The composition of the essential oil not only varies in the number of molecules but also in the stereoisomeric types of the extracted molecules, depending on the extraction method used (Vairinhos & Miguel, 2020; Virgiliou et al., 2021). To achieve greater recovery of the essential oil from B. oleracea var. italica seeds, extraction was performed using a rotator after initial recovery was not significant. For this extraction, pure ethanol and hexane were used in a ratio of 1:1:1 as organic solvent to obtain a greater amount of extract and compounds. Table 1 shows the different components of the essential oil of B. oleracea var. italica seeds. Identifying the components of the essential oil of these seeds is important due to the unsuccessful efforts made to characterize it because of the low yields obtained (0.009%).
The composition of the essential oil can be affected by various factors, including environmental and geobotanical conditions (climate, altitude, soil type, precipitation, etc.), cultivation method, harvest time, plant part collected, handling and storage of plant material (dry, refrigerated, etc.), plant age and growth stage, as well as the method of oil extraction (Gimenes et al., 2021). The antimicrobial activity of essential oils has been recognized for many years. The antimicrobial activity of essential oils from Tagetes pusilla, Senecio tephrosiodes, Lepechinia meyenii, Eucalyptus globules, and Cymbopogon citratus, among others, has been particularly studied.
Although there is little information on the structure-activity relationship of these oils, some authors agree that the antimicrobial activity is largely due to the presence of terpenoids (Moein et al., 2015), especially those containing functional groups such as alcohol, aldehyde, and ketone (Porras et al., 2021; Dumas et al., 2021). Brassica species are rich in polyphenols, flavonoids, and glucosinolates (Alves et al., 2016; Paul et al., 2019; Zhang et al., 2019); the antimicrobial activity of extracts obtained with different solvents, such as methanol, ethanol, and acetone, is apparently attributed to phenolic compounds, as shown in Tables 3 and 4.
Furthermore, certain components of essential oils have been observed to have the ability to penetrate bacterial cells and affect their DNA and RNA, disrupting their ability to replicate and ultimately leading to cell death (Pavelková et al., 2014; Kang et al., 2018). These effects can be attributed to the ability of essential oil compounds to intercalate into the structure of DNA and RNA, inhibiting their replication and transcription (Silva et al., 2013; Brun et al., 2019). In addition, some studies have shown that certain essential oils can interfere with the gene expression of bacterial cells, inhibiting the production of proteins essential for their survival (Buchberger et al., 2018; Rúa et al., 2019). [36, 37] These properties make essential oils a promising alternative to conventional antimicrobials and an attractive option for combating antibiotic resistance.
Moreover, it has been demonstrated that essential oils can interfere with protein synthesis, nutrient transport, and DNA replication in bacterial cells (Pavelková et al., 2014; Kang et al., 2018; Wei et al., 2019). For example, thymol, a phenolic compound present in oregano essential oil, inhibits protein synthesis by binding to the 50s ribosomal subunit (Brun et al., 2019; Winska et al., 2019). Another example is eugenol, a phenolic compound present in clove essential oil, which interferes with cell wall synthesis by inhibiting the UDP-N-acetylglucosamine-2-epimerase enzyme (Silva et al., 2013). These mechanisms of action are important in the fight against antibiotic-resistant bacteria and make essential oils a promising alternative as antimicrobial agents.
In summary, essential oils and their components have been shown to have effective antimicrobial activity against various types of bacteria. These compounds act by damaging the integrity of the cell membrane and altering homeostasis, leading to the loss of macromolecules, lysis, and cell death.
The obtained data demonstrate that both major and minor components of the essential oil and extracts could play an important role in the antimicrobial activity. Therefore, new antimicrobial agents based on the essential oil from Brassica oleracea var. italica seeds could be a possible alternative against pathogens. Moreover, since these compounds have been recognized as GRAS by the US FDA, they can be used for food preservation without additional risks.