Ecology of tannin-tolerant streptococci in the rumen

Full text: Tannins are polyphenolic compounds that can be found in significant quantities in plant tissue, although the chemical structures are highly variable. In extensive ruminant grazing systems, particularly in developing nations, protein rich forage trees and shrubs are often integrated into grazing systems as protein supplements but these plants often contain tannins which have a negative impact of tannins on ruminal microbiota and nutrient digestibility. Condensed tannins from the pasture legume Lespedeza cuneata inhibit pectinases and cellulases that are often associated with the bacterial cell wall. Ruminal bacteria that are inhibited include Fibrobacter succinogenes, Butyrivibrio fibrisolvens, and Ruminobacter amylophilis and are correlated with ultra-structural changes of the cell wall, while these cell wall changes do not occur with the tolerant Prevotella ruminicola. Other organisms which appear to tolerate tannins, are Streptoccus gallolyticus, Clostridium spp., and some proteobacteria. Tannin tolerant S. gallolyticus 2.2 produces an extracellular polysaccharide (EPS) coat in response to tannic acid and condensed tannins from Acacia angustissima and is probably the electro dense layer typically visible at the cell surface in the presence of tannins. We have examined the molecular basis of tolerance to condensed tannins in this organism, and to do this we measured expression levels of EPS genes in the presence of tannins. We found that there was a significant increase in cellular polysaccharides and EPS gene expression in response to tannic acid, acacia tannin, and calliandra tannin in S. gallolyticus 2.2 but not S. bovis JB1. We also found that tannin tolerant streptococci could be distinguished from tannin sensitive streptococci based on 16S-rDNA oligonucleotide probes. These probes were applied to ruminally extracted rRNA from animals consuming diets containing acacia or calliandra. To examine the response in vivo we fed sheep either A. angustissima, or C. callothyrsus and measured the abundance of S. bovis and S. gallolyticus populations using 16S-rRNA based oligonucleotide probes that differentiated these two populations. The S. bovis population declined significantly when acacia was fed at 37% of the diet but recovered when polyethylene glycol (PEG) was added to the ration; PEG has a neutralising effect on tannins. The S. gallolyticus population was more resistant to the presence of tannins in the diet but this effect was diminished when PEG was added to the diet. Similar population dynamics were observed with sheep fed calliandra at 30% of the ration. In conclusion, it appears that EPS production is correlated with tannin tolerance in vitro. Tannin resistant S. gallolyticus is more abundant in the rumen in the presence of tannins than is S. bovis indicating the presence of tannin resistant organisms may have significant nutritional consequences for the animal. (author)