Ancestral Lineages of Human Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a common cause of diarrhea among children living in and among travelers visiting developing countries. Human ETEC strains represent an epidemiologically and phenotypically diverse group of pathogens, and there is a need to identify natural groupings of these organisms that may help to explain this diversity. Here, we sought to identify most of the important human ETEC lineages that exist in the E. coli population, because strains that originate from the same lineage may also have inherited many of the same epidemiological and phenotypic traits. We performed multilocus sequence typing (MLST) on 1,019 ETEC isolates obtained from humans in different countries and analyzed the data against a backdrop of MLST data from 1,250 non-ETEC E. coli and eight ETEC isolates from pigs. A total of 42 different lineages were identified, 15 of which, representing 792 (78%) of the strains, were estimated to have emerged >900 years ago. Twenty of the lineages were represented in more than one country. There was evidence of extensive exchange of enterotoxin and colonization factor genes between different lineages. Human and porcine ETEC have probably emerged from the same ancestral ETEC lineage on at least three occasions. Our findings suggest that most ETEC strains circulating in the human population today originate from well-established, globally widespread ETEC lineages. Some of the more important lineages identified here may represent a smaller and more manageable target for the ongoing efforts to develop effective ETEC vaccines.Enterotoxigenic Escherichia coli (ETEC) infections are an important cause of childhood diarrhea and diarrheal deaths among young children in developing countries (59) and of diarrhea among travelers to these countries (6, 54). Human ETEC strains are E. coli that produce one or more of three plasmid-encoded protein enterotoxins called human heat-stable toxin (STh or STaII), porcine heat-stable toxin (STp or STaI), and heat-labile toxin (LT or LT-I). The enterotoxins induce secretion of salts and water into the intestinal lumen (29). Many ETEC strains also produce surface appendages, called colonization factors (CFs), which help anchor the bacteria to the small intestinal wall (20). The toxins and all but one known CF are plasmid encoded (20, 34).Human ETEC strains are phenotypically and epidemiologically diverse: more than 20 different CFs have thus far been described (20), the characterization of ETEC strains collected from different parts of the world has yielded 117 different serotypes (57), and some ETEC strains appear to be more pathogenic than others (9, 36, 46). This diversity poses a challenge for the ongoing efforts to develop effective ETEC vaccines (7). Many studies have shown that ETEC have emerged from E. coli on several occasions, probably through horizontal transfer of the enterotoxin-encoding virulence plasmids, and that some of these ETEC lineages appear to be widespread (4, 11, 31-33, 37, 38, 43, 47, 51). Because strains that originate from the same ETEC lineage may also have inherited many of the same epidemiological and phenotypic traits, identifying and defining these lineages may improve our understanding of the ETEC diversity and may lead to the identification of lineage-specific protective antigens that can be used in vaccines. To identify these lineages, we performed multilocus sequence typing (MLST) and phylogenetic analyses on a collection of ETEC strains that had been isolated from humans in different countries. We also estimated each lineage''s age as a measure of how stable and well established these lineages are in the E. coli population. If the ancestral origin of the human ETEC population changes frequently, it would complicate efforts to identify new, chromosomally encoded antigens capable of inducing protective immune responses against ETEC. In that case, today''s main vaccine development strategy of targeting plasmid-encoded virulence factors, such as the toxins and CFs, would probably continue to be the best approach for developing effective ETEC vaccines.