Abstract- Extended-spectrum ?-lactamases (ES?Ls) are a rapidly evolving group of ?-lactamases which share the ability to hydrolyze thirdgeneration
cephalosporins and aztreonam yet are inhibited by clavulanic acid. In this study, the aim was to determine the susceptibility of
ES?L producing clinical isolates of Escherichia coli across various antimicrobial agents according to the Clinical Laboratory Standard Institute
(CLSI) new breakpoints. Sixty five clinical isolates of Escherichia coli were collected from a Tertiary Hospital in Nigeria of which forty six ES?L
-producing isolates emerged. Presence of ES?L was determined by double- disk synergy test, DNA extraction and amplification of ES?L
genes- blaSHV, blaTEM, blaCTX-M by Polymerase Chain Reaction. Susceptibility of the ES?L-producing isolates was determined by the CLSI agar
diffusion method. Utilizing the CLSI new breakpoints, 76.09%, 73.91%, 73.91% and 63.04% were resistant to Cefotaxime, Ceftazidime, Aztreonam
and Cefepime respectively. While, 19.6%, 15.2%, 21.7% and 40% were susceptible to Cefotaxime, Ceftazidime, Aztreonam and
Cefepime respectively. Furthermore, though a significant number of ES?L-producing isolates were susceptible to carbapenem; ES?L isolates
that harboured the blaSHV, blaTEM, blaCTX-M genes showed resistance to carbapenem. Thus, carbapenem-resistant ES?L-producing isolates
emerged from this study. In conclusion, Nigeria is a developing country affected by the spread of bacterial strains harbouring ES?L and with
the emergence of carbapenem resistance; it is certain to create significant therapeutic problems as carbapenem is the drug of choice for serious
infections caused by ES?L-producing Escherichia coli.