commonly known as "Albahaca de campo" or "Albahaca silvestre", used by local people for traditional medicine or culinary purposes [95,96]. This plant has a height of 1 m and contains essential oil components with two types of aromatic leaves, namely glandular trichomes, peltate, and capitate. In addition, O. campechianum contains components of flavonoids, polyphenols, and tannins [97,98]. Traditionally, this plant is used as a decoction of leaves, ointments, and for the treatment of fever, cough, bronchitis, diarrhea, dysentery, and hypertension. In addition, O. campechianum can also be used as an emmenagogue that helps childbirth [99]. In terms of pharmacological effects, this plant extract is known to have antifungal, antioxidant, antiradical, antiproliferative, and analgesic activities [96,100,101]. In addition, it also has potential as a natural larvicide and pesticide [102]. The data of chemical constituents in O. campechianum can be seen in Table 4, Figure S4.  (45) 1014 (140) 1032 -Neoalloocimene (179) 1140 -Terpinolene (95) 1086 -Limonene (48) -  (103) 1439

Ocimum sanctum
O. sanctum is a plant from the Lamiaceae family, commonly known as basil or tulsi, which is native to India and is widely distributed as a cultivated plant throughout Southeast Asia [106]. It is known as a sacred plant in India and a symbol of purity. It got the name of "Tulasi" from Tulasi Devi, one of the Lord Krishna's eternal consorts. Tulasi was a Gopi who was said to have fallen in love with Krishna and was cursed by his wife Radha. He is very similar to Vishnu [107]. In India, it is used for religious plants and important events such as weddings [108]. O. sanctum is 30-75 cm tall with an herbaceous shape, and is erect, more-branched, and hairy-soft [107]. It has pointed or blunt leaves, the leaves are oval, and the flowers are tightly coiled with a pale red or dark red color [109].

Antibacterial Activity of Ocimum Species
Tests of antibacterial activity against Ocimum species have been carried out on both Gram-positive and Gram-negative bacteria. Based on the data in Tables 6-8, the Ocimum extracts that were widely tested and gave relatively high activity were essential oil extracts. In general, the five Ocimum species have good antibacterial activity. However, based on the inhibition zone data in Table 6     Furthermore, MIC is an antibacterial test used to determine its inhibitory activity. Based on de Aguiar et al. [157], MIC concentrations in the range 101-500 µg/mL have strong activity and in the range 500-1000 µg/mL have moderate activity. As for Table 7, testing of O. basilicum against several bacteria, such as S. mutans, S. aureus, B. cereus, L. monocytogenes, E. coli, P. aeruginosa, and S. typhi, showed MIC values below 100 µg/mL, which means that they provide very strong activity, as well as in O. sanctum. The other three Ocimum species only showed strong to weak activity.   The MBC data were used to show the ability of the compound to kill bacterial growth. Based on the MBC data in Table 8, the species from Ocimum that had the best activity in killing bacterial growth was O. sanctum which could be seen in the MBC values against B. cereus, E. herbicola, and P. putida. Therefore, based on the data of inhibition zone, MIC, and MBC, the Ocimum species that have more potential as natural antibacterials are O. basilicum and O. sanctum.

Interaction of Essential Oils to Bacteria
Essential oils usually contain chemical components in the terpenoid and phenylpropanoid groups. The five Ocimum species were proven to contain many essential oil components, especially in the monoterpenoid and sesquiterpenoid groups, as well as some phenylpropanoids [167,168]. Therefore, studies on antibacterial activity have also been carried out on the essential oil components. Essential oils of a plants can inhibit both Gram-positive and negative bacteria. However, some studies reported that essential oils were more sensitive to Gram-positive bacteria, but others reported more sensitive to Gramnegative bacteria [169]. Based on El-Shenaway et al. [170], essential oils were more sensitive against Gram-positive bacteria. This was because of differences in cell wall structures. In Gram-negative bacteria, there was an external capsule which prevented penetration of essential oils into microbial cells. The capsule is composed of a more complex bacterial cell wall with a 2-3 nm thick peptidoglycan layer, where there is an outer membrane on the outer layer of peptidoglycan. The presence of the outer membrane is one of the differences between the cell walls of Gram-negative and Gram-positive bacteria. Peptidoglycan and this outer membrane have strong covalent bonds to Braun lipoproteins. This causes the hydrophobic structure of the essential oil to more easily penetrate the cell walls of Gram-positive bacteria [169].
The ability of essential oils to inhibit bacterial growth is due to their hydrophobicity. It increases cell permeability and leak cell constituents [171]. Essential oils will cross cell wall and cytoplasmic membrane which arrange a lot of polysaccharide layers, fatty acids, and phospholipids. As a result, the interaction between lipophilic compounds from essential oils and various structures found in cell walls and membranes causes a cytotoxic effect [172]. Furthermore, essential oils can also agglomerate in the cytoplasm and cause damage to the protein and lipid layers [173].
In the cell membrane, there is a process of ATP production. The action of essential oils can affect changes in intracellular and external ATP balance. This results in a disturbance with ion loss and a decrease in membrane potential, a decrease in the amount of ATP, and a collapse of the proton pump. [174,175]. This will compromise vital functions such as energy systems, synthesis of structural macromolecules, and secretion of enzymes for growth [171]. In addition, essential oils can affect the pH conditions of bacteria. The presence of essential oils on the membrane can disrupt pH homeostasis and cause a significant decrease in pH. Then, the membrane will lose its capacity to block protons. This is because at low pH, the hydroxyl groups in essential oils do not dissociate so that their hydrophobicity will increase, resulting in easier interaction with bacterial cell membrane lipids [176,177].
There are several methods for extracting essential oils from Ocimum species, such as hydrodistillation, steam distillation, solvent extraction, enfleurage, cohobation, and maceration [178]. However, this method requires a fairly long process if it is to be consumed simply in the community. As an alternative, the Ocimum plant has been widely used traditionally to cure bacterial infections. Ocimum can be processed into juice to relieve toothache and treat otitis. This plant can also be made into an infusion for mouthwash. Decoction of this plant can provide an anesthetic effect and act as an antiseptic. Decoction of the leaves and stems can treat diarrhea, fever, and inflammation of the mucous membranes of the nose [179].

Conclusions
O. americanum, O. basilicum, O. gratissimum, O. campechianum, and O. sanctum are five types of Ocimum species that have abundant chemical components and antibacterial activity. Based on data on the chemical components of the five Ocimum species, it is known that the most abundant compounds are terpenoids. O. basilicum is the most commonly used and widely available on the market. However, the activity data of the five Ocimum species show that this plant can be a natural product that has potential as a natural antibacterial agent. Data Availability Statement: The study did not report any data.