Selective medium for enumeration of tannin-protein complex-degrading Streptococcus spp. in faeces of koalas
Osawa, RO & Mitsuoka, T 1990, Applied and Environmental Microbiology, vol. 56, no. 11, pp. 3609-3611.
The authors of this study developed a selective agar plate medium that can successfully enumerate strains of tannin-protein complex-degrading Streptococcus bovis bacteria in faecal samples from koalas.
It was previously found that the S. bovis biotype I found in faecal samples from koalas grew on a tannin-treated brain heart infusion agar (T-TBHIA) medium and formed a clear zone around the colony. Conversely, biotype II of the same bacterium grew on the T-TBHIA medium but did not form a clear zone around the colony. This finding indicated that, unlike S. bovis biotype II, biotype I can degrade tannin-protein complexes. Tannins are biomolecules that bind to proteins to form chemical complexes and occur in eucalypt leaves in high concentrations. S. bovis thus plays an important role in breaking down these complexes to facilitate the digestion of dietary protein in the koala. The purpose of this study was to develop a selective medium for enumerating the strains of S. bovis biotype I in koala faeces. A Columbia blood agar (BA) was used as a control medium that was not selective for any growth type, and BA treated with a supplement containing colistin sulfate and oxolinic acid (COBA) was used as a control medium that was selective for Streptococcus species. Brain heart infusion agar media were treated with tannic acid and either further treated with the Streptococcus selective supplement colistin-oxolinic acid (COT-TBHIA) or remained nonselective (T-TBHIA). Both the T-TBHIA and COT-TBHIA were equally effective for the growth of S. bovis biotype I, with the addition of colistin-oxolinic acid having no negative effects on colonial morphology or clear-zone formation. Both media also entirely inhibited the growth of other streptococcal species. The COT-TBHIA medium proved superior to T-TBHIA, however, in its ability to inhibit the growth of enterobacteria. The growth of enterobacteria such as Escherichia coli and Proteus mirabilis on T-TBHIA was a significant hindrance to the enumeration of S. bovis biotype I on this medium, and therefore COT-TBHIA was the preferred medium of those trialled.
The identification of a suitable medium for enumerating S. bovis biotype I from koala faeces assists in monitoring tannin-protein complex-degrading activities in the gut microflora of koalas, thereby improving our knowledge of the species’ unique digestive mechanisms.
Summarised by Joanna Horsfall
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