*3.1.1 Mechanisms of probiotic action*

Probiotics deliver their beneficial effects to the host through direct inhibition against potential pathogenic bacteria, improving the epithelial barrier function, stimulating the host immune system and re-establishing the commensal microbial community [37]. Probiotic bacteria can directly inhibit pathogens by producing

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microbiota.

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

antimicrobial compounds such as lactic acid and bacteriocins. They can also prevent adherence of pathogenic bacteria to the host cells via competitive exclusion, as probiotic strains use the same binding sites as the pathogenic bacteria or downregulate the expression of pathogen-binding sites [37]. Probiotic *Lactobacilli* have also been reported to preserve intestinal epithelial barrier function through stimulating mucin secretion, strengthening tight junction and preventing epithelial cell death

In addition, probiotics have the potential to boost host immune defenses against pathogens by modulating immune response [39]. Probiotics regulate innate and adaptive immune response by modulating immune cells and cytokine production via toll-like receptor-regulated signaling pathways [40]. Probiotic-induced alterations in the functions of dendritic cells, macrophages and T lymphocytes have been documented. For example, probiotic *Lactobacillus* strains modulated dendritic cells and thereby altered cell surface antigen expression and cytokine production in dendritic cells [37, 40]. Additionally, the impact of lactobacillus strains on macrophage function and its TNF-α production capacity has also been reported [41]. Probiotic bacteria *L. acidophilus* influenced the activity of regulatory T cells (Tregs) *in vitro* and *in vivo* in mice [42]. Tregs play a vital role in suppressing inflammation and

Modulating the balance of pro- and anti-inflammatory cytokine production is one of the most important mechanisms through which probiotics protect the host from pathogen-induced injury and inflammation [43]. Probiotic bacteria have induced anti-inflammatory cytokine, IL-10, in dendritic and regulatory T cells [43]. IL-10, also known as the master regulator of immunity to infection, plays an essential role in facilitating the optimal pathogen clearance by inhibiting the activity of Th1 cells, NK cells and macrophages [44]. In addition, probiotics inhibit pro-inflammatory cytokine production and thereby prevent excessive inflammation. Probiotic strain *L. rhamnosus* GG inhibited the LPS-stimulated TNF-α production in murine macrophages [45]. Also, *L. rhamnosus* GR-1 strain has significantly or partially reduced LPS-induced number of proinflammatory cytokines including TNF, IFN-γ, IL-1β, IL-2 and IL-6 in human decidual cells [46]. Of note, the immune modulation properties of probiotics are species- and strain-specific, and therefore, different species or different strains within a species are expected to have different

The role of probiotics on the local microbiota starts to be better understood due to the completion of the human genome project and the development of next generation DNA sequencing platforms that enabled a deeper understanding of the structure and composition of the host microbiome. Studies suggest that probiotics may re-establish the composition of the gut microbiota and confer beneficial effects on the gut microbial communities [47, 48]. However, there is limited information available with respect to the effects of probiotics on respiratory microbiota. A very recently published study showed that the oral probiotics alter respiratory microbiota of healthy cats [49]. The orally ingested probiotics were detected in the respiratory tract of the cat and were also associated with changes in richness and the overall composition of colonizing microbial populations of the respiratory tract. This observation points out that oral probiotics could alter the respiratory

Beneficial effects of probiotics in the prevention and control of human respiratory tract infections have been studied. For example, probiotic strain *Streptococcus salivarius* K12 can mitigate pharyngitis by inhibiting the colonization of pathogen

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

and thereby inhibit pathogen translocation [37].

maintaining immune tolerance.

immunomodulation properties [37].

*3.1.2 Using probiotics to mitigate BRD bacterial pathogens*
