Microbial colonisation of the esophagus

Our study povides the first evidence of microbial colonisation on the esophagus surface of the important domesticated mammals. By applying cryo SEM an authentic display of the microorganisms covering the surface was achieved (compare figures at 4.1.1). In the same way it was shown that innate defence mechanisms are required in the esophagus where the epithelium has to control and ward off bacterial threats. The epithelial surface revealed an occupation by a thin layer of bacteria and fungi. Species specific differences in the quantity of the microbiota could not be determined. In order to quantify the number of microorganisms further studies are necessary, whereby they should begin by establishing a standardised method for collecting samples.

The interest in the microbial composition of the regular digestive tract developed in the 19^th century; during the lifetime of Louis Pasteur, who believed that a symbiotic relationship between man and bacteria was essential for life. The development of experimental (animal) models allowed the research on this relationship in absolutely germ-free mice, demonstrating the role of commensal microflora, i.e. a function in vitamin synthesis (vitamin K and B12), the metabolism of nitrogen compounds and lipids, and especially the participation as barrier against the invasion of pathogenic microorganisms (MACKROWIAK 1982; ZILBERSTEIN et al. 2007). The latter authors studied the microbial flora of the human digestive tract in healthy volunteers. They collected samples from the mouth, esophagus, stomach, duodenum, jejunum, ileum, colon, and rectum, and determined the flora composition. The bacteria most frequently encountered in the human esophagus were Streptococcus sp. (40%) and Staphylococcus sp. (20%). Furthermore, the esophagus revealed a colonisation by Corynebacterium sp., Lactobacillus sp. and Peptococcus sp. (all 10%). The authors claimed that the esophageal microbiota are mainly transitory, similar to that detected in the mouth. However, they stated that when alimentary stasis occurs, as for example in neoplasia, there is an excessive increase in bacterial growth (LAU et al. 1981). In order to determine whether the microbial colonisation of the esophagus of the species studied in this thesis is only transitory or also indigenous, it is necessary to begin by characterising the microbial flora of the oral cavity of every animal. Comparing the microbial flora of the human esophagus (ZILBERSTEIN et al. 2007) with that of

the only other species studied, the horse (MEYER et al. 2009), revealed no similarities. Such finding is not surprising, since humans and horses have completely different nutrition habits. For a comparison of the human microbiota with the microflora of an animal species, the porcine digestive tract should be investigated in detail, as swine are often used as models for humans also due to similarities regarding the digestive organs.

In contrast to the esophagus, a lot of information is available on the microbial colonisation of other parts of the human digestive tract. Information about bacterial communities is expected to provide crucial information how to develop therapies for various gastrointestinal diseases. Furthermore, scientists hope to generate knowledge about the next generation of probiotic bacteria as part of functional food (SINGH et al. 2009). The gut bacterial microbiota has established multiple mechanisms to influence the eukaryotic host, generally in a beneficial way. The prokaryotic genomes of the human microbiota encode a spectrum of metabolic capabilities beyond that of the host genome, making the microbiota an integral component of human physiology (HOOPER et al. 2001; SCHENK and MUELLER 2008; NEISH 2009). Modulating bacterial numbers and diversity is entrusted to the gastrointestinal immune system. Although bacteria are distributed throughout the intestine, the major concentration of microorganisms can be found in the large intestine (SINGH et al. 2009). The diversity of species encompasses around 400-500 different bacteria (LOTZ et al. 2007). The small intestine has microbial communities with 1x10⁸ CFU g^-1 ileal content. Through the remainder of the digestive tract, bacterial concentrations gradually increase, reaching 1x10¹¹ CFU g^-1 (SINGH et al. 2009). Within the 400-500 species mentioned, 30-40 species make up for 99% of the total population of the intestine (GUARNER 2006). The results of several studies indicate that five genera are predominant in the large intestine of humans: Bacteroides, Eubacterium, Bifidobacterium, Peptococcus, and Fusobacterium (SANDBORN et al. 2003; SINGH et al. 2009). It is important to note that although immunostimulatory structures recognized by the innate immune system are generally produced by both, commensal and pathogenic bacteria, the synthesis of low stimulatory hypoacytylated LPS from Bacteroides might contribute to homeostasis (WEINTRAUB et al. 1989).

With regard to our own observations, one difference was most obvious between the colonisation of the intestinal tract and the esophagus, i.e. the quantity of microorganisms detected. In contrast to the resorpative intestine, only a thin layer of microbiota covered the epithelial surface of the esophagus. These findings are in line with observations of LOTZ et al. (2007) from the human esophagus, which is covered with only low to very low numbers of bacteria. As the esophagus mucosa does not fulfil such digestive tasks, as the absorption of nutrients or synthesis of vitamins, the colonisation with commensal bacteria here seems not essential for nutrient absorption. As a result, much less bacteria, in comparison to the intestine, can be observed. Nevertheless, due to the fact that we demonstrated the existence of pattern recognition receptors in the esophagus epithelium, we also showed that already a relatively low number of microorganisms is able to evoke the response of the innate immune system. It is possible that such a low number of microorganisms maintains homeostasis in the esophagus, whereby an excessive colonisation with pathogenic bacteria may be avoided.