The effect of condensed tannins fromLotus pedunculatusandLotus corniculatuson the growth of proteolytic rumen bacteria in vitro and their possible mode of action
Five strains of proteolytic rumen bacteria were treated with condensed tannins (CT) purified from Lotus pedunculatus and Lotus corniculatus to investigate their effect on the growth of these bacteria in vitro. Streptococcus bovis NCFB 2476, Eubacterium sp. C124b, Prevotella bryantii B14, Butyrivibrio fibrisolvens H17c, and Clostridium proteoclasticum B316Twere tested against 200, 400, and 600 µg CT·mL1extracted from L. pedunculatus and L. corniculatus. In the absence of CT, all bacterial strains showed typical growth and reached maximum optical density (OD) after 68 h of incubation in a plant protein medium. Growth of Eubacterium sp., P. bryantii, and B. fibrisolvens was inhibited (P < 0.010.001) more by the CT from L. pedunculatus than by the CT from L. corniculatus. All strains continued to grow in the presence of 200 µg·mL1of the CT from L. pedunculatus, but attained significantly (P < 0.050.01) lower maximum OD600values than (minus CT) controls, except for S. bovis. At 400 and 600 µg·mL1, the addition of CT from L. pedunculatus inhibited (P < 0.050.001) the growth of all bacterial strains tested compared with controls. The growth of Eubacterium sp. and P. bryantii was stimulated for the first 46 h of incubation (P < 0.001) by 200 µg·mL1of CT from L. corniculatus, but then declined leading to a significant difference in OD values compared with the controls. At 400 µg·mL1, the CT from L. corniculatus reduced (P < 0.050.01) the growth of all strains except S. bovis, while 600 µg·mL1inhibited (P < 0.010.001) the growth of all strains. To study the mechanism of CT action, the degradation of the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; Fraction 1 Leaf protein) was followed after bacterial cells or Rubisco were preincubated with CT extracted from L. corniculatus and L. pedunculatus. Both preincubations decreased LSU degradation, but they differed in their response to polyethylene glycol (PEG) addition. Addition of PEG to CTRubisco preincubations negated the effects of CT, while PEG addition to CTbacteria preincubations did not. This implies that the CTbacterial interaction is stronger than the CTRubisco interaction or the interaction is of a different type. Also, L. pedunculatus CT reduced the degradation of the LSU to a greater extent than the CT from L. corniculatus when preincubated with bacteria.Key words: condensed tannins, growth, in vitro, proteolytic rumen bacteria, mode of action, Rubisco.