An improved primer design for the loop-mediated isothermal amplification (LAMP) method to detect oxacillinase (OXA)-48 β-lactamase genes in Gram-negative bacteria for clinical applications

Introduction: Recently, increased frequencies of carbapenemase-producing Enterobacteriaceae have been reported worldwide. Among multiple genetic subtypes, oxacillinase (OXA)-48 β-lactamase-producing strains have been associated with inbound infection because they have been detected predominantly in patients who traveled outside of Japan. However, a recent case report of OXA-48 β-lactamase-producing Enterobacteriaceae suggested the latent spread of domestic infections. Due to a lack of specific inhibitors, culture-based detection of OXA-48 β-lactamase-producing bacteria is difficult. Thus, DNA-based detection methods, including PCR, direct sequencing and loop-mediated isothermal amplification (LAMP), have been employed. Among these methods, LAMP detection is more favorable than other methods because of its technical simplicity and low cost. Methods: We designed novel LAMP primers to detect OXA-48 β-lactamase-producing bacteria and investigated their possible clinical applications with bacterial genome-spiked human materials (cerebrospinal fluid, blood, feces, urine, and sputum). We evaluated the specificity of the LAMP primers using 37 bacterial strains: 8 standard, 9 reference, and 20 clinical Gram-negative strains. Results: Our LAMP primers detected 10 copies of the OXA-48 type β-lactamase gene and exhibited no cross reactivity with other β-lactamase genes. Sensitivity was not influenced in any clinical sample, in contrast to PCR detection, which was strongly inhibited by substances in fecal samples. Conclusions: These results suggest the superior performance of LAMP compared with conventional PCR for detecting the OXA-48 type β-lactamase gene in various clinical samples.