Effects of Thymbra capitata essential oil on in vitro fermentation end-products and ruminal bacterial communities

An in vitro trial using batch cultures inoculated with ruminal digesta and incubating two different basal diets [high forage (F) and a high concentrate (C) diet] was carried out to investigate the effects of Thymbra capitata essential oil (NEO) and its main compounds, including carvacrol, cymene, ?-terpinene given alone or in a synthetic combination (SEO), on ruminal fermentation and bacterial community. After 24 h of incubation, primary fermentation end-products (gas, methane, volatile fatty acids (VFA) production, and ammonia concentration) and rumen microbial diversity were determined. Limited but statistically significant inhibition of ruminal fermentation was observed with NEO for the C diet, as total VFA production was reduced (P<0.05) compared to the control. This effect was not detected when NEO was added to the F diet, but SEO and carvacrol decreased (P<0.05) total VFA production in this case. Moreover, NEO and carvacrol decreased (P<0.05) propionate molar proportion when using the F diet. Methane production was not affected (P>0.05) by any of the experimental treatments or diets evaluated. Microbial diversity analysis showed only a moderate effect of carvacrol and SEO on 13 genera, including, among others, Atopobioum and Lysinibacillus, that might be involved in the development of Subacute Ruminal Acidosis. In conclusion, T. capitata EOs trigger only minor effects on rumen microbiota and a slightly inhibited ruminal fermentation even at the lower doses tested in the present study, suggesting limited potential as a feed additive to attain nutritional or environmental benefits.