## 4 Discussion  
Milk is the foundational raw material of the dairy food chain, and its quality is crucial for the safety of dairy products. As a rich and complex nutritional matrix, milk provides an excellent medium for nearly all microorganisms. The health of the udder and the production site of milk significantly impact its hygienic status, especially in cases of subclinical or clinical mastitis. When mastitis occurs, treatment with antibiotics typically follows, curing the udder, and after the withdrawal period, the milk can be safely consumed again. This research aims to study the prevalence of pathogenic _E. coli_ strains on the first day after the end of the withdrawal period when the milk re-enters the food chain. Tables 2 and 8 display the prevalence of pathogenic _E. coli_ strains across different experimental groups. Masitis groups (B groups) notably harbor a high abundance of such strains, which, although significantly reduced, persist in milk after the withdrawal period (C groups). In group A, originating from healthy udders, the prevalence of _E. coli_ isolates is 4%, a figure relatively lower or comparable to that reported by other researchers [24; 59; 60; 61]. While some authors have reported higher prevalence rates ([PERSON] et al., 2016, 74.4%; [PERSON] et al., 2020, 61%), they often refer to bulk tank samples [2; 61; 24]. The reported prevalence of _E. coli_ in samples from udders with clinical mastitis varies widely from 7.3% ([PERSON] et al., 2007) to 55.7% ([PERSON] et al., 2022), and thus our result of 39.33% should be considered within this range [62; 63]. It is important to note that _E. coli_ is not commonly regarded as a major causative agent for mastitis in ewes [64]. Pathogenic _E. coli_ O157 was not found in group A (healthy udders), consistent with findings from other studies by researchers such as [PERSON] et al. (2015), [PERSON] et al. (2017), and [PERSON] et al. (2020) [2; 31; 65]. It was detected in group B (4 strains, 2.67% of the samples) and maintained a similar occurrence (2-4%) in the C groups despite antibiotic treatment. Additionally, other pathogenic non-O157 _E. coli_ isolates were isolated. The \"big six\" serotypes (O26, O45, O103, O111, O121, O145), known for their ability to cause severe illness, were present in all groups [66; 67], along with other serotypes associated with illness such as O104 and O152 [68; 69; 70]. The classification of \"O\" serotypes is pivotal in epidemiology for linking cases to their source. Currently, a total of 176 \"O\" antigen structures of _E. coli_ have been identified [71], and the conventional serotyping method, which involves agglutination specific to each serotype, continues to be utilized [72]. In this study, 15 different \"O\" serotypes were isolated, 10 of which are considered hazardous. Since our samples were directly obtained by milking the udder and the microorganisms showed a low occurrence rate, it suggests that the infected udder is the source of these serotypes. When mastitis occurs, the bacterial populations of the normal microflora are significantly disrupted, allowing various commensals like _E. coli_ to enter the ecosystem. Although most serotype prevalence rates decreased after treatment, not all did. For instance, in subgroup B3, three strains of O111 were isolated, while in subgroup C3, the number increased to four. This indicates contamination of environmental origin during the withdrawal period. The duration of the withdrawal period depends on the antibiotic class, and while it ensures the absence of antibiotic metabolites in milk, it does not guarantee the return of bacterial microflora to its normal equilibrium. Consequently, various opportunistic pathogens and commensals may still be present in the milk consumed by humans. The virulence of _E. coli_ relies on various factors including adhesion, biofilm formation, nutrient acquisition, competition with other bacteria, toxin production, and evasion of host defense mechanisms [73].