الفهرس 
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Abstract
Bovine mastitis is a significant economic threat for the dairy sector of animal production worldwide including Egypt and is one of the multifactorial diseases in dairy farms, which has many risk factors that provoke the disease. The objectives of this study are, assess the effects of births, season, and lactation on the occurrence of clinical and subclinical mastitis, also the identification of the most common bovine intramammary infection-causing bacterial pathogens through isolation and molecular detection. A total of 295 pathogens of 6 different species were isolated from udder in dairy farms in five different governorates. The pathogens founded in this study were Escherichia coli contributed for (41.3%) of all cultured pathogens followed by Staphylococcus aureus (25.8%), Streptococcus agalactiae (0.7%), Streptococcus dysgalactiae (16.9%), other staphylococci (5.4%) and other non-identified isolates (9.8%). The antibiotic resistance patterns were determined for 11 antibiotic drugs using the disc diffusion method following Clinical and Laboratory Standards Institute recommendations, the minimum inhibition concentration (MIC) using ofloxacine, cefquinome, tetracycline and amoxicillin clavulanate drugs and the molecular detection of udder pathogens and their virulence genes as rfbEO157 encoding gene as Shiga toxin-producing E. coli (STEC), coagulase gene and methicillin-resistance encoding gene (MecA). The multidrug resistance rates were higher among Streptococcus species than among E. coli strains, S. aureus, and other Staphylococcus species (86%), (54.3%), (41.6%) and (28.6%) respectively. Tetracycline resistance was the most common in E. coli, while cefotaxime in Streptococcus species and penicillin in Staphylococcus species, were approximately 45.6, 90 and 75% respectively. Molecular investigation of extended-spectrum β-lactamases (ESBLs) and plasmid-mediated AmpC β-lactamases (PABLs) encoding genes among E .coli were reported in (100%) of the isolates encoded TEM-type ESBLs, none of which (0%) encoded OXA-type ESBLs, on the other hand, CTX-M-type ESBLs and SHV-type β-lactamases were encoded in (53.9%) and (4.7%) of the ESBL isolates, respectively and 27% exhibited CMYІІ-type PABLs. Plasmid-mediated colistin resistance encoding gene (mcr-1) was expressed in 1.6% of E. coli isolates. Finally, the minimum inhibition concentration (MIC) was performed on the milk itself as in vivo study. MIC values of four antibiotics, three in pure form (ofloxacine, tetracycline and cefquinome) and one in combination (amoxicillin clavulanate) were determined on milk samples as in vivo study.