Outbreak of meropenem-resistant Serratia marcescens comediated by chromosomal AmpC beta-lactamase overproduction and outer membrane protein loss

Abstract

The aim of this study was to investigate the mechanisms involved in the meropenem resistance of Serratia marcescens clinical isolates. Meropenem-resistant (MIC range, 16 to 32 μg/ml) S. marcescens isolates were recovered from nine patients in a tertiary hospital in Seoul, South Korea, from June to November 2005. All the isolates shared identical or similar (>85% similarity) SpeI macrorestriction patterns, indicating clonal spread. PCR experiments did not detect any carbapenemase in those isolates. They carried the bla(CTX-M-22) gene located on a 150-kbp plasmid of the incompatibility group L/M; however, the addition of clavulanic acid exhibited few effects on meropenem MICs. Although meropenem MICs were reduced 4- to 16-fold with the addition of boronic acid, no plasmid-borne AmpC β-lactamase gene was detected in PCR experiments. Real-time quantitative PCR experiments showed that expression levels of the chromosomal ampC gene in those isolates were 87.06 to 155.76 times higher than that of the reference strain ATCC 8100. SDS-PAGE showed a lack of the 42-kDa outer membrane protein (OmpF). In combination with the overproduction of the chromosomal AmpC enzyme, the loss of OmpF may have played a role in the acquisition of meropenem resistance in our isolates.

FIG. 1.

SpeI macrorestriction patterns of nine S. marcescens clinical isolates and a comparison of AmpC gene expression levels with carbapenem MICs. MEM, meropenem; IPM, imipenem.

FIG. 2.

Diagrammatic scheme of IS1 insertion into the ompF gene. The ompF gene of S. marcescens was interrupted by the insertion of the IS1 sequence at the 129th nucleotide position of the ompF gene. Putative promoter sequences of the opmF gene are indicated by boldface and underlined type. Right and left inverted repeats (IRR and IRL, respectively) of the IS1 sequence are also indicated by boldface and underlined type. Direct repeats (DRs) flanking the IS1 region are indicated by boxes.