## 3 Results  
Clarification: We would like to clarify that, for the sake of simplicity and economy, the subgroups (B1, B2, and B3) within group B are not mentioned in some tables. Instead, only the central subgroup, referred to as B, is listed. These animals exhibited clinical mastitis but had not yet started treatment, so they were considered as one group (B). Subsequently, three subgroups with different therapeutic approaches were created depending on the drug of choice (B1, B2, and B3). These three groups correspond to the three subgroups of the main group C (C1, C2 and C3), respectively. Table 2 illustrates the distribution of the different types of _E. coli_ isolated from raw milk among different groups and subgroups. The occurrence of the microorganism (total) in the milk from the healthy udders (group A, 5%) is significantly lower compared to milk from udders affected by mastitis (group B, 29.50%), which is very high, indicating a substantial difference (OR = 12.32, CI 95% 4.73-32.08, \(p\) < 0.001). Likewise, the prevalence of _E. coli_ in the milk from animals treated with antibiotics (total of groups C1 + C2 + C3 = 17.00%) is lower than in the mastitis group B (OR = 2.29, CI 95% 1.39-3.81, \(p\) = 0.0013), but increased with respect to the controls (OR = 5.36, CI 95% 2.01-14.26, \(p\) = 0.0008). frequencies of non-O157:H7 _E. coli_ strains, their distribution does not differ significantly among the different groups (\(p\) = 0.8172). Tables 3 and 4 display the metabolic and virulence characteristics of the isolated strains of non-O157 _E. coli_, as well as the sorbitol-negative (\(-\)) and sorbitol-positive (\(+\)) _E. coli_ O157 strains, respectively. There was a diversity of these traits among the different strains as expected but it could not be associated with the groups. However, the strains of _E. coli_ O157 H7 sorbitol (\(-\)) showed a higher uniformity since all of them were amylase and lecithinase producers, reacted positively to the gelatinase test, had a positive serum resistance reaction, and all produced the serotonin (STX) 2, while none of them was a lipase producer. Table 5 shows the resistance to antibiotics of the isolated _E. coli_ strains in the different groups. The prevalence of the susceptible and resistant strains changed significantly during and after the treatments, from the B to the C groups. [PERSON]'s rank test revealed a statistically significant monotonicity (r values > 0.80, \(p\) < 0.001) among the sensitive and resistant strains in each group. However, in every group, the antibiotic substance used for  
the treatment of mastitisti s led to an increased prevalence of resistant strains and a reduced prevalence of sensitive ones. Table 5 also displays the distribution of extended-spectrum \(\beta\)-lactamase (ESBL) phenotype production, which did not display a consistent pattern. For example, in groups C1 and C2, the majority of strains did not demonstrate ESBL activity, whereas the opposite trend was observed in group C3. Table 6 shows the resistance of the nine _E. coli_ O157:H7 strains classified by group and by their reaction to sorbitol. All strains were found to be resistant to amoxicillin, and all were found to be sensitive to sulfonamides. In Table 7 the capacity of biofilm formation of the isolated _E. coli_ strains is presented. Most of these strains (52.17%) had the capacity to form biofilm. Of the 48 strains able to form a biofilm, 27 of them (56.25%) possessed this property in high intensity.