Based on cryo-SEM, standard and high resolution TEM, glycoconjugate histochemistry, and microbiological differentiation, the present study demonstrates the colonisation of the epithelium of the equine oesophagus with microorganisms. As particularly apparent using cryo-SEM to illustrate natural conditions, the present microbiota were clearly dominated by bacteria, forming a one-layer system, as attached to and embedded in concentrated mannose/mannan substances covering the outer stratum corneal cells. Bacterial numbers ranged from 5600 to 7200 per mm(2) in the central part of the oesophagus, the number of fungi was less than 1% of the amount of bacteria. The compact stratum corneal cells showed numerous short protrusions sometimes as part of desmosomal contacts, but mainly projecting into distinct intercellular spaces, containing a mixture of acid and neutral glycoconjugates. The outermost corneal cells exhibited intact mitochondria and cytoplasmic vesicles, and a number of short cell processes toward the oesophageal lumen; i.e. into the glycoconjugate layers on the surface of the oesophagus. The diverse spectrum of bacteria found indicated a permanent mucosal flora, predominated by facultative and obligate anaerobic species. The genera isolated most frequently and in highest numbers included streptococci, Prevotella spp., Fusobacterium spp. and Actinobacillus equuli. Only two groups of Enterobacteriaceae (Escherichia coli, Pantoea spp.) were regularly found and their abundance was lower than that of the other bacterial groups mentioned above. Yeasts were very rarely identified as the typically present fungi.
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