Tolerance mechanisms of streptococci to hydrolysable and condensed tannins

In vitro growth assays demonstrated that Streptococcus bovis JB1 was sensitive to tannic acid (TA), acacia (AT), and calliandra tannins (CT), while Streptococcus gallolyticus 2.2 was tolerant. At low levels of tannin, S. bovis JB1 exhibited some tolerance but this was always less than for S. gallolyticus 2.2. Exopolysaccharide (EPS) has been hypothesised to be produced by S. gallolyticus in response to tannins and may form a protective layer around the cells. The study found that there was a significant increase in cellular polysaccharides and EPS gene expression in response to TA, AT, and CT in S. gallolyticus 2.2 but not S. bovis JB1. To examine the response in vivo, sheep were fed either Acacia angustissima, or Calliandra callothyrsus and the abundance of S. bovis and S. gallolyticus populations was measured 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 300 g kg−1 diet. 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 that the presence of tannin-resistant organisms may have significant nutritional consequences for the animal.