**3.2 Essential oils**

Essential oils (EOs) from aromatic and medicinal plants are receiving increased scientific attention because of their long history of being sources of natural antimicrobial substances for the treatment of infectious diseases [55]. Thanks to their natural mixture of very complex chemical composition, EOs have shown a broad range of antimicrobial activities against both Gram-negative and Gram-positive bacterial pathogens and have long been used for respiratory tract infections (**Figure 2**) [56, 57]. For example, respiratory pathogens including *Streptococcus pyogenes*, *S. pneumonia* and *Escherichia coli* were effectively inhibited by EOs of cinnamon bark, thyme and spotted beebalm *in vitro* [58].

Interestingly, EOs exhibit minimal effects on LAB including *Lactobacilli* and *Bifidobacteria* that are commonly used as probiotics [59], suggesting EOs may have limited negative effects on beneficial bacteria within the host microbiota. The EOs showed higher minimal inhibition concentration (MIC) values for the probiotic bacteria, whereas it was effective in much lesser concentration against pathogenic bacteria in gastrointestinal tract [59]. Saguibo et al. [60] reported that some probiotic LAB have selective resistance against inhibitory effect of several plant extracts that displayed a strong inhibition on pathogenic bacteria. These evidences suggest the possible combination of the probiotics with EOs and combat with pathogenic bacteria. Probiotics accomplish their antimicrobial activities mainly through producing bacteriocin. In most cases, the probiotics inhibit proliferation of pathogens by generating acidic environment and thereby lower the chance of pathogens' survival. EOs exhibit their antibacterial effect by inducing morphological changes in the target bacterial cells as well as producing reactive oxygen species (ROS) within the bacterial culture [58]. The EOs can accomplish a complete killing of the pathogens even at a lower dosage as the pathogens are normally lack of counteracting mechanism against the effects of EOs [59]. Combining EOs with probiotics is expected to increase the efficacy of probiotics in controlling the bacterial pathogens owing to their synergistic effect which is normally higher than the two individual effects due to their complementary actions [61].

Immune stimulatory effects of EOs have also been well documented. EOs extracted from *Eucalyptus globules* stimulated the innate cell-mediated immune response [62]. Inhibition of cytokine production and arachidonic acid metabolism by a compound of eucalyptus EO has been observed in human blood monocytes *in vitro* [63]. The same authors also reported the anti-inflammatory effects of eucalyptus EO in bronchial asthma [64]. Likewise, vapors of EOs showed

**25**

**Figure 2.**

*Bovine Respiratory Disease in Feedlot Cattle: Antimicrobial Resistance in Bovine Respiratory…*

anti-inflammatory effect on the trachea and reduced asthma [65]. In addition to the antimicrobial and immune modulation properties, the antibiotic resistance modifying activity of EOs has recently been gaining research interest [66–68]. Some EOs and their major components, such as thymol and geraniol, have shown to improve the efficacy of antibiotics against multidrug-resistant bacterial pathogens and therefore have been suggested to be used as antibiotic adjuvants [67, 69, 70].

Although antimicrobial activity of EOs against human respiratory bacterial pathogens has been well documented, limited information is available with respect to the effects of EOs against bovine respiratory pathogens. We have recently published data showing that EOs inhibit the BRD bacterial pathogens *M. haemolytica*, *P. multocida* and *H. somni* [16]. The EOs of ajowan, thyme and cinnamon leaf completely or partially inhibited these BRD pathogens in both vapor and liquid phases [16, 17]. These EOs did not display any noticeable cytotoxicity to bovine turbinate cells of the upper respiratory tract [17] and also exhibited minimal antimicrobial activity on six commensal *Lactobacillus* strains that were isolated from the nasal pharynx of a healthy feedlot cattle [17]. This suggests that EOs will have limited negative effects on the commensal bacterial community within the bovine respiratory tract, when they are administered to target pathogens. In addition, Kissels et al. [71] evaluated four different EO components, including carvacrol, thymol, transanethole and 1,8-cineole, as antibacterial agents or as adjuvants for the antibiotics doxycycline and tilmicosin against *M. haemolytica* and *P. multocida*. Carvacrol and thymol inhibited the growth of both of these tested pathogens with MIC values ranged from 0.63 to 2.50 mM. These two EO compounds also displayed an additive effect when one of them was combined with tilmicosin. In addition, combination of thymol with dexycycline displayed synergetic effect against tested BRD pathogens. These studies demonstrated that EOs can be used to control bovine respiratory pathogens in feedlot cattle. Volatile nature of EO makes the EO more promising

*3.2.1 Using EOs to mitigate BRD bacterial pathogens*

*The antibacterial properties of essential oils (EOs).*

*DOI: http://dx.doi.org/10.5772/intechopen.81718*

*Bovine Respiratory Disease in Feedlot Cattle: Antimicrobial Resistance in Bovine Respiratory… DOI: http://dx.doi.org/10.5772/intechopen.81718*

**Figure 2.** *The antibacterial properties of essential oils (EOs).*

*Bacterial Cattle Diseases*

**3.2 Essential oils**

beebalm *in vitro* [58].

