Serratia marcescens enzyme SME-2 isolated from sputum in New Zealand

INTRODUCTION: The Serratia marcescens enzymes (SMEs) are chromosomally encoded Ambler Class A carbapenem-hydrolysing β-lactamases, which distinctively express resistance to carbapenems while remaining susceptible to extended-spectrum cephalosporins. Global reports of SMEs are infrequent. Here we describe the isolation of an SME-2-producing S. marcescens from the sputum of a patient who was hospitalized at Christchurch Hospital, New Zealand. METHODS: An immunosuppressed asthmatic patient who presented with shortness of breath and hypoxia grew S. marcescens from a sputum culture. Antimicrobial susceptibilities were determined by Phoenix, with MICs of meropenem and imipenem determined by Liofilchem(®) MIC gradient strips and interpreted according to EUCAST breakpoints. Investigation for carbapenemase was performed using Carba NP, modified CIM (mCIM) and GeneXpert(®) Carba-R. WGS was performed using the Illumina DNA Prep library kit and sequenced using MiSeq. RESULTS: The isolate showed an unusual susceptibility profile, including high-level resistance to meropenem and imipenem, while remaining susceptible to extended-spectrum cephalosporins. The Carba NP and mCIM were positive and WGS demonstrated the presence of a bla(SME-2) gene located on the chromosome within the SmarGI1-1 genomic island. In addition, a bla(SRT)-like class C β-lactamase, aac(6′)-Ic aminoglycoside-modifying enzyme and various multidrug efflux mechanisms were found. Phylogenetic core-genome analysis indicated no matching genome with RefSeq database strains. CONCLUSIONS: S. marcescens is an opportunistic pathogen of concern, harbouring a variety of intrinsic resistance mechanisms, including the potential for stable AmpC hyperproduction. Globally, SME-type carbapenemases have been infrequently reported; however, isolates carrying this mechanism could have limited treated options, having implications for patient management. To the best of our knowledge this is the first report of SME in New Zealand.