Dissemination of 16S rRNA methylase in AmpC producing Enterobacter cloacae, Citrobacter freundii and Serratia marcescens in Korea

Abstract number: p1274

Lee  S., Park  Y-J., Yu  J-K., Lee  K., Arakawa  Y.

Objectives: 

A new type of mechanisms, methylation of 16S rRNA, conferred by plasmid, has been reported in a few clinical isolates of gram-negative bacteria and it can confer high-level broad-range resistance to aminoglycosides except neomycin and streptomycin in gram-negative human pathogens. In this study, we investigated (1) the prevalence of high-level aminoglycoside resistance and plasmid-mediated 16S rRNA methylase in Enterobacter cloacae, Citrobacter freundii and Serratia marcescens in Korea, and (2) its association with ESBLs.

Methods: 

A total of 413 consecutive, non-duplicate isolates, including E. cloacae (158), C. freundii (126), and S. marcescens (129), collected from clinical specimens at 11 clinical laboratories, were used in this study. For isolates showing high-level resistance (MICs of 512 mg/ml) to amikacin or arbekacin, a search for the 16S rRNA methylase genes (armA, rmtA, and rmtB) was performed by PCR amplification with the following sets of primers: rmtA, rmtB, armA. For detection of extended-spectrum b-lactamase genes, the pairs of primers specific for blaTEM, blaSHV, bla CTX-M-1, M-2, M-9, and blaPER-1 were used.

Results: 

Of the total of 413 isolates, 49 were highly resistant to both amikacin and arbekacin. One E. cloacae isolate was highly resistant to arbekacin but susceptible to amikacin (MIC of 16 mg/ml). The prevalence of the high level resistance to amikacin or arbekacin was 9.5% (15/158), 10.3% (13/126), and 17.1% (22/129) in E. cloacae, C. freundii and S. marcescens, respectively and almost all of them (14 E. cloacae, 12 C. freundii and 21 S. marcescens isolates) harboured armA. In E. cloacae and C. freundii, all but one of the armA producers also harboured ESBLs (13 out of 14 E. cloacae, 11 out of 12 C. freundii) but in S. marcescens, 14 out of 21 armA-producing S. marcescens harboured ESBLs. The most common type of ESBL was CTX-M (Table 1). One C. freundii isolate harboured rmtB gene and it also harboured CTX-M type ESBL. None harboured rmtA gene.

Conclusion: 

The prevalence of high-level aminoglycoside resistance was quite high (12.1%), and it was highest in S. marcescens and followed by C. freundii and E. cloacae. Co-existence of ESBL was higher in E. cloacae (92.9%) and C. freundii (81.8%) than in S. marcescens (71.4%). CTX-M type was the most common. One C. freundii harboured rmtB gene and it also harboured CTX-M gene.