Escherichia coli producing extended-spectrum beta-lactamase (ESBL) are antimicrobial- -resistant Enterobacteriaceae important in the livestock production sector, especially dairy cows because these antimicrobial-resistant bacteria can be transferred to consumers via consumption. If antimicrobials are continually used in dairy farms, this may result in antimicrobial resistance. Therefore, investigation of antimicrobial resistance and finding new alternative methods for inhibiting ESBL-producing E. coli is essential. Hence, the aim of this study was to examine the ability of selected antimicrobials to inhibit E. coli ATCC 25922, control bacteria and ESBL-producing E. coli isolated from dairy farms. We also investigated the capacity of polyphenol extract from 10 varieties of medicinal plants to inhibit ESBL-producing E. coli using a broth microdilution method. It was found that control bacteria were susceptible to all antimicrobial agents, i.e., ampicillin, cefotaxime, ciprofloxacin, chloramphenicol, gentamycin, imipenem, nalidixic acid, tetracycline, and sulfamethoxazole/ trimethoprim. However, ESBL-producing E. coli exhibited both susceptibility and resistance to selected antimicrobials. The polyphenol extracted from Psidium guajava Linn at the lowest concentration was 4.5 mg/mL, which could inhibit control bacteria, but at the same concentration could not inhibit ESBL-producing E. coli. These phenomena indicated that ESBL-producing E. coli had both susceptibility and resistance to antimicrobials. Polyphenol, which could inhibit non-resistant E. coli, could not inhibit ESBL-producing E. coli.