The objectives of this study were to determine the effect and mode of action of Saccharomyces cerevisiae (YST2) on enteric methane (CH4) mitigation in pigs. A total of 12 Duroc×Landrace×Yorkshire male finisher pigs (60±1 kg), housed individually in open-circuit respiration chambers, were randomly assigned to two dietary groups: a basal diet (control); and a basal diet supplemented with 3 g/YST2 (1.8×1010 live cells/g) per kg diet. At the end of 32-day experiment, pigs were sacrificed and redox potential (Eh), pH, volatile fatty acid concentration, densities of methanogens and acetogens, and expression of methyl coenzyme-M reductase subunit A gene were determined in digesta contents from the cecum, colon and rectum. Results showed that S. cerevisiae YST2 decreased (P<0.05) the average daily enteric CH4 production by 25.3%, lowered the pH value from 6.99 to 6.69 in the rectum, and increased the Eh value in cecum and colon by up to −55 mV (P<0.05). Fermentation patterns were also altered by supplementation of YST2 as reflected by the lower acetate, and higher propionate molar proportion in the cecum and colon (P<0.05), resulting in lower acetate : propionate ratio (P<0.05). Moreover, there was a 61% decrease in Methanobrevibacter species in the upper colon (P<0.05) and a 19% increase in the acetogen community in the cecum (P<0.05) of treated pigs. Results of our study concluded that supplementation of S. cerevisiae YST2 at 3 g/kg substantially decreased enteric CH4 production in pigs.

The digestion of plant biomass by symbiotic microbial communities in the gut of herbivore hosts also results in the production of methane, a greenhouse gas that is released into the environment where it contributes to climate change. As methane is exclusively produced by methanogenic archaea, various research groups have devoted their efforts to investigate the population structure of symbiotic methanogens in the gut of herbivores. In this review, we summarized and compared currently available results from 16S rRNA gene clone library studies, which cover a broad range of hosts from ruminant livestock species to wild ruminants, camelids, marsupials, primates, birds and reptiles. Although gut methanogens are very diverse, they tend to be limited to specific phylogenetic groups. Overall, methanogens related to species of the genus Methanobrevibacter are the most highly represented archaea in the gut of herbivores. However, under certain conditions, archaea from more phylogenetically distant groups are the most prevalent, such as methanogens belonging to either the genus Methanosphaera, the order Methanomicrobiales or the Thermoplasmatales-Affiliated Lineage C Comparisons not only highlight the strong influence of host species and diet in the determination of the population structure of symbiotic methanogens, but also reveal other complex relationships, such as wide differences between breeds, as well as unexpected similarities between unrelated species. These observations strongly support the need for high throughput sequencing and metagenomic studies to gain further insight.