Variation in antimicrobial action of proanthocyanidins from Dorycnium rectum against rumen bacteria

The proanthocyanidin polymer fractions of the leaves of the forage legume Dorycnium rectum were analysed by acid catalysis with benzyl mercaptan, NMR and ES-MS. The results showed that D. rectum differs from other temperate proanthocyanidin-containing forage legumes in that the range of polymers extends up to very high degrees of polymerisation. Three fractions were characterised as low, medium, and high molecular weight proanthocyanidin fractions with mean degree of polymerisations of 10.3, 41 and 127, respectively. Epigallocatechin was the most abundant extension unit and the terminating flavan-3-ols comprised largely catechin and gallocatechin units in equal proportions. Formation of thiolyated dimer products showed the interflavan-linkages of the lower molecular weight proanthocyanidins to be predominantly C4-->C8 with a small amount of C4-->C6. ES-MS spectra distinguished lower from higher polymeric proanthocyanidins from M2- to M8(2)-. The antibacterial activity of proanthocyanidin fractions against pure cultures of microbes selected from the ruminal population to represent fibre degrading, proteolytic and hyper ammonia producing bacteria in broth culture was evaluated. The activity of proanthocyanidin fractions against Clostridium aminophilum, Butyrivibrio fibrisolvens and Clostridium proteoclasticum was significantly dependent on their structure but not so against Ruminococcus albus and Peptostreptococcus anaerobius. The latter observation was unique in that they were sensitive to all proanthocyanidin fractions evaluated, even at the lowest concentration (100 microg/ml). The results suggest the effects of the extractable proanthocyanidins on rumen microbes should be considered when evaluating an alternative proanthocyanidin-containing forage source for ruminants, such as D. rectum.