Metabolism of polyunsaturated fatty acids and their toxicity to the microflora of the rumen

Ruminal microorganisms hydrogenate polyunsaturated fatty acids (PUFA) present in forages and thereby restrict the availability of health-promoting PUFA in meat and milk. The aim of this study was to investigate PUFA metabolism and the influence of PUFA on members of the ruminal microflora. Eleven of 26 predominant species of ruminal bacteria metabolised linoleic acid (LA; cis-9,cis-12–18:2) substantially. The most common product was vaccenic acid (trans-11–18:1), produced by species related to Butyrivibrio fibrisolvens. α-Linolenic acid (LNA; cis-9,cis-12,cis-15–18:3) was metabolised mostly by the same species. The fish oil fatty acids, eicosapentaenoic acid (EPA; 20:5(n − 3)) and docosahexaenoic acid (DHA; 22:6(n − 3)) were not metabolised. Cellulolytic bacteria did not grow in the presence of any PUFA at 50 μg ml⁻¹, nor did some butyrate-producing bacteria, including the stearate producer Clostridium proteoclasticum, Butyrivibrio hungatei and Eubacterium ruminantium. Toxicity to growth was ranked EPA > DHA > LNA > LA. Cell integrity, as measured using propidium iodide, was damaged by LA in all 26 bacteria, but to different extents. Correlations between its effects on growth and apparent effects on cell integrity in different bacteria were low. Combined effects of LA and sodium lactate in E. ruminantium and C. proteoclasticum indicated that LA toxicity is linked to metabolism in butyrate-producing bacteria. PUFA also inhibited the growth of the cellulolytic ruminal fungi, with Neocallimastix frontalis producing small amounts of cis-9,trans-11–18:2 (CLA) from LA. Thus, while dietary PUFA might be useful in suppressing the numbers of biohydrogenating ruminal bacteria, particularly C. proteoclasticum, care should be taken to avoid unwanted effects in suppressing cellulolysis.