depending on genotype, age, environmental factors, diet and the use of antimicrobials. However, despite this variation, ~90% of the contributing organisms are of the phyla Bacterio- detes and Firmicutes. The gut microbiota can profoundly influ- ence the health of the host and it has been implicated in many disease states, including insulin resistance, pancreatic disease and cardiovascular disease, to name but a few conditions. Ani- mals are born with no effective gut microbiota and inoculation and colonisation begins to occur immediately after birth. Col- onisation both educates the immune system and moderates its reaction to antigens, while providing nutrients, such as some vitamins and amino acids, as well as short-chain fatty ac- ids. It is well established that the gut microbiota can have a significant role in food-component digestion and absorption and can have a positive effect on energy release from the diet.

A diverse population of commensal bacteria can inhibit en- teropathogens colonising and infecting the gut; this is called ‘colonisation resistance’. Effective colonisation resistance re- quires a highly diverse and complex microbiota, and animals with low diversity are likely to be more susceptible to enteric pathologies. Reduced gut microbial diversity increases the risk of colonisation of pathogenic bacteria, resulting in gut in- flammation and a potential systemic response. There is evi- dence to suggest modulation of the immune function by the gut microbiota. Gut microbes modulate the expression of cer- tain receptors in the gut that affect gut permeability. Imbal- ances in gut microbiota can lead to increased gut permeabili- ty and unregulated proinflammatory cytokines, as well as metabolic endotoxaemia and insulin resistance.

Factors affecting the gut microbiota These can be broadly categorised into three classes: host-asso- ciated, biotic and abiotic. Host-associated factors include the host’s own genetic make-up, and heritable taxa have also been

Figure 2. Illustra- tion of changes in microbial di- versity with anti- biotic use fol- lowed by rehabilitation.

identified. The host’s immune system also plays a role. By far, the most influential factors are environmental and the food source. Host lifestyle and diet play a key role in the develop- ment and maintenance of the gut microbiota, with environ- mental sources of microbes contributing to colonisation imme- diately after birth. Diet and medication have a significant impact on the microbial community within the gut, with medi- cations accounting for the most significant variation between individuals. For example, diet is the major determinant of ru- men bacterial community structure. Medication − including, but not limited to, antibiotics − has a significant impact on gut microbiota and, subsequently, host health. Antibiotic use re- sults in changes in the intestinal microbiota (Figure 2) and a re- duction in diversity and complexity due to their non-specific action. Subsequently, colonisation resistance decreases and animals are rendered more susceptible to enteric pathogens. This reduction in colonisation resistance has been implicated in one of the greatest threats of this and subsequent generations: antimicrobial resistance (AMR). Worryingly, even with a decline in the use of antimicrobials, there is still antibiotic resistance in animals that have not received antibiotic growth promoters (AGP), and thus simple restrictions are not enough to promote microbial diversity and combat resistance (Figure 2).

The rumen microbiota is a diverse and complex ecosystem that dictates rumen health and function. Rumen bacteria are probably the microbial group that has been studied most. However, it is only in the last few years that we are starting to get a picture of the extent of the diversity and nature of ru- men bacterial strains. The gut microbiota has a significant in- fluence on host health, immunity and physiology and can play a role in helping to reduce reliance on antimicrobials. However, a diverse and complex microbiota is required in or- der to maximise resistance to colonisation by pathogenic species and reduce the susceptibility to disease.


▶ ALL ABOUT FEED | Volume 28, No. 8, 2020


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