Aims: To isolate predominant ruminal bacteria that produce trans-10, cis-12 conjugated linoleic acid (CLA) from linoleic acid (LA). Methods and Results: Mixed bacteria from ruminal contents of a cow fed grain were enriched with DL-lactate and trypticase. They produced more trans-10, cis-12 CLA than those that were not enriched (7 vs 2 lg mg protein )1 , P < 0AE05). Enrichments had an abundance of large cocci that produced trans-10, cis-12 CLA from LA. Strain YJ-4 produced the most trans-10, cis-12 CLA (approx. 7 lg mg protein )1 ) and 16S rDNA sequencing indicated that YJ-4 was a strain of Megasphaera elsdenii. Megasphaera elsdenii T81 produced approx. 4 lg trans-10, cis-12 CLA mg protein )1 while strains B159, AW106 and JL1 produced < 0AE5 lg mg protein )1 . The trans-10, cis-12 CLA production of YJ-4 was first order with respect to cell concentration (0-800 lg protein ml )1 ), but kinetics were not first order with respect to substrate concentration. Conclusions: Some M. elsdenii strains produce significant amounts of trans-10, cis-12 CLA. Significance and Impact of the Study: Trans-10, cis-12 CLA appears to cause milk fat depression in cattle fed diets supplemented with grain and polyunsaturated fatty acids, but predominant ruminal bacteria that produced trans-10, cis-12 CLA from LA had not previously been isolated.

The objectives of this study were to identify the factors and procedures responsible for increasing the conjugated linoleic acid (CLA) content in fermented milk. Fourteen lactic acid bacteria were screened for CLAproducing ability using sunflower oil (containing 70% linoleic acid) as a substrate. Among the screened strains, Lactococcus lactis I-01 showed the highest CLA-producing ability. The optimal concentration of sunflower oil for CLA production was 0.1 g/L in whole milk, which accounted for 0.25% of total milk fat. Our results demonstrated that CLA formation in fermented milk could be affected by numerous factors such as bacterial strain, cell number, optimal substrate concentration, and the period of incubation at neutral pH.