*Streptococcus pyogenes* and stimulating anti-inflammatory response in epithelial cells [50, 51]. A recent study revealed that the relative abundance of nasopharyngeal LAB in cattle entering the feedlot was significantly greater in animals that remained healthy compared with those that developed BRD [52], suggesting that a certain LAB are important to bovine respiratory mucosal health. Furthermore, an *in vitro* pilot study that we conducted to test antimicrobial properties of commercially available LAB strains (*Lactobacillus*) against *M. haemolytica* demonstrated the possibility of using probiotics to mitigate BRD pathogens [53]. The *Lactobacillus* strains isolated from the nasopharynx of healthy feedlot cattle displayed antimicrobial activity against the growth of *M. haemolytica in vitro* [54]. We also recently reported that the intranasal inoculation of *Lactobacillus* spp. strains inhibits the colonization

of *M. haemolytica* S1 into the nasopharynx of dairy calves challenged with

an alternative to antimicrobial metaphylaxis.

effects due to their complementary actions [61].

*M. haemolytica* S1 (Amat et al., unpublished data). These studies suggest the potential application of probiotics to mitigate BRD bacterial pathogens in feedlot cattle as

Essential oils (EOs) from aromatic and medicinal plants are receiving increased scientific attention because of their long history of being sources of natural antimicrobial substances for the treatment of infectious diseases [55]. Thanks to their natural mixture of very complex chemical composition, EOs have shown a broad range of antimicrobial activities against both Gram-negative and Gram-positive bacterial pathogens and have long been used for respiratory tract infections (**Figure 2**) [56, 57]. For example, respiratory pathogens including *Streptococcus pyogenes*, *S. pneumonia* and *Escherichia coli* were effectively inhibited by EOs of cinnamon bark, thyme and spotted

Interestingly, EOs exhibit minimal effects on LAB including *Lactobacilli* and *Bifidobacteria* that are commonly used as probiotics [59], suggesting EOs may have limited negative effects on beneficial bacteria within the host microbiota. The EOs showed higher minimal inhibition concentration (MIC) values for the probiotic bacteria, whereas it was effective in much lesser concentration against pathogenic bacteria in gastrointestinal tract [59]. Saguibo et al. [60] reported that some probiotic LAB have selective resistance against inhibitory effect of several plant extracts that displayed a strong inhibition on pathogenic bacteria. These evidences suggest the possible combination of the probiotics with EOs and combat with pathogenic bacteria. Probiotics accomplish their antimicrobial activities mainly through producing bacteriocin. In most cases, the probiotics inhibit proliferation of pathogens by generating acidic environment and thereby lower the chance of pathogens' survival. EOs exhibit their antibacterial effect by inducing morphological changes in the target bacterial cells as well as producing reactive oxygen species (ROS) within the bacterial culture [58]. The EOs can accomplish a complete killing of the pathogens even at a lower dosage as the pathogens are normally lack of counteracting mechanism against the effects of EOs [59]. Combining EOs with probiotics is expected to increase the efficacy of probiotics in controlling the bacterial pathogens owing to their synergistic effect which is normally higher than the two individual

Immune stimulatory effects of EOs have also been well documented. EOs extracted from *Eucalyptus globules* stimulated the innate cell-mediated immune response [62]. Inhibition of cytokine production and arachidonic acid metabolism by a compound of eucalyptus EO has been observed in human blood monocytes *in vitro* [63]. The same authors also reported the anti-inflammatory effects of eucalyptus EO in bronchial asthma [64]. Likewise, vapors of EOs showed

**24**

anti-inflammatory effect on the trachea and reduced asthma [65]. In addition to the antimicrobial and immune modulation properties, the antibiotic resistance modifying activity of EOs has recently been gaining research interest [66–68]. Some EOs and their major components, such as thymol and geraniol, have shown to improve the efficacy of antibiotics against multidrug-resistant bacterial pathogens and therefore have been suggested to be used as antibiotic adjuvants [67, 69, 70].
