Use of Boronic Acid Disk Tests To Detect Extended- Spectrum β-Lactamases in Clinical Isolates of KPC Carbapenemase-Possessing Enterobacteriaceae

During the last decade, carbapenem resistance has emerged among clinical isolates of the Enterobacteriaceae family, and this is increasingly attributed to the production of β-lactamases capable of hydrolyzing carbapenems (23). Among these enzymes, a new type of Ambler class A β-lactamase, the Klebsiella pneumoniae carbapenemase (KPC), has been rapidly spreading among K. pneumoniae isolates and other Enterobacteriaceae in the northeastern regions of the United States and has now spread to several regions of North and South America, as well as in Israel, China, and Greece (2, 13, 16, 21). The current spread of KPC enzymes makes them a potential threat to currently available antibiotic-based treatments. These enzymes confer various levels of resistance to all β-lactams, including carbapenems, even though cefamycins and ceftazidime are only weakly hydrolyzed (15, 18). KPC-possessing strains frequently carry extended-spectrum β-lactamase (ESBL) genes (1, 3, 8, 13, 24), which could possibly contribute to the expression and dissemination of the β-lactam resistance trait (8, 18, 21). It should be also noted that KPCs and ESBLs are mostly plasmid-encoded determinants that can easily disseminate to other enterobacterial strains (3, 7, 15, 18, 26). Therefore, the phenotypic detection of ESBLs in KPC-producing isolates of the Enterobacteriaceae is of potential interest for epidemiological purposes as well as for limiting the spread of the underlying resistance mechanisms. The CLSI recommends a phenotypic confirmatory test for ESBL production that consists of measuring the growth-inhibitory zones around both cefotaxime (CTX) and ceftazidime (CAZ) disks with or without clavulanate (CA) for K. pneumoniae, Klebsiella oxytoca, Escherichia coli, and Proteus mirabilis (4). Different double-disk synergy tests (DDSTs) based on the synergy of amoxicillin (amoxicilline)-clavulanate (AMC) with extended-spectrum cephalosporins and aztreonam have also been extensively used for the detection of ESBLs (7). However, strategies for the laboratory identification of ESBLs need to be reviewed and adjusted as additional mechanisms of resistance to β-lactams coexist in enterobacterial strains (7). KPCs hydrolyze several β-lactam antibiotics, and hence, the presence of an ESBL can be masked by the expression of a KPC. Moreover, the weak inhibition of KPCs by the β-lactam inhibitors (15, 18, 30) may interfere with the interpretation of ESBL detection methods and KPC enzymes may be mistaken for ESBLs. Thus, there is a need to accurately detect ESBLs in the presence of coexisting KPC expression. Boronic acid (BA) compounds were recently reported to be reversible inhibitors of KPCs (6, 16, 27). In particular, we have shown that BA disk assays are considered positive for the detection of the KPC enzyme when the growth-inhibitory zone diameter around a meropenem, imipenem, or cefepime disk with phenylboronic acid is 5 mm or greater of the growth-inhibitory zone diameter around the disk containing meropenem or cefepime alone (27). The results of this study also showed that BA affected the activity of CAZ in ESBL-negative KPC-producing isolates but not in SHV ESBL-positive KPC-producing isolates, most likely due to the presence of the SHV ESBL, which is not restrained by BA (27). BA-based tests with disks of CAZ and CTX have also been successfully employed for the identification of ESBLs in AmpC producers (11, 25). These observations led us to design a modified CLSI ESBL confirmatory test using antibiotic disks containing BA as well as different DDSTs employing BA for the accurate detection of ESBLs in KPC-producing enterobacterial isolates.