الفهرس 
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Abstract
Escherichia coli is implicated as a major cause of diarrhea in young calves with significant economic losses worldwide. The emergence of antimicrobial resistance within E. coli is considered a major challenge to veterinary and public health. The phenotypic and genotypic characteristics of antibiotic multi-resistance within different E. coli pathotypes obtained from diarrheic calves have been performed. Furthermore, the role of M. domestica and milk has been identified in the transmission of pathogenic multi-resistant strains. A total of 213 E. coli strains were isolated from 157 diarrheic calves with molecular detection of ETEC, EPEC, STEC and AE-STEC at 13.6%, 14.1%, 15.5% and 13.1%, respectively. Hybrid pathotypes of ETEC/EPEC, ETEC/STEC and ETEC/ AE-STEC were identified at 3.3%, 5.2% and 3.8%, respectively. Molecular detection of AE-STEC O157 strains was performed on 137 rectal swabs from 157 diarrheic calves and was presented at 8.02%. Virulence genes profile of AE-STEC O157 revealed the presence of eae, stx1, stx2 and ehylA in percentages of 93.3%, 73.3%, 20% and 13.3%, respectively with the predominance of eae+stx1 combination at 66.7%. The association of M. domestica and milk with the transmission of pathogenic multi-resistant strains was determined on 110, 80 and 26 E. coli strains isolated from 70 rectal swabs from 157 diarrheic calves, 60 milk samples and 20 M. domestica, respectively from dairy farms located on the Giza and Cairo-Alex desert road. Molecular pathotyping of these strains revealed the detection of pathogenic E. coli within diarrheic calves, M. domestica and milk at 81%, 76.9% and 33.7%, respectively. Phenotypic antimicrobial resistance revealed that 99.1% of fecal strains were antibiotic multi-resistant, while M. domestica and milk multi-resistant strains were found at 100% and 85%, respectively. The higher resistance for β-lactams was detected within different E. coli strains except for cefquinome that exhibited low resistance in M. domestica and milk strains at 30.8% and 30%, respectively. ESC resistant strains within fecal, M. domestica and milk strains were detected at 69.1%, 73.1% and 71.3%, respectively confirming the exact role of M. domestica and milk in the transmission of ESCs resistance. On the contrary, milk strains conferred lower resistance for non β-lactams, particularly for nalidixic acid and ciprofloxacin, which were the most effective antibiotics for milk strains. Molecular detection of ESBLs and PABLs encoding genes revealed the predominance of the blaTEM gene in different E. coli strains, while none of these strains harbored the blaOXA gene. The highest percentages for blaCTXM and blaCMYII genes were detected in M. domestica strains illustrating their role in the transmission of β-lactamases genes to diarrheic calves. Concerning colistin resistance, the mcr-1 gene was not detected in M. domestica strains, while fecal and milk strains carried this gene at 35.5% and 15%, respectively. E. coli phylotyping revealed a higher frequency of phylogroup B2 within fecal and M. domestica strains, while milk strains were mainly assigned to B1 phylogroup. Pathogenic E. coli strains with the same phenotypic and genotypic antimicrobial resistance and phylogroups were identified for both diarrheic calves and M. domestica, which indicates that M. domestica serves as a multi-resistance reservoir and contributes to the dissemination of virulence and antimicrobial resistance in dairy farms.