## 3 Discussion
A similar outcome was observed on _E. coli_ strains for type II TA system antitoxin genes _mazE_ and _hipB_, which were downregulated under NaCl-induced osmotic stress [37]. None of the other systems here studied were influenced by this condition in any of the tested bacterial growth phases. Special attention must be given to the involvement of the acid environment with the activation of chromosomally expressed type II TA systems in a hybrid _E. coli_ since it may activate three out of the five selected systems, namely, _yha V-prlF, yoeB-yefM_, and _pasTl_. These three systems act on cellular translation and protein synthesis. The PasT toxin specifically targets the 50S ribosomal subunit, while the YoeB and YhaV toxins target mRNA. Therefore, the activation of these systems can influence the decrease in protein synthesis within the cell during acidic stress, consequently favoring cell survival through the induction of dormancy or cellular persistence [38; 39; 40]. The association between TA systems and UPEC had great importance in the colonization of the niche and the stress response; it was described that during the infectious process of the urinary tract in a mouse model, the PasT-PasI system promoted the colonization and persistence of bacteria in the kidneys, while the TA YefM-YoeB and YhaJ-Hha systems promoted an increase in colonization in the bladder [41]. As our hybrid _E. coli_ harbors virulence factors from ExPEC, its capacity to survive and thrive in low pH conditions may facilitate a successful colonization of the urinary tract, a step necessary to cause urinary tract infections [42]. Further studies are necessary to precisely determine the specific mechanisms activated within each TA system, as these may vary significantly. In this study, we demonstrated that TA system activation is environment dependent, which may benefit the hybrid strains by enhancing their ability to colonize and replicate in diverse host niches. Our findings also reveal that in the stationary phase, under acid conditions, the BA1250 strain exhibits an increased activation of several systems, underscoring their potential importance in survival and maintenance in this environment. After bacterial invasion, immune cells, such as neutrophils and macrophages, engulf the pathogens, sequestering them within specialized compartments known as phagosomes. Inside these phagosomes, bacteria are subjected to an adverse environment characterized by significant nutrient deprivation, which impairs their survival and leads to cellular death [43; 44]. Additionally, proteins associated with innate immunity, such as LPS recognition by TLR4, trigger further responses, including acid stress (via phagosome acidification), thiol, metal, and oxidative stress (through the production of reactive oxygen and nitrogen species, ROS/NOS) [43; 44; 45; 46]. The combination of nutritional deprivation and stress responses creates a hostile environment for invading bacteria, promoting their elimination. Therefore, the PasT-PasI system may act as a virulence factors, enhancing resistance to these conditions, and increasing bacterial survival and pathogenicity. In contrast, the YhaV-PrIF system plays a role in bacteriostasis, and its activation may be an adaptive response to improve bacterial survivability in acid and nutrient-reduced environments. The massive bacterial genome sequencing revealed that chromosomal toxin-antitoxin systems are widespread, frequently with multiple copies present in a given replicon [4; 47], located mostly in genomic islands (prophages, conjugative elements, or transposons) or constitute small genomic islets by themselves [48]. Recent advances in the field suggest alternative views for the roles and functions of these highly abundant and mobile elements in the light of genome evolution. The significant activation of chromosomal type II systems in a condition that would be faced by a hybrid strain led us to investigate the distribution of these systems in a broader range of hybrid, intestinal, and extraintestinal _E. coli_ strains.