Prospective Molecular and Phenotypic Characterization of blaCTX-M and blaTEM ESBL Genes in Uropathogenic Escherichia coli from ICU Patients at Three Tertiary-Care Hospitals in Tabriz, Iran.

BACKGROUND: Urinary tract infections (UTIs) caused by extended-spectrum β-lactamase (ESBL)-producing Escherichia coli have become a major therapeutic challenge worldwide, especially in intensive care units. In northwestern Iran, the molecular epidemiology of the key resistance genes blaCTX-M and blaTEM, along with their potential co-carriage on plasmids, remains poorly understood. This gap limits the development of effective local empirical treatment guidelines and infection control strategies.

METHODS: A cross-sectional study was conducted at three tertiary-care hospitals in Tabriz, Iran (2025). We identified 100 non-duplicate Escherichia coli isolates recovered from ICU patients with UTIs using standard microbiological methods. Antimicrobial susceptibility testing was performed by the disk diffusion method according to CLSI 2024 guidelines. ESBL phenotype was confirmed using the combined disk test with Ceftazidime and Clavulanic acid. Genotypic detection of blaTEM and blaCTX-M was performed by conventional PCR. Chi-square and Fisher's exact tests were used to assess statistical associations.

RESULTS: Among 135 culture-positive urine specimens, E. coli was the predominant pathogen (74.07%), followed by Klebsiella pneumoniae (11.85%). Universal resistance to Cefotaxime (100%) and high resistance to Ceftazidime (95%) were observed. Imipenem retained the highest in vitro activity (82% susceptibility). Phenotypic ESBL production was detected in 48/100 (48%) isolates. Among ESBL producers, blaTEM was present in 28 (58.3%), blaCTX-M in 20 (41.7%), and both genes co-occurred in 18 isolates (37.5%). The observed co-occurrence significantly exceeded expected values under independence (observed: 18 vs. expected: 11.7; Chi-square p = 0.0005; Fisher's exact p = 0.00025), demonstrating non-random co-occurrence consistent with, but not proof of, plasmid-mediated co-carriage.

CONCLUSION: ESBL-producing E. coli carrying both blaTEM and blaCTX-M are highly prevalent among ICU-associated UTI in Tabriz. The statistically significant non-random co-occurrence of these genes has direct implications for surveillance strategies, necessitating simultaneous genotyping of both genes. Carbapenems remain effective, underscoring the urgency of their stewardship to prevent the emergence of carbapenem-resistant Enterobacterales (CRE) in this setting.