**2. Results**

In Table 1, the essential oil yields from three different frankincense gum cultivars, called Najdi, Sahli and Houjri, are shown. The samples were subjected to three successive hydrodistillations (2, 4, 6 h) to obtain three corresponding essential oils that were called Grade 1, Grade 2 and Grade 3. The essential oils from *B. sacra* oleogum resins were obtained in yields of 8.92–12% (*w*/*w*) in Grade 1. Regarding Grade 2 essential oils, Najdi recorded the highest percentage of 2.20% (*w*/*w*), followed by Sahli, of 2.08% (*w*/*w*). Grade 3 showed the lowest yield percentages of 0.40–0.72% (*w*/*w*) of essential oils (Table 1). However, Grade 1 essential oil samples recorded much higher yield values in comparison to values mentioned in the literature [11]. The essential yields of hydrodistilled *B. sacra* oleo gum resin are comparable to a previous study [10].

**Table 1.** Percentage yields of essential oils obtained from three different frankincense gum cultivars called Najdi, Sahli and Houjri, subjected to three successive hydrodistillations.


All the essential oil samples were investigated and compared for the presence of volatile components, see Table 2. The data obtained from gas chromatography–mass spectrometry (GC/MS) analysis of Grade 1 essential oil indicate the presence of more than 30 components. These chemical components were grouped into three categories, namely monoterpenes, diterpenes and sesquiterpenes.

**Table 2.** Chemical constituents identified in Grade 1, 2 and 3 (respectively 2, 4 and 6 h hydrodistillation time) oleogum resin essential oils. RI = retention index relative to C8–C36 *n*-alkanes on ZB-WAX column, MS = NIST and Wiley library and the literature, tr. = trace (<0.01%).



**Table 2.** *Cont*.

#### *2.1. Composition of Resin Essential Oils*

In the case of the cultivar Najdi, a total of 23 active components were identified in the Grade 1 essential oil by GC/MS analysis. The highest percentage was reported for α-pinene, followed by δ-3-carene, camphene and β-pinene (79.59%, 9.94%, 3.23%, and 2.39%, respectively). In the Grade 1 essential oil of Sahli, GC/MS data showed that 29 constituents were present in the essential oil. The major components were α-pinene (78.69%), sabinene (7.78%), camphene (2.66%), limonene (2.31%) and β-pinene (2.66%). In the case of Houjri, GC/MS reports 37 volatile components, which is the highest number compared to Najdi and Sahli with α-pinene, followed by sabinene, camphene, β-pinene, δ-3-carene and cis-verbenol (71.09%, 7.63%, 3.00%, 2.17%, 2.16% and 1.85%, respectively). Data are shown in Table 2.

In Grade 2 essential oil, a sensible reduction of monoterpenes was reported in comparison to Grade 1 essential oil. In all cultivars the major components α-pinene, camphene, β-pinene and δ-3-carene decrease, and the sesquiterpens β-elemene and β-eudesmene increase with a successive 4 h of hydrodistillation.

Grade 3 essential oils show a% reduction in monoterpens (α-pinene, camphene, β-pinene and δ-3-carene) in three cultivars, while at the same time the content in % of sesquiterpenes (β-elemene, β-eudesmene, γ-cadinene) is increased. The highest percentage was recorded for α-pinene. Table 2 shows the results.

#### *2.2. Antibacterial Activity of Essential Oils*

All the essential oils from the different frankincense cultivars were tested against two relevant pathogens, *S. aureus* and *P. aeruginosa*, included in the global priority list of antibiotic-resistant bacteria from WHO/OMS [12], and against some reference strains of dermatological interest: *Staphylococcus epidermidis* ATCC 12228, *Staphylococcus hominis* ATCC 27,844 and *Propionibacterium acnes* ATCC 11827. The results are expressed in terms of minimal inhibitory concentration (MIC) in percentage *v*/*v* or in mg/mL and are reported in Table 3. The results indicate that Grade 2 essential oil of Najdi showed an MIC of 52 mg/mL for *S. aureus* ATCC 6538, *P. aeruginosa* ATCC 9027 and *P. aeruginosa* ATCC15442. The essential oil of Grade 2 Sahli was active against all reference strains of *S.aureus* and *P. aeruginosa*, with MIC values ranging from 440 to 110 mg/mL. It showed the best MIC values, ranging from 6.16 to 0.264 mg/mL, against bacterial strains of dermatological interest *S. epidermidis*, *S. hominis* and *P. acnes*.


**Table 3.** Antibacterial activity of essential oils expressed in terms of minimum inhibitory concentration (MIC) in %*v*/*v* or mg/mL (between brackets).

#### *2.3. Antifungal Activity of Essential Oils*

The antifungal activity was determined against *C. albicans* ATCC 10,231 and *Malassezia furfur* ATCC 14521. All tested samples showed an interesting antifungal activity against *C. albicans* and *M. furfur*, with MIC values ranging from 54.56 to 0.240 mg/mL. In particular, Grade 2 essential oil from Najdi and Grade 1 essential oil from Sahli showed MIC values at the lowest tested concentration corresponding to a percentage *v*/*v* of 0.03 (≤0.252 mg/mL) against both fungal pathogens. The data were shown in the Table 4.

**Table 4.** Antifungal activity MIC values in %*v*/*v* and in mg/mL between brackets.

