## 4 Discussion
Table 5 also highlights the capacity of isolated _E. coli_ strains to produce ESBL, with 43.47% of strains demonstrating this phenotype in vitro. However, the expression of this phenotype in vivo depends on other genetic and environmental parameters, which were apparently not favorable in our study, as evidenced by the relatively lower resistance frequencies against Cephalothin (CEP) and cefotaxime (CEF). ESBL genes are known to circulate in the environment, with some studies suggesting sheep as reservoirs for these genes [83]. A meta-analytical study by [PERSON] et al. (2020) concluded that human intestinal carriage of ESBL _E. coli_ globally increased by a factor of eight [84]. Our results align with those of [PERSON] and [PERSON] (2022) [85]. In group C2, animals were administered oxytetracycline. Interestingly, while the resistance frequency to tetracycline decreased--a paradoxical finding--the resistance frequency to gentamicin increased, while resistance to amikacin remained unaffected. These three substances share the same mode of action (binding to the 30s subunit of the ribosome) but have different structures. The increased resistance to gentamicin may be attributed to a cross-resistant effect, and a similar explanation could apply to amikacin, a newer medication that might be less susceptible to standard mechanisms of resistance. It is possible that strains resistant to oxytetracycline were eliminated by other mechanisms, such as antagonism with other bacteria, preventing their survival and subsequent counting. Finally, animals in experimental group C3 were treated with enrofloxacin, resulting in a significant increase in resistance frequency to ciprofloxacin. Interestingly, the resistance rate to nalidisic acid, which shares the same mode of action with quinolones by inhibiting the gyrase, decreased. This phenomenon could be attributed to the possibility, similar to that seen with oxytetracycline, that resistant bacteria were unable to survive due to antagonism with other microorganisms. All isolates demonstrated sensitivity to the combination of trimethoprim/sulfamethoxazole (TRI/SU) and sulfathiazole (SUF), likely because neither of these substances nor any structurally or functionally related antibiotics (which interfere with the metabolism of folic acid) were included in the therapeutic protocols of local veterinarians. A total of 41.30% of the _E. coli_ isolates were classified as multidrug-resistant (MDR) as they exhibited resistance to more than two classes of antibiotics. It is concerning that 20 out of 38 MDR strains (52.63%) were isolated from the C groups. Among O157 _E. coli_ isolates, all four from group B were multidrug-resistant (Figure 2), while three out of six strains from the C groups also demonstrated multidrug resistance. Interestingly, the two strains not classified as MDR were sorbit-positive. While molecular methods offer greater accuracy and precision in microbial characterization, conventional methods remain valid and widely used for classifying _E. coli_ isolates [86]. Bacterial identification using selective media is cost-effective and less complex, although it lacks the specificity and sensitivity of molecular biology techniques. Consequently, selective culture media are not highly reliable for identifying bacteria from diverse environments, and results must be verified using more accurate methods. Despite this limitation, selective culture media serve as a robust screening tool for preliminary bacteria selection before further molecular analyses.