Streptococcus agalactiae infections may cause clinical or subclinical Mastitis in dairy cows by invading the mammary gland. This research included the isolation of 29 strains of S. agalactiae from 425 milk samples obtained from cows affected by clinical Mastitis in Chaharmahal and Bakhtiari province, Iran. The antimicrobial sensitivity of S. agalactiae strains was determined using 16 antibiotics from seven different classes. The epidemiological spread of S. agalactiae was determined by identifying the serotypes of isolates using multiplex PCR. In addition, the presence of antibiotic-resistance genes and virulence genes were investigated to infer the pathogenicity and antibiotic resistance of S. agalactiae using the multiplex PCR method. A total of 29 strains of S. agalactiae, which constitute 6.82% of the samples, were identified based on phenotypic traits, biochemical properties and dltR gene amplification. Multiplex serotype polymerase chain reaction study showed that most of the isolates belonged to Type III serotype. Phenotypically, 100% of the isolated strains were resistant to tetracycline and penicillin. The frequency of resistance to beta-lactams (penicillin and amoxicillin) was 100% and 82.75%. tetM, tetO and tetT genes, responsible for resistance to tetracyclines, were found in all samples, corresponding to the drug-resistant phenotype. Among the genes related to the virulence factor, 100% of the isolates had the dlts gene. The examination of virulence genes revealed that the majority of isolates included the cfb, pavA and scPb genes. This data has the potential to assist in the prevention and management of Mastitis and enhance our comprehension of epidemiological patterns in dairy cows affected by S. agalactiae in Chaharmahal and Bakhtiari province.
