Antibiotic resistance of Escherichia coli isolated from dairy cow milk associated with subclinical mastitis in the Licin Subdistrict, East Java, Indonesia

Background: The incidence of bacterial resistance is a serious global health concern because it reduces the effectiveness of antibiotics and complicates the treatment of infectious diseases. Mastitis is one of the most common and economically significant diseases in the dairy industry, leading to reduced milk yield, altered milk quality, and financial losses for farmers. Antibiotics are commonly used for mastitis treatment, but their excessive or inappropriate use can accelerate the development of antimicrobial resistance (AMR). Escherichia coli, a major foodborne pathogen, is also recognized as an important causative agent of bovine mastitis, with contamination often originating from milk and dairy products. Aim: This study investigated the antimicrobial resistance of E. coli isolated from dairy cows with subclinical mastitis in the Licin Subdistrict to provide information on suitable antimicrobial treatment for mastitis. Methods: Exploratory laboratory research was conducted using purposive sampling. Milk samples from lactating Holstein Friesian dairy cows were screened for subclinical mastitis using the California mastitis test (CMT). Samples with CMT scores of +++ and ++++ were processed for E. coli isolation using eosin methylene blue agar, Gram staining, and Indole, Methyl Red, Voges Proskauer, and Citrate biochemical tests. Resistance testing against different antibiotics was conducted using the Kirby–Bauer disc diffusion technique. Results: A total of 29 milk samples were examined; 17 (58.6%) samples tested positive for subclinical mastitis. Among the 17 subclinical mastitis-positive samples, 4 (23.5%) were positive for E. coli. Antibiotic resistance testing of the E. coli isolates showed 100%, 25%, and 25% resistance to erythromycin, tetracycline, and streptomycin, respectively. All E. coli isolates (100%) were sensitive to aztreonam, chloramphenicol, trimethoprim, and ciprofloxacin. Conclusion: The antibiotic resistance patterns observed in E. coli isolates pose significant challenges for mastitis treatment in dairy farms. Although complete resistance to erythromycin limits therapeutic options, the maintained sensitivity to aztreonam, chloramphenicol, trimethoprim, and ciprofloxacin provides viable alternatives. These findings emphasize the importance of antimicrobial stewardship and routine resistance monitoring to preserve effective treatment options and ensure sustainable dairy production.