Metabolism of tannin-protein complex by facultatively anaerobic bacteria isolated from koala faeces
Osawa, R, Walsh, TP & Cork, SJ 1993, Biodegradation, vol. 4, no. 1, pp. 91-99.
Isolates of tannin-protein complex-degrading enterobacteria (T-PCDE) and Streptococcus bovis biotype I from koala faeces can degrade protein complexed with the hydrolysable tannin gallotannin, but cannot degrade protein complexed with the condensed tannin quebracho. These bacteria could also metabolise gallic acid to pyrogallol, as could the faecal isolates Klebsiella pneumoniae and K. oxytoca, although these strains could not degrade tannin-protein complexes (T-PCs). None of the strains examined were able to further degrade pyrogallol to phloroglucinol, although pyrogallol did not occur in fresh koala faecal samples.
Tannins are biomolecules that bind to proteins to form chemical complexes that are highly resistant to degradation by digestive enzymes in the mammalian gut. They occur in various plants including eucalypts in high concentrations. Tannins can be categorised into two groups according to their structure: hydrolysable tannins, which contain carbohydrates at their core that can be esterified by phenolics like gallic acid; and condensed tannins, which have a more complex structure, cannot be hydrolysed, and contain no carbohydrates. In koalas, the bacteria S. bovis biotype I and T-PCDE are both known to play an important role in degrading protein complexed with hydrolysable tannin to facilitate digestion of dietary proteins. It wasn’t previously known, however, whether the same bacteria could degrade protein complexed with a condensed tannin or, more broadly, how the process of T-PC degradation by these bacteria occurred. In the context of previous studies that have described some metabolic pathways for the degradation of T-PCs, those of several facultatively anaerobic bacteria within koala faeces were examined in this study. As it is known that the enzyme tannase can hydrolyse gallotannin, the degradation of protein complexed with gallotannin by T-PCDE and S. bovis in this study indicates that these strains exhibit positive tannase activity. None of the strains examined in this study were able to transform pyrogallol to phloroglucinol, yet pyrogallol was not found in koala faeces despite the animal consuming this toxin in high levels. This implies that pyrogallol may be degraded by other types of microflora and that this process occurs either in the animal’s alimentary tract or the liver.
The findings presented by this study support the notion that some hydrolysable tannin escapes degradation in the koala’s stomach and continues along the alimentary tract, in which different microflora cooperate in T-PC degradation and the metabolism of the phenolics that are produced as a consequence.
Summarised by Joanna Horsfall
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