Phenotypic Cefotaxime-Resistant Escherichia coli Screening and Detection of Associated Extended-Spectrum β-Lactamase Genes in Online-Marketed Fish from Kochi, India

Abstract

Online marketplaces are booming, and in cities like Kochi, Kerala, many households rely on them for supplies, including fish. Antimicrobial resistance is a growing trend across the food sector. Therefore, this research investigated the presence of cefotaxime resistant Escherichia coli in fish sold through these online platforms. A total of 100 fish samples obtained from online retailers in Kochi, India, were screened for cefotaxime-resistant E. coli using MacConkey agar supplemented with cefotaxime (1 μg/mL). Confirmed isolates underwent antimicrobial susceptibility testing (17 antibiotics), double disc synergy testing (DDST), and PCR detection of ESBL-associated genes (bla_CTX-M-1, bla_CTX-M-2, bla_CTX-M-9, bla_TEM, bla_SHV, bla_OXA-1-like). Multiple antibiotic resistance (MAR) index and multidrug resistance (MDR) of the isolates were calculated. Cefotaxime-resistant E. coli was detected in 28% of samples, yielding 57 isolates. Among these, 85.9% (49/57) were multidrug resistant (MDR). High resistance was observed to amoxicillin-clavulanic acid (98%), followed by cefpodoxime (84%) and ceftazidime (60%). Phenotypic ESBL production was confirmed in 67% (36/ 57) of isolates. However, ESBL-associated genes were detected in a smaller proportion: bla_CTX-M-1 (16.6%), bla_TEM (20.8%), and both genes (4.1%). No isolates carried bla_SHV or bla_OXA-1-like genes. The MAR index ranged from 0.059 to 0.824, with 37% of isolates exhibiting MAR > 0.2. While cefotaxime resistance enabled selection of multidrug-resistant isolates, molecular confirmation demonstrated that only a subset carried ESBL genes, suggesting additional resistance mechanisms were involved. These findings highlight the importance of surveillance of antimicrobial-resistant E. coli in emerging food distribution systems such as online seafood markets.