## 3 Biological Activities
### Antibacterial Activity
Its ability to disrupt bacterial virulence factors and inhibit quorum-sensing mechanisms underscores its potential as a valuable option in the fight against infections of this bacteria. Citral antibacterial activity was evaluated against four clinically isolated strains of bovine mastitis pathogens, including _S. aureus_, _S. agalactiae_, _B. cereus_, and _E. coli_. The results showed that _S. agalactiae_ and _B. cereus_ are more susceptible to citral than _S. aureus_ and _E. coli_. Citral appears to have multiple targets in the bacterial cell, depending on the concentration and number of its components [48]. Another study evaluated the growth kinetics of _L. innocua_ and _L. monocytogenes_ exposed to citral-carvacrol combinations. The terpene combinations exhibited antibacterial activity against both bacteria, and the effects depended on the concentration of terpen in the culture medium. When terpene-treated _L. innocua_ and _L. monocytogenes_ were incubated, significant lag phase and growth rate differences were observed between low and high inoculum concentrations, indicating that the inoculum level should be considered in modeling studies. When bacterial cells were exposed to terpen, the proportion of sublethal injured cells increased with increasing terpene dose, showing that citral combined with carvacrol used at 25% MIC can control the growth of _Listeria_ species. [51]. Possible mechanisms that could lead to synergistic effects between citral and carvacrol are that citral and carvacrol target different sites in the bacterial cell; (i) carvacrol increases the permeability of the outer membrane, which makes it easier for citral to enter the cytoplasm and interact with proteins and nucleic acids; (ii) the combination of the two substances could improve their solubility or bioavailability and thus enhance the antibacterial effect [61]. Citral was also tested against _Salmonella enterica_ serovar Typhimuriumiumium at sublethal concentrations (MIC = 3.1 mM) and found that citral exerts changes in the fatty acid composition of the membrane, showing limitation of the cyclization of unsaturated fatty acids to cyclopropane fatty acids when cells enter the stationary phase and saturation of the bacterial membrane [53]. The combination of citral with other natural compounds, and in some cases, antibiotics, holds considerable significance in antimicrobial therapy for several reasons. Firstly, it can enhance the antimicrobial efficacy, potentially synergizing the action against a broader spectrum of pathogens. Secondly, this approach may help reduce the risk of antibiotic resistance development, as it can allow for lower antibiotic doses, reducing selective pressure. Thirdly, it offers the potential to simultaneously target multiple mechanisms of bacterial resistance, making it more challenging for bacteria to adapt. Moreover, combining natural compounds like citral with antibiotics can mitigate the adverse side effects associated with high antibiotic doses, thereby improving the overall safety profile of the treatment. In this sense, these combinations can broaden the therapeutic options for infectious diseases, especially in cases of multidrug-resistant pathogens, providing alternative strategies for effective treatment. Nonetheless, it is crucial to conduct rigorous research to assess the safety and efficacy of such combinations before clinical application, considering potential interactions and side effects. The mechanism of action of citral, a monoterepenic aldehyde, is likely similar to that of other aldehydes. In general, it is known that at low concentrations, an aldehyde can cross link amino groups in the cell wall and cytoplasm and inhibit enzymes with a thiol group in the cytoplasmic membrane. Citral in high concentrations could target the cytoplasm, causing coagulation and precipitation of the cytoplasmic components. At intermediate concentrations, aldehydes can also cause cytoplasmic clotting [48].