Methyl-mannosylerythritol lipid (MEL), a new sugar esterified lipid synthesized by Pseudozyma aphidis, was assessed for its functionality in modulating rumen fermentation and microbiota toward more propionate and less methane production. A pure culture study using rumen representatives showed that MEL selectively inhibited the How to cite this article: Okuhira K, Koike S, Ito S, Kobayashi Y. The bio-surfactant mannosylerythritol lipid acts as a selective antibacterial agent to modulate rumen fermentation.