WHO Collaborating Centre for Reference and Research on Escherichia and Klebsiella
Department of Microbiology & Infection Control
Statens Serum Institut
Artillerivej 5, DK-2300 København S, Danmark
Characterization of Pathogenic E. coli and Research Needs
Complete serotyping of E. coli isolates includes O:K:H serotyping and has for the past half century been the backbone in the characterization of pathogenic strains. In 1983, O:H serotyping was used as one of the first typing methods to introduce the concept that certain E. coli strains have a clonal connection.
Some strains have the capability to cause disease in humans and are subdivided into pathotypes depending on which part of the body they affect and of their particular pathogenic mechanism. The serotype does not predict the pathotype in itself, which is largely due to the acquisition of virulence factors and –genes. Clinical syndromes may either be associated with extraintestinal infections (urinary tract infection, sepsis/meningitis), or enteric/diarrheal disease. The first are referred to as Extraintestinal Pathogenic E. coli (ExPEC), the latter as Diarrheagenic E. coli (DEC). DEC are subdivided into different pathotypes based on their adhesion/colonization mechanism and the toxins produced: Enteropathogenic E. coli (EPEC), Enterotoxigenic E. coli (ETEC), Enteroinvasive E. coli (EIEC), Enteroaggregative E. coli (EAEC), Shiga toxin-producing E. coli (STEC), which are also referred to as Vero cytotoxin-producing E. coli (VTEC), and Diffusely Adherent E. coli (DAEC). A recently described pathotype includes Adherent Invasive E. coli (AIEC), which is thought to be associated with Crohn's disease. For the majority of these pathotypes a minimum number of factors (molecular and/or phenotypic) can be used to define the pathotype. Phylogenetic lineages are to some degree associated with specific virulence factors; but the plasticity of the E. coli genome combined with the ability to acquire and exchange mobile genetic elements and cassettes can result in hybrid strains. Such new combinations of defining virulence factors poses great challenges for characterization and typing of each E. coli isolate. Depending on the purpose for typing and characterization many different methods ranging from phenotypic assays to molecular techniques such as gene detection, DNA fingerprinting to whole genome sequencing can be used. The past two decades has seen a plethora of different approaches and a major challenge in the characterization of E. coli is the comparability of the results generated by these different techniques and methods. Furthermore, our understanding of the principles that govern exchange of genes and the relations between individual traits in terms of their significance for virulence and host adaptation requires ongoing research. The combined efforts should contribute to a general IDEA (Index of Diversity in Evolution and Adaptation) which will bridge the phylogenetic approach based on the clonal concept on one side and the horizontal transfer of gene cassettes on the other side.