Essential oils (EOs) derived from plants are promising antimicrobial compounds, with over ~300 commercially available EOs. Many EOs (e.g., cinnamon oil, clove oil and lavender essential oil) exhibit antibacterial, antibiofilm and antifungal properties which have a wide range of applications in the food and dietary supplement industry [91,92,93,94,95]. EOs are also reported to prevent biofilm formation on abiotic surfaces, which has encouraged the development of alternative disinfection strategies, targeting contaminated surfaces and equipment used in food processing [96,97,98,99]. Moreover, EOs have been added to animal feed and water as taste enhancers for livestock nutrients and as growth promoters, particularly in poultry and porcine farming [94,100,101]. Here, we describe some compounds that exhibit promising antimicrobial and antibiofilm activities against campylobacters.

Cinnamon oil (Cinnamomum cassia) and clove oil (Eugenia caryophyllus) are reported to have bioactive compounds such as cinnamaldehyde (CA), eugenol (EG) and carvacrol (CR) [92]. These compounds act as antimicrobial and antibiofilm agents against many pathogens including P. aeruginosa, Salmonella Typhimurium, Streptococcus mutans and Listeria monocytogenes [102,103,104,105]. CA, EG and CR also exhibit an ability to significantly decrease Campylobacter spp. biofilms and remove the biofilms from stainless steel and polystyrene surfaces [71,72,73,74,106]. Several studies revealed the effectiveness of CR to reduce C. jejuni in vitro and in vivo [107,108,109,110,111,112]. For instance, Wagle et al. [106] found that the minimum inhibitory concentration (MIC) of CR (at 0.002%) was able to reduce the C. jejuni adhesion to primary chicken enterocytes (in an in vitro model of chicken intestinal physiology) up to 1.5 log cfu/mL as compared with control. Interestingly, CR downregulated the expression of C. jejuni colonisation factors, critical for persistence in the chicken gut, such as chemotaxis (aspartate chemoreceptor, CcaA), interactions with host cells (aspA) and anaerobic respiration (NapB). Similar to that, šimunović et al. [112] demonstrated that CR (MIC 0.0032%), as a pure compound or in synergistic combinations with thymoquinone, and rosmarinic acid, not only has antimicrobial activity against C. jejuni but also can increase the antibiotic susceptibility of C. jejuni by inhibiting the efflux pump activity. Unfortunately, further attempts to determine antibacterial properties of CR against C. jejuni using the broiler chicken model were inconsistent. Arsi et al. [113] reported that CR supplemented feed at 0.5–1% could significantly reduce Campylobacter counts in broiler chicks, either alone or in combination with thymol. However, their results could not be replicated in other trials, reportedly due to absorption of those compounds before they reach their target, the small intestine and caeca of chickens, or effects of other enteric microflora [109]. To improve the in vivo outcomes, Allaoua et al. [109] used a CR-based product, solid galenic CR formulation, designed to delay the CR release to allow it to reach the caeca of broiler chickens in order to control C. jejuni. This new formulation was aimed to preserve the antibacterial efficacy of CR against C. jejuni by allowing CR to reach the caeca and large intestine at an effective concentration (at MIC 0.02 mg/mL), which significantly decreased the C. jejuni caecal load (by 1.5 log). Kelly et al. [108] also reported that CR was able to reduce Campylobacter cell adhesion and invasion of chicken intestinal primary cells and also biofilm formation in vitro. They also showed that CR was able to delay colonisation of chicken broilers by inducing changes in gut microflora. Campylobacter spp. was only detected at 35 days of life in the treatment groups compared with the control group where the colonisation occurred at 21 days. Reducing the number of campylobacteria in the chicken intestine is a goal of most studies as quantitative risk assessment models indicate that a reduction of C. jejuni numbers on a broiler carcass by 100-fold (or 2 log units) could result in a significant reduction, by 30 times, in the incidence of campylobacteriosis [114]. Even a relatively small reduction in C. jejuni numbers in the chicken cecum by 1 log₁₀ CFU can reduce the public health risk by more than 50% [8]. In addition, CR had a significant effect on E. coli numbers in the cecum of the chickens in treatment groups. Similarly, Szott et al. [111] found that CR additive could reduce C. jejuni counts in vivo by 1.17 log (up to 28 days of age); however, CR did not successfully reduce Campylobacter caecal colonisation in 33-day-old broilers. Interestingly, addition of CR to the diet decreased feed intake increased feed conversion rates and body weight at all levels of supplementation [115]. Similarly, combining basic diet with cinnamon oil (0.3 g of cinnamon oil per kg) could enhance daily weight gain of broiler chickens by 5.1% [116]. One more potential advantage of using CR is its effect on probiotic bacteria where the additional proliferation of probiotic bacteria such as Lactobacillus and Bifidobacteria spp. has been proposed to be a potential mechanism of inhibiting avian colonisation by disease-causing organisms such as Campylobacter spp. [91,117]. The important benefit, all studies agree, is that CR is safe to use as a dietary supplement in the chicken diet and could improve poultry health, feed efficiency, and delay Campylobacter colonisation in chickens.

Lavender essential oil (LEO) has antiviral activity against Herpes simplex virus type 1 [118]; antibacterial activity against piperacillin-resistant E. coli J53 R1, chloramphenicol-resistant L. monocytogenes L120, S. aureus MRSA and P. aeruginosa [119,120,121,122]; and antifungal activity against Aspergillus niger and Aspergillus tubingensis [123]. LEOs also show an antibiofilm activity against C. jejuni with MIC ranged from 0.2 mg/mL to 1 mg/mL [124]. LEOs were reported to downregulate a range of genes (i.e., Cj0719c, kpsS, lgt, maf4, waaC and Cj1467), involved in the initial attachment of Campylobacter spp. cells to abiotic and biotic surfaces. Adaszynska et al. [122] have evaluated the effect of LEO on chicken production by adding LEO to drinking water given to broiler chickens. The results of the experiments not only showed a significant inhibition of microbial growth, but also a significant increase in the body weight of the chickens in the groups receiving LEO as compared with the control group. Similarly, juniper essential oil (JEO) had shown potent anti-adherent effects against C. jejuni [67,74,76,125], where flavonoid-rich fractions from juniper, at 1 mg/mL, were able to inhibit attachment of C. jejuni cells to polystyrene by up to 70–99%, and reduced the invasion of INT407 cells by 76%. α- and β-pinene are another example of essential oil components from Alpinia katsumadai seeds that can have antimicrobial, antimalarial, and antioxidant effects [77,126,127,128]. The antimicrobial activities of (-)-α-pinene were reported against Campylobacter spp. in vitro; however, (-)-α-pinene alone showed a low efficacy with MIC₅₀ > 500 mg/L required to inhibit 50% of the strains, but when (-)-α-pinene was combined with antibiotics ciprofloxacin and erythromycin, strong potentiating effects against different Campylobacter strains were observed. The concentrations of antibiotics could be decreased from 1 mg/mL to 0.002 mg/mL for ciprofloxacin, and from 512 mg/mL to <1 mg/mL for erythromycin [129]. Possible applications of such natural compounds could be in food packaging to maintain food quality and reduce cross-contamination, or as feed additives to increase weight gain of chickens and by reducing the costs associated with antimicrobial feed additives.