Effects of fatty acid oxidation products (green odor) on rumen bacterial populations and lipid metabolism in vitro

This study investigated the effects of green odor fatty acid oxidation products (FAOP) from cut grass on lipid metabolism and microbial ecology using in vitro incubations of rumen microorganisms. These compounds have antimicrobial roles in plant defense, and we hypothesized that they may influence rumen lipid metabolism. Further, they may partially explain the higher levels of conjugated linoleic acid cis-9, trans-11 in milk from cows grazing pasture. The first of 2 batch culture experiments screened 6 FAOP (1 hydroperoxide, 3 aldehydes, 1 ketone, and 1 alcohol) for effects on lipid profile, and in particular C18 polyunsaturated fatty acid biohydrogenation. Experiment 2 used the most potent FAOP to determine effects of varying concentrations and identify relationships with effects on microbial ecology. Batch cultures contained anaerobic buffer, rumen liquor, and FAOP to a final concentration of 100 μM for experiment 1. Triplicates for each compound and controls (water addition) were incubated at 39°C for 6 h. The hydroperoxide (1,2-dimethylethyl hydroperoxide, 1,2-DMEH) and the long chain aldehyde (trans-2 decenal) had the largest effects on lipid metabolism with significant increases in C18:0 and C18:1trans and reductions in C12:0, C14:0, C16:0, C18:1cis, C18:2n-6, C18:3n-3, C20:0 and total branch and odd chain fatty acids compared with the control. This was associated with significantly higher biohydrogenation of C18 polyunsaturated fatty acid. In experiment 2, 1,2-DMEH was incubated at 50, 100, and 200 μM for 2, 6, and 24 h. Increasing 1,2-DMEH concentration resulted in a significant linear increase in C18:1trans-10, trans-11, conjugated linoleic acid, and C18:0 and a linear decrease in C18:2n-6 and C18:3n-3, although the scale of this response declined with time. Microbial profiling techniques showed that 1,2-DMEH at concentrations of 100 and 200 μM changed the microbial community from as early as 2 h after addition, though microbial biomass remained similar. These preliminary studies have shown that FAOP can alter fatty acid biohydrogenation in the rumen. This change was associated with changes in the microbial population that were detected through DNA and branched- and odd-chain fatty acid profiling approaches.