is enteropathogenic for man and rodents. Previous studies provided evidence that invades the lymphoid follicles of the Peyer's patches (PP) of the small intestine. In this study -induced tissue alterations of the follicle-associated epithelium (FAE) and the underlying PP tissue were analysed by scanning (SEM) and transmission electron microscopy (TEM) as well as by conventional histological examination. For this purpose, an experimental mouse infection model including orogastric infections as well as ileal loop experiments were used. A rapid and selective colonisation of the FAE after orogastric yersinia infection was observed by SEM. TEM studies confirmed that adhered closely to the FAE including M cells and enterocytes. Histological studies and TEM revealed that selectively invaded the PP M cells but not other cells of the FAE. One day after infection the FAE as altered and small micro-abscesses comprising yersiniae expressing the major outer-membrane protein YadA were observed immediately beneath the FAE. Adjacent villi were dilated from lymphangiectasis and transmigrating polymorphonuclear leucocytes (PMNL) were found within the epithelium. At 5-7 days after infection the FAE and parts of PP were destroyed. Profound alterations of the cyto-architecture of the PP were due to the enormous recruitment of PMNL. By day 5 after infection, abscesses were found in the mesenteric lymph nodes. However, TEM studies revealed evidence that may disseminate from the PP not only the lymphatics but also by invasion of blood vessels. Taken together, the results of this study demonstrate that the FAE is the primary site of host-pathogen interaction in infection and that this pathogen penetrates M cells and subsequently induces destruction of the PP.


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