Sukolrat Boonyayatra*, Pornwimon Pata, Panittiya Nakharuthai and Wasana Chaisri
Streptococcus agalactiae is a contagious mastitis pathogen commonly found in dairies in northern Thailand. During infection, S. agalactiae may form biofilms which is known to be associated with increased antimicrobial resistance of bacteria. We aimed to investigate changes in antimicrobial resistance of biofilm producing S. agalactiae associated with bovine mastitis. We measured biofilm formation, Minimum Inhibitory Concentrations (MIC) and Minimal Bactericidal Concentrations (MBC), and Minimum Biofilm Eradication Concentration (MBEC) of 56 archived isolates from bovine milk in Chiang Mai, Thailand. Quantitative biofilm evaluation found no (0%) strong, 21 (37%) moderate, and 30 (54%) weak biofilm producers, as well as 5 (9%) non-biofilm producers. Qualitative biofilm assay found only 11 isolates (20%) to be biofilm producers; these were further investigated for resistance to ampicillin, cloxacillin, cephalexin, gentamicin and tetracycline. All 11 isolates showed higher MBECs compared to MICs and MBCs. Some S. agalactiae strains from cows with clinical or subclinical mastitis can produce biofilms in vitro, and these appear more resistant to common antibiotics. Such resistance can be an obstacle in the eradication of S. agalactiae from infected herds. Determination of biofilm formation by S. agalactiae cultured from milk may be useful for creating an effective treatment plan and prognosis of bovine mastitis.
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