**10. Phylogenetic groups of** *E. coli*

According to older phylogenetic studies, the *E. coli* strains were classified into four main phylogenetic groups: A, B1, B2, and D. However, recent studies showed that there are more phylogenetic groups seven (A, B1, B2, C, D, E, and F) belong to *E. coli* sensu stricto, whereas the eighth is represented by cryptic Clade I. Apart from clade I, also clades II, III, IV, and V are known to exist [46]. The majority of strains that cause extraintestinal infections belong to the phylogenetic group B2, whereas as strains belonging to the phylogenetic groups A and B1 are known to have low extraintestinal pathogenicity potential but beside commensal strains, strains also cause diarrhea (**Figure 12**). According to Doumith M, et al., *E. coli* strains belonging to various phylogenetic groups displayed diverse phenotypic and genotypic features thought to support fitness in various ecological settings, resulting in niche preference according to scientific findings [48]. To determine *E. coli* phylogroups, several approaches have been described. Polymerase chain reaction (PCR)-based tests, multi-locus sequence typing (MLST), ribotyping, and sequencing of the 16S rRNA gene are among them [49]. For the determination of the original four different phylogroups (A, B1, B2, and D), the Clermont triplex PCR phylogroup method was used [50].

However, research has revealed that this method can only confirm 80–85% of all *E. coli* phylogroups, and in 2013 Clermont et al. [51], proposed a revisited method, the quadruplex PCR, which can be used to classify *E. coli* in the seven phylogenetic groups and clade I [52]. Clermont et al. [53] also proposed a PCR method for the detection of clades II–V.
