**5.4 Population structure of EAEC**

EAEC subtypes are defined based on the presence of virulence plasmid pAA that carries *aggR* gene that regulates the expression of other virulence determinants located on the plasmid. This and the high serotype diversity, as well as other accessory genes contribute to the high heterogeneity noted for this pathotype. An earlier study on the phylogenetic analysis of EAEC revealed that isolates belonged to multiple lineages [156]. A similar observation was noted with MLST where 150 Nigerian EAEC strains were clustered into 96 STs [157]. Indeed, EAEC strains are known to belong and spread across four *E. coli* phylogroups (A, B1, B2, and D) (**Figure 4**) with diverse serotypes [144, 156].

In a recent study [146], of the 97 EAEC strains analyzed using MLST, 42% were reported to belong to phylogroup B1, while the majority of the few strains that belonged to phylogroup A lack the AAF-associated genes. Although serotype diversity is high in this pathotype, this study also noted that EAEC strains that belonged to phylogroup D were clustered into three serotype-specific lineages (lineage 1–3). All strains in lineage 1 were O166:H15 and belonged to ST349, lineage 2 consisted of serogroups O44, O73, and O17/O77 in combination with either H18 or H34 and ST130, while lineage 3 carried O153:H30 serotype and ST38 [146]. Contrarily, in India, EAEC strains implicated in diarrhea were more prevalent in phylogroup D [158] suggesting that the diversity in the pathotype is not limited by geography. While EAEC subtypes are believed to differ in their virulence determinants content, this hypothesis and comparative phylogenetic analysis of aEAEC and tEAEC are underexplored. Also, large-scale phylogenomic and phylogeographic analyses of this pathotype are scarce. Further studies should focus on this.
