الفهرس 
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Abstract
Introuction: Mastitis is a complex disease, involving many different factors. However, a bacterial infection exists in most cases of mastitis, Streptococcus spp. are a genus of gram-positive bacteria where S. dysgalactiae, S. agalactiae and S. uberis are the most important mastitis pathogens Aim of the work: Isolation and identification of streptococcal species recovered from clinical and subclinical mastitis in cows and buffalo with detection of virulence genes (cylE and mig) of S. agalactiae and S. dysgalactiae by polymerase chain reaction and studying the effect of different antibiotics against streptococcal isolates and application of minimal inhibitory concentration test. Material and methods: Two hundred and eighty-eight milk samples were collected from apparently healthy and mastitic cows and buffalo reared in households’ smallholders and large dairy farms in Dakahlia and Damietta Governorates, Egypt. All milk samples were subjected to bacteriological examination. Results: the overall prevalence of subclinical mastitis was higher than clinical mastitis in cows and buffalo, S. agalactiae isolated with the highest percentage (59.37%), while S. dysgalactiae with percentage (40.62%). The sensitivity of S. agalactiae isolates against 15 different antibiotic discs revealed that S. agalactiae isolates showed higher resistance to trimethoprim, tetracycline, minocycline, Isolates of S. agalactiae and S. dysgalactiae were sensitive to penicillin and amoxicillin. Conclusion: The present investigation showed that a higher prevalence of S. agalactiae than S. dysgalactiae in various mastitic animals (cows and buffalo) in Egypt. Furthermore the development of antibiotic-resistant and virulent strains of S. agalactiae and S. dysgalactiae occurred. Also, the slightly elevated MIC indicated the emergence of resistant streptococcal species. Consequently, continuous surveillance of antimicrobial-resistant mastitis pathogens is vital for the right decisions on bovine mastitis treatment.