Rodríguez-González, María José. Universidad Nacional de Costa Rica. Escuela de Medicina Veterinaria. Programa de Investigación en Enfermedades Tropicales, Heredia; Costa Rica.Jiménez-Pearson, María Antonieta. Instituto Costarricense de Investigación y Enseñanza en Nutrición y Salud, San José; Costa Rica.Duarte, Francisco. Instituto Costarricense de Investigación y Enseñanza en Nutrición y Salud, San José; Costa Rica.Poklepovich, Tomás. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas; Argentina.Campos, Josefina. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas; Argentina.Araya-Sánchez, Luis Nazario. Universidad Nacional de Costa Rica. Escuela de Medicina Veterinaria, Heredia; Costa Rica.Chirino-Trejo, Manuel. University of Saskatchewan. Western College of Veterinary Medicine, Saskatoon, Canadá.Barquero-Calvo, Elías. Universidad Nacional de Costa Rica. Escuela de Medicina Veterinaria. Programa de Investigación en Enfermedades Tropicales, Heredia; Costa Rica.Objective: This study aimed to determine the prevalence of fecal carriage of antibiotic-resistant Escherichia coli of healthy household dogs with an emphasis on extended-spectrum β-lactamases (ESBL), AmpC-type β-lactamases and resistance to quinolones. Materials and Methods: Rectal swabs were collected from 74 dogs without any clinical evidence of gastrointestinal disease. Samples were cultured on MacConkey agar plates and MacConkey supplemented with 2 μg/mL cefotaxime or 5 μg/mL ciprofloxacin. Isolates were identified with Vitek 2 Compact and susceptibility testing performed by Kirby Bauer disk diffusion method. Minimal inhibitory concentration (MIC) was done on isolates resistant to cefotaxime, ciprofloxacin, and nalidixic acid. PCR amplification was performed to detect CTX-M and CMY-2. Isolates positive for CTX-M and/or CMY-2 were selected for whole-genome sequencing. Results: Multiresistance was detected in 56% of the isolates. A high percentage of resistance was detected for cefazolin (63%), ampicillin (54%), streptomycin (49%), nalidixic acid (42%) and tetracycline (38%). The MIC50 and MIC90 for isolates resistant to cefotaxime (24%) was determined as 16 and >250 μg/mL, respectively; for ciprofloxacin (18%), 125 and 250 μg/mL, respectively. ESBL (CTX-M type) and AmpC (CMY-2 type) were detected in 6 (7.1%) and 14 (19%) of the isolates, respectively. Whole-genome sequence analysis showed high genetic diversity in most of the isolates and a large variety of resistance mechanisms, including mobile genetic elements. Conclusion: The frequency of multidrug-resistant E. coli is worrying, mainly because of the presence of many isolates producing ESBL and AmpC β-lactamases. Based on the "One Health" concept, considering the relationships between animals, humans, and the environment, these data support the notion that companion animals are important reservoirs of multidrug-resistant bacteria.