## 4 Discussion
Recent research has shown that _stx_ subtypes have a predilection toward different receptors: the Stx B subunit recognizes Gb3 as its principal receptor and to a lesser extent Gb4. [PERSON] and [PERSON] 2019 [51] highlighted the relevance of this interaction as a key mechanism in the pathogenicity of STEC. Stx1a interacts strongly with Gb3 on the human glomerular endothelium. On the other hand, the subtype _stx2e_ shows predilection for Gb4 and Gb5 present in the glomerular endothelium of ruminants and pigs. In this work, the _stx2g_ gene was predominant, followed by _stx2b_. These subtypes are not associated with HUS and HC development in humans, which could represent a low hazard to establish disease. In contrast, the presence of _stx2c_ and _stx2d_ genes that were not reported in this investigation that boost the development of HUS and HC, were reported in sheep carcasses from Turkey and Switzerland. [PERSON] et al. (2014) [36] found _stx2c_ and _stx2d_ genes in backyard sheep in Mexico. [PERSON] et al. (2011) [8] demonstrated that isolates harboring _stx2g_ gene obtained from humans, animals and environmental sources had a close phylogenetic relationship, reinforcing the idea of human infections as a potential zoonotic disease. Identification of _stx_ subtypes is a priority, as it allows for an early prediction of the virulence potential of each STEC isolate. This observation generated enough evidence to know that _stx2a_ and _stx2d_ genes are crucial determinants in the severity of HUS; furthermore, the mere presence of the _stx2a_ gene is considered an independent risk factor to the developed HUS in multivariate analysis. Therefore, the identification of _stx_ subtypes should be performed routinely in diagnostic laboratories [52]. Pathotype EPEC was the second most frequent (3.3%) and is responsible for neonatal diarrhea in human and animal populations [53], however it also affects adult sheep in Australia and Brazil [34; 45]. The isolates in this study did not express the _bfp_ gene, so they were categorized as a-EPEC [6]. ETEC was the third most frequent pathotype (2.2%) and is considered one of the main diarrhea pathogens in lambs and calves [54]. In Kenya, it was also reported as the third most frequent DEC (10%) in slaughtered goats, while an investigation in Mexico rated it as the second most frequent in bovines, the same as in Burkina Faso (4%) [14; 16; 46]. Previous reports about ETEC were described in bovines with/without diarrhea in Brazil, Vietnam (with the same number of isolates as this work), and Burkina Faso [55; 56; 57]. There is a lack of information regarding this pathotype in slaughtered sheep in Mexico. The last reported pathotype was EIEC with only 1.1%. This low percentage is also observed by other authors in comparison with other pathotypes in other species; for example, [PERSON] et al. (2018) [16] only discovered 11.5% of EIEC in bovine feces, while [PERSON] et al. (2012) [14] found 1% of this pathotype in slaughtered poultry in Burkina Faso. In this work EAEC was not detected, however other investigations report this pathotype along with STEC, EPEC, and ETEC in Mexico, Iran, and Burkina Faso in bovines and goats [14; 16; 47]. The presence of most of the DEC pathotypes of public health concern can be isolated from sheep, goats, and bovines, which raises the relevance of livestock as a reservoir of these pathogens [58]; precarious hygiene conditions make it possible for DEC to contaminate meat products with feces during different processes in slaughterhouses in Mexico. The high percentage (\(\sim\)90-100%) of isolates belonged to phylogenetic group B1 (compensal _E. coli_), which is similar to that reported in other countries in isolates from sheep, goats, and bovines [59; 60; 61; 62]. Antimicrobial resistance (AMR) was observed in STEC isolates against AMP (72%), TET (30%), and SXT (9%); these percentages were lower in comparison to a study carried out in Turkey where a higher frequency of AMR to AMP and TET (100% and 50% respectively) was reported [63].