Interrelations Between Probiotics, Gut Microbiota, Intestinal Barrier, and Immune Response; Focusing on Diarrhea in Dairy Calves1

The interplay between gut microbiota and host health has attracted significant interest in the animal science community. Maintaining gut microbiota homeostasis by supplementing probiotics to treat clinical conditions like calf diarrhea is an emerging area of research nowadays because of increased concerns regarding antimicrobial resistance (AMR) and drug residues in animal products. Probiotics reduce the incidence of calf diarrhea by increasing the gut microbiota diversity and richness with more commensal bacteria such as Lactobacillus and Bifidobacterium that produce antimicrobial compounds, as well as modulating the immune response by increasing cytokines, Interleukin-2 (IL-2), IL-4, IL-6, IL-10, and reducing tumor necrosis factor-α (TNF-α), by increasing production of antibodies, especially immunoglobulin E (IgE), also IgG, differentiating naïve Th lymphocytes (Tho) into Th1, hence stimulate innate immunity and prime the adaptive immune response. Specific probiotic strains of bacteria and yeast (Saccharomyces cerevisiae) derived probiotics maintain the integrity of the intestinal barrier. In this review, data is being organized to address the role of probiotics in treating calf diarrhea by modulating gut microbiota and stimulating an immune response against notorious pathogens, to present animal and veterinary scientists and nutritionists with a new concept to treat infectious diseases from the perspective of the gut microbiota, increasing animal health, performance, and welfare. In conclusion, health status and gut microbiome are strongly interlinked. Research data indicated a significant reduction in the incidence of diarrhea after probiotic administration. If interrelations between probiotics and existing gut microbiota are explored more quantitatively, novel antibiotic substitutes can emerge in the future.