The effect of condensed tannins from Lotus corniculatus on the proteolytic activities and growth of rumen bacteria

Eleven strains of ruminal bacteria (Streptococcus bovis NCFB 2476 and B315, Butyrivibrio fibrisolvens strains WV1 and C211, Prevotella ruminicola 23, Prevotella-like strain C21a, Eubacterium spp. C12b and C124b, Clostridium proteoclasticum B316T, Ruminococcus albus 8, and Fibrobacter succinogenes S85) were used to determine the effect of condensed tannins (CTs) from Lotus corniculatus (LC) on the proteolysis of the large (LSU) and small (SSU) subunit of ribulose-1,5-bisphosphate carboxylase (Rubisco) protein extracted from white clover (Trifolium repens), and on bacterial growth in vitro. The effects of CTs were determined with and without polyethylene glycol (PEG; 1.7 mg PEG/mg CT), which binds and inactivates CTs. Proteolysis of the LSU and the SSU of Rubisco (with 1.5 mg CT/ml) was determined in vitro and measured using a sodium dodecylsulphate polyacrylamide electrophoresis (SDS-PAGE) procedure. Bacterial growth in a plant protein medium containing 0, 50, 100, 200, 400, and 600 μg CT/ml was also measured after 0, 2, 4, 8, 12 and 24 h of incubation. Proteolytic bacterial cultures without CTs or with CTs + PEG degraded LSU and SSU rapidly (6–8%/h), but proteolysis varied markedly between species. Cultures of P. ruminicola C21a degraded both the LSU (11.3%/h) and SSU (9.8%/h) when grown in the presence of 1.5 mg CT/ml. Bacterial strains S. bovis B315 (11.5%/h) and strain NCFB 2476 (10.6%/h), Eubacterium sp. C12b (5.1%/h) and C124b (5.5%/h) appeared to degrade SSU more effectively than the LSU (2.6, 2.3, 3.8, and 2.6%/h, respectively) in the presence of 1.5 mg CT/ml. F. succinogenes S85, R. albus 8, P. ruminicola 23, C. proteoclasticum B316T, B. fibrisolvens WV1 and C211a had low (0.3%/h) to moderate (3–4%/h) rates of proteolysis when CT was included in the medium. In the absence of CT, all bacterial strains showed typical growth. Addition of 200, 400 and 600 μg CT/ml significantly (P < 0.05) reduced the growth rate and maximum optical density of most bacterial strains tested compared to the minus CT controls. Some strains (C. proteoclasticum B316T; P < 0.05 and R. albus 8; P = 0.09), however, showed transient increases in their growth rate at low (50–100 μg/ml), but not at high (>200 μg/ml) concentrations of CT. It was concluded that CTs from LC reduced the rate of proteolysis and inhibited the growth of proteolytic rumen micro-organisms.