1. The hydrogenation of a range of double-bond positional and configurational octadecenoic acid isomers (cis (δ2 and δ4 to δ13) and trans (δ2 and δ5 to δ13)) to stearic acid by a rumen Fusocillus sp. were examined.
2. The cis and trans δS to δ13 isomers were all hydrogenated to some extent by late-log-phase cultures added to suspensions of individual isomers and incubated for a further 3 h. Of the cis-isomers, δ5 to δll (79–73% conversion to stearic acid) were the preferred substrates. δ24s- (30%) and δ13-cis-isomers ( 5 % ) were poorly hydrogenated. Of the trans-isomers, δ8, δ9 and δ10 were 45% converted to stearic acid, the other isomers were poorly hydrogenated. These results are in agreement with less extensive studies using sheep rumen micro-organisms.
3. When cultures were grown from small inocula in media containing individual isomers more extensive hydrogenation was found than with late-log-phase cultures. At 24 h, cis δ2, δ4 and δ5 gave the highest conversions to stearic acid (90%) followed by the cis δ6 to δ12 and trans δ8 to δ10 isomers (approximately 75%), although at 6 and 12 h δ9-trans gave higher yields of stearic acid than δ9-cis, probably because the growth of the cis cultures showed a longer log-phase.