Enteropathogenic Escherichia coli (EPEC) causing microvillus damage in weaned pigs

The authors give an overview of the enteropathogenic Escherichia coli (EPEC) strains mainly causing severe diarrhea of children (earlier named dyspepsia of newborn) under one year of age. At present these diseases occur mainly in some developing countries. By the year 1980, it has been widely accepted that infections caused by enteropathogenic E. coli (EPEQ are characterised by the formation of attaching and effacing (AE) lesions on intestinal epithelial cells, L e. by the intimate attachment of the bacteria to the enterocyte membrane and by the effacement of the microvilli. In this regard, they are similar to the enterohaemorrhagic E. coli (EHEC) strains, which produce additionally toxins (with synonymic designation of Vero- or Shiga-toxins), thereby inhibiting protein synthesis of the host cells. Most of the genes of EPEC and EHEC, required for the formation of the AE lesion are clustered on a chromosomal pathogenicity island called the locus of enterocyte effacement (LEE) (Figure 3). The middle part of the LEE contains two genes (eae and tir) that are involved in intimate adhesion, which is followed by reorganisation of the cytoskeletal actin underneath the adherent bacteria and by AE lesion formation (Figure 1. and Figure 2a, b). The eae (intimin) gene encodes for a 94-97 kDa outer membrane protein, which is required for intimate adherence and AE lesion development. Today at least 10 different types of intimin have been defined (from eae-alpha to eae-theta). The receptor for intimin is produced by the EPEC bacteria in form of the "translocated intimin receptor" (Tir), and is injected into the host cell by components of Type III secretion system, acting as a molecular syringe (Figure 4). In case of the typical EPEC strains, this multistep process is initiated by the binding of boundle forming pili (BFP) to the intestinal microvilli (Figure 1). Production BFP is governed by the EPEC adhesion factor (EAF) plasmid. EPEC infections producing AE lesion have been described among several species of domestic animals but an unequivocal pathologic significance have been described only in case of rabbit EPEC. Few data are available regarding occurrence and pathogenetic significance of EPEC in pigs. The authors briefly report on the literature data regarding porcine EPEC, including their recently published (23), and so far unpublished, results. As a first step, a collection of 204 Austrian and 220 Hungarian hemolytic E. coli isolates from diarrhoeal suckling and weaned pigs was investigated for the presence of eae gene by polymerase chain reaction (PCR). Results were negative for eae in all cases, but an overwhelming majority of the strains proved to be enterotoxigenic E. coli (ETEC), or verotoxigenic E. coli (VTEC). In order to explore further the genetic and phenotypic characteristics of porcine eae(+) E. coli, a case-control study was performed. This second study aimed to compare serotypes and intimin types of eae(+) E. coli strains in healthy and diarrhoeal weaned pigs based on intestinal and faecal samples collected from 13 farms, regardless of their haernolytic character. Detection of the eae gene was performed on E coli strains, isolated from 221 diarrhoeal and clinically healthy weaned pigs. As a result, 37 intestinal and fecal EPEC strains were isolated representing 13% of the pigs regardless of clinical picture. Only 3 of these 37 strains were hemolytic. They were characterized by eae-beta intimin gene (85% of intestinal and 53% of fecal strains). Interestingly, the dominant Hungarian type was the O123:H11, eae-beta which was not reported earlier. Besides, there were strains of typical porcine (045) and of other (049, 028, 0108) EPEC serogroups. The eae-gamma type strains were more frequently (23%) isolated from fecal samples, indicating that the preferential site of colonization by these eae-gamma type strains may be different from those of eae-beta strains. Based on the genomic analysis of strains by pulse field electrophoresis (PFGE) the strains of serogroup 0108 could be clearly distinguished. Besides, there were two, genetically distinct subgroups (Canadian and Hungarian) of the two major serogroups 045 and 0123 (Figure 5). in conclusion, the porcine EPEC strains studied can be distinguished from the typical human EPEC strains as atypical EPEC, of which the dominant serotype O123:H11: eae-beta was first detected here. It seems that the intimin types of porcine atypical EPEC may differ according to the intestinal or rectal site of isolation. Results on PFGE analysis also bear an international relevance by indicating that PFGE is useful for identifying genomic subtypes of the same porcine EPEC serotypes, similarly to that of 0157 EHEC strains

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