Loop-mediated isothermal amplification assay targeting the blaCTX-M9 gene for detection of extended spectrum β-lactamase-producing Escherichia coli and Klebsiella pneumoniae.

Extended-spectrum β-lactamases (ESBLs) produced by Enterobacteriaceae are one of the resistance mechanisms to most β-lactam antibiotics. ESBLs are currently a major problem in both hospitals and community settings worldwide. Rapid and reliable means of detecting ESBL-producing bacteria is necessary for identification, prevention and treatment. Loop-mediated isothermal amplification (LAMP) is a technique that rapidly amplifies DNA with high specificity and sensitivity under isothermal conditions. This study was aimed to develop a convenient, accurate and inexpensive method for detecting ESBL-producing bacteria by a LAMP technique. ESBLs-producing Escherichia coli and Klebsiella pneumoniae were isolated from a tertiary hospital in Bangkok, Thailand and reconfirmed by double-disk synergy test. A set of four specific oligonucleotide primers of LAMP for detection of bla(CTX-M9) gene was designed based on bla(CTX-M9) from E. coli (GenBank Accession No. AJ416345). The LAMP reaction was amplified under isothermal temperature at 63°C for 60 min. Ladder-like patterns of band sizes from 226 bp of the bla(CTX-M9) DNA target was observed. The LAMP product was further analyzed by restriction digestion with MboI and TaqI endonucleases. The fragments generated were approximately 168, 177 and 250 bp in size for MboI digestion and 165, 193, 229, 281 and 314 bp for TaqI digestion, which is in agreement with the predicted sizes. The sensitivity of the LAMP technique to bla(CTX-M9) was greater than that of the PCR method by at least 10,000-fold. These results showed that the LAMP primers specifically amplified only the bla(CTX-M9) gene. Moreover, the presence of LAMP amplicon was simply determined by adding SYBR Green I in the reaction. In conclusion, this technique for detection of ESBLs is convenient, reliable and easy to perform routinely in hospitals or laboratory units in developing countries.