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

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.