Finding EPIs in Helichrysum italicum EOs that are effective against the efflux pro­cesses of gram-negative bacteria was the aim of the study conducted by Lorenzi et al. [56]. According to additional research, using 2.5% of H. italicum essential oil reduces the minimum inhibitory concentration (MIC) of chloramphenicol for the Enterobacter aerogenes MDR strain EA27 by eight times, from 1,024 to 128 mg/L [52]. The remaining, less active plants reduce the MIC of chloramphenicol by two to four times. This EO was selected for research by Lorenzi et al. (2009) because it was able to reduce EA27’s chloramphenicol resistance to a level equivalent to that of the control phenylalanine arginine-naphthylamide (PAN) [55]. According to this study, chemicals found in H. italicum essential oil target efflux pathways to change drug resistance in a variety of gram-negative bacterial species. The results show that EO reduces the chloram­phenicol MIC for strains of A. baumannii, P. aeruginosa, and E. aerogenes isolates. Fur­thermore, for a strain of E. aerogenes (CM-64) that overproduced the tripartite efflux pump AcrAB-TolC, this EO reduces the MIC of chloramphenicol. Compounds not previ­ously identified as modulators are present in the two most active fractions. Geraniol seemed to be the most potent of these substances at inhibiting efflux pathways. Curi­ously, the same geraniol that was initially tested for resistance to chloramphenicol was also found to be effective in inhibiting resistance to other clinically significant antibiotics, such as β-lactams and the fluoroquinolone norfloxacin. Additionally, an EPI activity ranking shows that geraniol is a more effective inhibitor of resistance in an acrAB mutant than PAN, suggesting that these two substances target different mol­ecules. Together, these results provide a new supply of drugs that may help cure the condition, and geraniol may help us understand MDR in gram-negative bacteria that continue to pose a threat to public health [56].

3. Inhibition of ATP and its enzyme

4. Inhibition of protein synthesis

5. The synergistic effect

1. Cell membrane destruction: Terpenoids primarily destroy the bacterial cell mem­branes by using their lipophilicity. They have bactericidal or antibacterial activities by diffusing inward via the phospholipid bilayer of bacteria [109]. Since the integrity of the cell membrane is crucial to bacterial biological processes, terpenoids’ damage to the membrane will impair the bacteria’s basic physiological functions and lead to the loss of vital components like proteins and enzymes, which will ultimately result in the anti­bacterial effect [110]. Table 12.2 shows some terpenoids that inhibit the growth of micro­organisms through this mechanism.

2. Intercellular communication is a function of the anti-quorum sensing (QS) system [108]. Bacteria use it as a communication tool to coordinate their interactions with other organisms, which is also the primary cause of antibiotic resistance [111]. The literature has presented and provided illustrations of the gram-positive and gram­negative bacteria’s group sensing signal loop [112]. Research has demonstrated that the QS action between bacteria can be efficiently inhibited by a low quantity of cinna­mon aldehyde [113]. QS can be efficiently inhibited by low quantities of carvacrol and thymol, which block the bacterial self-inducer acyl homoserine lactone (AHL) [114].

-ATPase, which targets the cell membrane and causes in­tracellular acidification and cell death [115]. In another study, the researchers used the MIC of carvacrol to treat the target infection. A luminometer (Biotek) was used to test the samples’ levels of extracellular ATP. Based on absorbance analysis at 260 nm, this study discovered that carvacrol harmed the E. coli membrane and that potassium and ATP ions were also discharged [116].