en Anti-microbial agents, resistance and treatment protocols Anti-microbial agents, resistance and treatment protocols Original article Original article <strong>Introduction:</strong> Heteroresistance is the phenomenon wherein subpopulations of microbes exhibit a range of resistance to an antibiotic and are prevalent in a broad range of microorganisms. Not much data on carbapenem and &beta;-lactams heteroresistance among extra-intestinal pathogenic <em>Escherichia coli</em> (ExPEC) in blood infections is available. This study aimed to investigate the frequency of heteroresistance to &beta;-lactams and carbapenems in ExPEC strains isolated from patients&#39; blood in Tehran, Iran. <strong>Methods</strong>: Identification of <em>E. coli</em> isolates was performed using standard biochemical tests, and PFGE was employed to determine the <em>E. coli</em> isolates genotypes. Antibiotic resistance pattern to carbapenems (ertapenem, meropenem, and imipenem), cefotaxime, and cefepime was determined by measuring minimum inhibitory concentration (MIC). The results were used to assess &beta;-lactam and carbapenem heteroresistance in <em>E. coli</em> isolates. <strong>Results:</strong> In this study, for the first time, heteroresistance to the antibiotics carbapenem (ertapenem, meropenem, and imipenem), cefotaxime, and cefepime was observed in <em>E. coli</em> isolates from blood samples. Sixteen samples had heteroresistance to antibiotics. Three and two isolates showed heteroresistance to imipenem and ertapenem, respectively. Simultaneous heteroresistance to two carbapenem family antibiotics in three isolates was detected. Also, three isolates showed heteroresistance to cefepime and five to cefotaxime.&nbsp;The imipenem-resistant isolates exhibited a higher level of heteroresistance than those resistant to other carbapenem antibiotics.<strong> Conclusions:&nbsp;</strong>The present study shows that heteroresistance is increasing in Iran. Furthermore, this study demonstrated increasing resistance to the carbapenem family. Extraintestinal Pathogenic Escherichia coli, Heteroresistance, Broth microdilution, Agar dilution 170 177 http://jommid.pasteur.ac.ir/browse.php?a_code=A-10-249-9&slc_lang=en&sid=1 Mohammad Taha Ebrahimi mtebrahimi@ymail.com 0000-0003-3177-7990 Yes Department of Microbiology, School of Medicine, Tehran University of Medical Sciences Mohsen Nazari mohsennazari911@gmail.com 0000-0001-8990-7036 No Department of Bacteriology, Pasteur Institute of Iran, Tehran, Iran Niloofar Mobarezpour 0000-0003-4569-0597 No PPD Tuberculin Department, Razi Vaccine & Serum Research Institute, Karaj, Iran Hamed Afkhami 0000-0002-1110-6447 No Department of Medical Microbiology, Faculty of Medicine, Shahed University of Medical Science, Tehran, Iran Sepideh Fereshteh 0000-0002-9622-2497 No Department of Bacteriology, Pasteur Institute of Iran, Tehran, Iran