Detection of a novel SME-6 Carbapenemase in Serratia ureilytica in Germany

Abstract

Background Carbapenem resistance in Serratia species is occasionally mediated by the serine carbapenemase Serratia marcescens enzymes (SMEs). During microbiological diagnostics, we identified a carbapenem-resistant Serratia ureilytica isolate in which resistance was not mediated by any known SME variants or other characterized carbapenemases. Objectives To identify and characterize the underlying resistance mechanism in a carbapenem-resistant Serratia ureilytica isolate that lacks known SME variants or other characterized carbapenemases. Methods Antimicrobial susceptibility testing (AST) was performed using Vitek, gradient strips, broth microdilution, and disc diffusion methods. WGS was performed to identify the resistance mechanisms. Growth curve analysis and RT-qPCR were performed at 30°C and 37°C. Results WGS identified a novel SME variant, SME-6, which differed from a known variant, SME-2, by two amino acids (G117R and G147E). AST showed carbapenem resistance at 30°C but susceptibility at 37°C. Growth curve analysis showed a shorter lag phase at 30°C compared with 37°C, and RT-qPCR showed a ∼3-fold higher blaSME expression at 30°C. Conclusions To the best of our knowledge, this study reports the first identification of SME-6 and the first detection of an SME-type carbapenemase in Germany. Resistance was found to be temperature-dependent, with faster growth and higher SME-6 expression at lower temperatures contributing to the phenotype. These findings suggest SME variants may be underdiagnosed using current diagnostic protocols, highlighting the need for adjustments to improve detection of temperature-sensitive resistance mechanisms.