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Microbiological studies of the intestinal microflora of the koala, Phascolarctos cinereus. I. Colonisation of the caecal wall by tannin-protein-complex-degrading enterobacteria

Osawa, R, Bird, PS, Harbrow, DJ, Ogimoto, K & Seymour, GJ 1993, Australian Journal of Zoology, vol. 41, no. 1, pp. 599-609.

Tannin-protein-complex-degrading enterobacteria (T-PCDE) normally occur in the bacterial layer bound to the wall of the caecum in koalas but are dangerously susceptible to antibiotic treatment. T-PCDE are suggested to maintain a symbiotic relationship with the host whereby the bacteria generate health benefits for the koala in exchange for its extensive colonisation of the intricate caecal wall.

  The authors of this study utilised tissue samples from the caecal walls of six deceased koalas to examine the T-PCDE that colonise the caecal wall. One of those koalas was treated with the antibiotic oxytetracycline before its death. It was found that a bacterial layer that includes T-PCDE normally covers both the caecal wall and the proximal colon of the koala. Because T-PCDE are susceptible to antibiotics, the koala treated with oxytetracycline lacked this bacterial layer. In the five untreated koalas, the colonisation pattern of T-PCDE on the caecal wall differed between specimens, with some displaying a sparse distribution of T-PCDE cells within the bacterial layer and others having tightly-packed populations of T-PCDE cells in an area where food remained. The T-PCDE were observed as filamentous rods that were positioned perpendicularly to the caecal surface. This spatial arrangement in combination with numerous folds in the mucous membrane that increase the internal surface area of the caecum allows T-PCDE to colonise the epithelium extensively. In addition to T-PCDE, several other types of bacteria were associated with the epithelium which may also contribute to digestive processes.

  Koalas feed primarily on eucalypt leaves which are rich in tannins that form complexes with proteins. The species, therefore, requires a mechanism by which these complexes can be degraded to facilitate the digestion of dietary proteins. T-PCDE fulfil this function and occur in the alimentary tract of the koala. Based on their observations, the authors of this study suggest that a symbiotic relationship may exist between T-PCDE and the koala. The considerable internal surface area of the koala’s caecum allows the epithelium to be extensively colonised by the bacteria, which in turn provide the koala with the benefits associated with T-PC degradation. These benefits include facilitating the metabolism of dietary proteins as well as harnessing nitrogen from degrading proteins for microbial protein synthesis in the hindgut. The benefits that T-PCDE in the caecum offer to the koala are highlighted by the poor body condition of the koala treated with oxytetracycline which destroyed the T-PCDE layer and consequently reduced the animal’s digestive capabilities.

  The findings of this study contribute to our growing understanding of the role and nature of T-PCDE in the alimentary tract of the koala and are also significant for improving knowledge of how antibiotic treatments can hinder digestion in the species.

 

Summarised by Joanna Horsfall

 

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