**1. Introduction**

The *Boswellia* genus of plants includes about twenty species distributed in northeastern coastal areas of Africa, the Arabic peninsula and the Indian subcontinent. *Boswellia sacra*, a small tree of up to 5 m of height with papery peeling bark and densely tangled branches with leaves clustered at the ends, is indigenous to the southern parts of Oman in the Dhofar region, and is also cultivated in other parts of Oman.

*Boswellia sacra* is well known for its oleo-gum resin named frankincense or olibanum, which is usually harvested from deep incisions made into the tree trunk. Generally, in almost all of the cultures where frankincense is traded, it is used for fragrance and fumigating objects used specifically for a religious purpose. In traditional medicine, frankincense has a unique place amongs<sup>t</sup> remedies for the treatment of many disorders (dermatological, gastric, hepatic, rheumatoid arthritis, etc.) [1].

Investigation of the pharmacological activities of essential oils of different *Boswellia* ssp. resins reveals a wide range of medicinal uses. In particular, *B. sacra* has been used as an analgesic, antioxidant, cardio protective substance and an anti-inflammatory [2–4]. Frankincense essential oils have been commonly used to treat microbial infections from as early as the 11th century, and some authors have studied the effect of frankincense on urinary tract infections [5]. Resin essential oils have an antimicrobial activity against important human pathogens, both bacterial and fungal organisms, such as *Staphylococcus aureus*, *Escherichia coli*, *Proteus vulgaris* and *Candida albicans* [6,7]. *B. sacra* oil also shows significant activity on *Aspergillus parasiticus* and *Aspergillus flavus* growth, and consequently suppresses aflatoxin production [8].

There are various compounds detected in *Boswellia* species based on several factors, which include color, purity, aroma, clump size, age of the tree, season of harvest and the geographical location of the plant source. The essential oil of *B. sacra* contains a high proportion of monoterpenes (97.3%). The common compounds include α- and β-pinene, limonene, myrcene, linalool and others.

Ba¸ser [9] claimed that octyl acetate (39.9%) was the main constituent, followed by 1-octanol (11.9%). Al-Harrasi [10] found limonene (33.5%) and (*E*)-β-ocimene (32.2%) to be the predominant compounds in olibanum from *B. sacra*. Camarda et al. [6] also reported limonene to be the dominant substance, albeit at half the abundance found by Al-Harrasi (18.2%). Furthermore, Camarda identified α-pinene as the second most abundant substance (15.1%). Al-Saidi identified α-pinene as the main volatile component, followed by octyl acetate (13.4%) [11].

Omani resin is commercially available in *B. sacra* different accessions, under the local names Houjri, Najdi, and Sahli or Shaebi (Figure 1), based on different geographic locations in Dhofar from where the resins are harvested. Houjri, the first grade resin, has the lightest color and a larger clump size, is collected from trees growing in the north of the Samhan mountains, and is the most expensive. Najdi, the second resin, has a pale yellow color and is collected from the plateau behind the Dhofar mountains. Finally, Sahli or Shaebi, which also has a darker color, is collected from the valleys and is the cheaper one.

**Figure 1.** Omani commercially available *B. sacra* resin samples: (**A**) Houjri, (**B**) Najdi, and (**C**) Sahli/Shaebi.

In the present investigation, we report on the comparative study of the constituents of frankincense essential oil from three different cultivars grown in various agro-climatic conditions in the Dhofar region in Southern Oman, on the eastern border with Yemen. In addition, the authors compared in vitro antibacterial and antifungal activities of *B. sacra* resin essential oils, with the goal to find new strategies in the struggle against antibiotic-resistant microorganisms.
