Son H. Ly , Tuan T. Pham , Han M. N. On , Bao D. Truong , & Thuong T. Nguyen *

* Correspondence: Nguyen Thi Thuong (email:

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This study aimed to determine subclinical mastitis (SCM) caused by bacterial species, using chromogenic culture media and to assess the antimicrobial resistance rate in the isolated bacteria. From March to December 2023, 143 milk samples were collected from 71 Holstein Friesian cows with SCM across seven dairy farms in Ho Chi Minh City and Binh Duong province. Milk samples were incubated in triplicate chromogenic culture media to identify SCM caused by microorganisms. Our study revealed that 39.2% (56/143) of the samples had the growth of a single morphology, 26.6% (38/143) exhibited growth of two distinct morphologies, 9.0% (13/143) were found to be contaminated, and 25.2% (36/143) showed no growth. The isolated Streptococcus species were Strep. agalactiae 34.3% (49/143), Strep. uberis 22.4% (32/143), and Enterococcus spp. 1.4% (2/143). Besides, S. epidermidis 20.3% (29/143), S. saprophyticus 14.7% (21/143), and S. aureus 4.2% (6/143) were frequently isolated among Staphylococcus species. For gram-negative bacteria causing SCM, E. coli 2.8% (4/143), Klebsiella spp. 1.4% (2/143), and Pseudomonas spp. 4.2% (6/143) were the most isolated. Regarding antimicrobial susceptibility testing, the resistance rate of each bacterial species to each antibiotic tested differed for Staphylococcus, Streptococcus, and gram-negative bacteria. Staphylococcus aureus was not resistant to gentamycin, florfenicol, and marbofloxacin. The resistance rate of S. epidermidis to gentamycin, florfenicol, trimethoprim sulfadiazine, and amoxicillin-clavulanic acid varied from 10.3% to 17.2%. Marbofloxacin and trimethoprim-sulfadiazine were excellent choices in treating SCM caused by S. saprophyticus because of their low resistance rate (10.3 - 13.3%). Streptococcus uberis was sensitive to the combined antibiotic amoxicillin-clavulanic acid. The resistance rate of Strep. agalactiae to this combined antibiotic (amoxicillin-clavuclanic acid) was the lowest (10%). Pseudomonas spp. was resistant to the tested antibiotics. Our study suggests that identifying bacterial species and conducting antimicrobial susceptibility tests play a crucial role in improving the treatment effectiveness for bovine SCM.

Keywords: Antimicrobial resistance, Chromogenic media, Dairy cow, Subclinical mastitis

Article Details


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