We evaluated highly sensitive methods using boronic acid (BA) to detect extended-spectrum ␤-lactamase (ESBL) production. A total of 182 clinical isolates of Klebsiella spp. (n ‫؍‬ 118) and Escherichia coli (n ‫؍‬ 64) were analyzed: 62 harbored only ESBLs, 80 harbored both ESBLs and plasmid-mediated AmpC ␤-lactamases (pAmpCs), and 40 harbored only pAmpCs. The CLSI confirmatory test detected all isolates that produce only ESBLs but detected 85% of isolates that produce both enzymes. When a >5-mm increase in the zone diameter of either the cefotaxime (CTX) or the ceftazidime (CAZ) disk in the presence of both clavulanic acid (CA) and BA was considered to be a positive result, the test detected all isolates that harbor ESBLs (؎ pAmpCs) but showed frequent false-positive results (50%) for isolates that produce only pAmpCs. Meanwhile, when a >3-mm increase in the zone diameter of either the CTX/BA or the CAZ/BA disk in the presence of CA was considered to be a positive result, the test also detected all isolates that harbor ESBLs (؎ pAmpCs) and showed less frequent false-positive results (5%) in isolates that produce only pAmpCs. The latter new interpretive guideline has enhanced detection of ESBLs in clinical isolates of Klebsiella spp. and Escherichia coli and allowed detection of an ESBL even when potentially masked by a pAmpC.

This work identifies an ISCR1-related bla CTX-M-14 gene, which has never been reported before, from a clinical isolate of Escherichia coli. The bla CTX-M-14 gene was preceded by an ISCR1 element that was followed by a class 1 integron containing three different insert gene cassettes, i.e., dfrA12, orfF, and aadA2.