Interactions between host and gut microbiota in domestic pigs: a review

Abstract

It is well established that pig gut microbiota plays a critical role in maintaining metabolic homeostasis as well as in a myriad of physiological, neurological and immunological functions; including protection from pathogens and digestion of food materials - some of which would be otherwise indigestible by the pig. A rich and diverse gut microbial ecosystem (balanced microbiota) is the hallmark of good health; while qualitative and quantitative perturbations in the microbial composition can lead to development of various diseases. Alternatively, diseases caused by stressors or other factors have been shown to negatively impact the microbiota. This review focuses primarily on how commensal microorganisms in the gastrointestinal tract of pigs influence biochemical, physiological, immunological, and metabolic processes within the host animal.

Keywords: Gut; host-microbe interactions; immunity; microbiome; pigs.

Figure 1.

A systems biology model for the brain-gut–microbiome interactions in mammals. The interconnected structural networks of the central nervous system influence via the autonomous nervous system to alter microbiota composition and function indirectly by regulating the microbial environment in the gut. The brain communicates with the gut microbiota indirectly through gut-derived molecules via afferent vagal and spinal nerve endings, or directly through microbe-generated signals. Alterations in these bidirectional interactions in response to perturbations like diet, medication, infections, and stress can alter the stability and behavior of this system resulting in brain-gut disorders.

Figure 2.

Factors affecting the gut microbiota in pigs. The diversity of gut microflora can be affected by a variety of factors. Sub-therapeutic doses of antibiotics used in commercial farming can negatively affect the microbiota. Similarly, different stressors such as high temperature, transportation, weaning, and overcrowding can also change the diversity of the microbiota for the worse. The inclusion of probiotics, prebiotics, and fiber appear to nullify these effects and improve diversity. Other factors, such as the age of the animal, its mode of the birth, breed and the environment it lives in, can influence the microbiota. Potentially pathogenic microbes are depicted in red and beneficial (and other commensals) microbes in green. The red arrows indicate a negative impact and a green arrow a positive impact. The gray arrows are an indication that different factors can affect the microbiota differently, either positively (e.g., if animals are bred in a healthy, growth-conducive environment) or negatively (e.g., following cesarean birth, devoid of any natural mother’s microbiota).

Figure 3.

Inducible heat shock proteins. Dietary nutrients, gut microbiota components, and certain diseases can induce the formation of iHSPs (HSP25 and HSP70) in the GIT epithelium either directly from the microbiota or indirectly through the microbiota secretions and/or metabolites. This increases protection for the host against stressors like oxidation or inflammation in the epithelium.

Figure 4.

Intestinal Alkaline Phosphatase (IAP) – roles in the GIT. IAPs secreted by the enterocytes travel bi-directionally into the blood circulation and to the intestinal lumen where they act to dephosphorylate LPS from Gram-negative bacteria. Membrane-bound IAPs also prevent the translocation of the LPS through the mucus layer.