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Helicobacters are indigenous to the human stomach: duodenal ulceration is due to changes in gastric microecology in the modern era
  1. Division of Infectious Diseases, A3310 Medical Center North, Vanderbilt University School of Medicine, Nashville, TN 37232–2605, USA

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Peptic ulcer disease has been considered to be a “disease of civilisation”.1 Yet Helicobacter pylori, which now is believed to play a critical role in this illness,2 ,3 has probably been part of the human biota since time immemorial,4 and peptic (especially duodenal) ulceration seems to have become epidemic in humans at a time whenH pylori was loosening its firm grip on the human stomach. These phenomena seem contradictory. It is the aim of this paper to describe the elements of the apparent paradox, and to consider several alternative explanations. Resolution of this question has substantial implications for human medicine.

Microbial niches in humans

The human body is the home of countless microbes5which usually are called the “normal flora”. Indigenous biota is a better term than normal flora, since bacteria are not plants,6 and normal is difficult to define. The major habitats of the indigenous biota include the gastrointestinal and upper respiratory tracts, the vagina, and the skin, and each niche shares several characteristics (table 1). In each locale, multiple different ecological niches are present. For example, the lumen of the colon is characterised by notable variation in redox potential, and each niche has predominant organisms. The predominant biota of the skin is anaerobic,7 a phenomenon that indicates the multiplicity of ecological niches present. The indigenous biota is acquired early in life, and there often is a succession of organisms as local microenvironments change. Examples of this phenomenon include the changes in the colonic and oral biota accompanying weaning and the eruption of teeth, respectively.8 The biota are numerous, with 103 to 1011 organisms per gram at each site. The colonising microbes are diverse. Bacteria predominate, but fungi and protozoans also are present. In several locales, many different species are present, …

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