Ruminal biohydrogenation (BH) corresponds to a microbial reduction of dietary unsaturated fatty acid. The linoleic acid (C18:2) BH is divided into three steps: first an isomerisation into conjugated linoleic acids (CLA), then a reduction producing mainly trans-octadecenoic acids (trans-C18:1), and a final reduction producing stearic acid (C18:0). Isomerisations of CLA and trans-C18:1 can lead to a number of positional and geometrical isomers. The control of BH reactions is of interest for researchers because BH directly affects the composition of fatty acids of milk and meat. In order to better understand C18:2 BH and its variations, the development of an enzymatic approach is necessary to ascertain if the action of modulators affects the bacterial enzyme activity or ruminal bacteria. The aim of this study was to investigate the C18:2 BH capacity of ruminal content after inactivation of bacteria by chloramphenicol (Cm), an inhibitor of protein synthesis in prokaryotes. The BH of C18:2 produced mainly cis9,trans11-CLA and trans10,cis12-CLA, and trans11-C18:1 and trans10-C18:1 isomers, as previously described (Jouany et al., 2007). The increase in cis12-C18:1 came from reduction of trans10,cis12-CLA, that of trans6+7+8-C18:1 from the reduction of minor CLA isomers not quantified in this study, like trans8,trans10-CLA (Shingfield et al., 2008). The trans11 pathway was rapid: the cis9,trans11-CLA production was maximal at about 1h of incubation while trans11-C18:1 accumulated throughout incubation. On the other hand, trans10 pathway was slow: trans10,cis12-CLA regularly increased during incubation, so that it was more abundant than cis9,trans11-CLA after 3h incubation, and trans10-C18:1 only began to increase after 2h of incubation. The amount of C18:0 began to increase in the media when trans11-C18:1 concentration was over 0.05 mg/mL. Such evolution of fatty acids involved in C18:2 BH was similar to that reported in vitro with living ruminal microorganisms by Harfoot et al. (1973) and Jouany et al. (2007). So, this enzymatic approach using Cm could be an interesting and valid method to study C18:2 BH, however 3h of incubation were not sufficient to study the final reduction
