Enterohemorrhagic Escherichia coli, EHEC.Microbiological diagnosis, characterisation and clinical bacteriological aspects

Abstract

Enterohemorrhagic Escherichia coli (EHEC) constitutes a group of E. coli that in the past two decades has been the cause of several outbreaks of gastrointestinal disease. The microorganism causes hemorrhagic colitis (HC) and haemolytic uremic syndrome (HUS) in 5-10% of the patients, 30% of the HUS patients develop remaining kidney injuries. The capacity to control EHEC disease and to limit the scale of outbreaks is dependent upon prompt diagnosis and identification of the source of infection. Cattle are the principal reservoir of EHEC and foodstuffs, which presumably have come into contact with domestic animal manure and/or are inadequately pasteurised, are important vehicles of infection. The predominating virulence factor of EHEC is the verocytotoxin first identified in an E. coli of serogroup O157:H7. In the present study, the PCR technique with primers detecting the verocytotoxin genes was shown to be a fast, sensitive and specific method to screen for and identify EHEC. The study contributes to the knowledge and understanding that also EHEC non-O157 are pathogenic and that all verocytotoxin producing E. coli should be looked after among patients with diarrhoea, HC and HUS. The importance of quick identification was exemplified in an outbreak of EHEC among the staff at the children s hospital in Göteborg, Sweden. As the outbreak was promptly identified, partly thanks to the PCR technique, no further spread to other staff members or patients occurred. In the present thesis, pulsed-field gel electrophoresis (PFGE) was used and the method was shown to be very useful in the epidemiological work to separate single cases of EHEC from outbreaks. In order to further characterise EHEC non-O157, the presence of additional chromosomal and plasmid genes probably involved in the pathogenesis were analysed as these genes have been found in EHEC O157. The gene coding for intimin, enabling tight attachment to epithelial cells of the intestine was also often present among EHEC non-O157. Moreover, the gene coding for an enterohemolysin possibly supporting the bacterial need for iron was especially often present among EHEC isolated from patients with severe symptoms. Further, the study has shown that the gene coding for aerobactin seems to be able to compensate for the absence of enterohemolysin and that also the type 1 fimbriae may be valuable to obtain a niche in the gut. A comparative study of verocytotoxin producing E. coli in cattle showed that there is a significantly lower amount of E. coli harbouring the intimin gene and the gene coding for aerobactin, supporting the assumption that not all verocytotoxin producing E. coli in cattle are pathogenic. In conclusion, methods identifying the verocytotoxines or their genes should be used to identify EHEC of all serogroups. PCR is preferable as it is a fast, sensitive and specific method. Genes coding for intimin, enterohemolysin and aerobactin adds to the pathogenicity of EHEC. These genes are not that common among isolates from cattle. PFGE is a most valuable tool for epidemiological tracing of EHEC.