Comparative studies on the metabolism of linoleic acid by rumen bacteria, protozoa, and their mixture in vitro.

Linoleic acid was differentially catabolized by the various rumen microbial fractions, such as rumen bacteria (B), protozoa (P), and their mixture (BP). The predominant isomer of conjugated linoleic acids (CLA) synthesized by B, P, and BP from linoleic acid was 9c11t-CLA. The formation of 9c11t-CLA was higher (P < 0.05) in P suspension (53.6 μg/mg microbial nitrogen) compared with B (38.3 μg/mg microbial nitrogen) and BP (28.8 μg/mg microbial nitrogen) suspensions by 12 h of incubation. The second most abundant CLA isomer was 10t12c. The accumulation of 10t12c-CLA in BP suspension was 2.3 times lower (P < 0.05) than that in B suspension (84.8 μg/mg microbial nitrogen) by 12 h of incubation. The accumulation of 10t-18:1 in BP suspension during 6- and 12-h incubation periods were not different (P > 0.05) than that in B suspension (6.8 and 14.0 μg/mg microbial nitrogen, respectively). However, the accumulation of 11t-18:1 in BP suspension at 6- and 12-h incubations were 2.7 and 3.3 times higher (P < 0.05), respectively, than that in B suspension. There were no significant accumulations of 11t-18:1, 10t-18:1, and 18:0 in P suspension throughout the incubation period. It was concluded that B, P, and BP metabolized linoleic acid to different isomers of CLA, whereas B, including BP, was only capable of biohydrogenating the CLA isomers to 18:0 by the reduction of 18:1 isomers. P was incapable of biohydrogenating LA, but its association with B in the BP suspension altered the biohydrogenation of LA significantly compared with B alone.