Genomic characterisation and epidemiology of nosocomial Serratia marcescens isolates resistant to ceftazidime and their plasmids mediating rare blaTEM-61

•Nosocomial spread of Tn1–blaTEM-61 transposon in two genotypes of Serratia marcescens.•A new polymorphic repeat unit was detected in the promoter of the blaTEM-61 gene.•Tn1–blaTEM-61 transposon was located on an S. marcescens narrow-host-range plasmid.•Intracellular translocation of Tn1–blaTEM-61 to a broad-host-range plasmid was confirmed. We determined the whole DNA sequences of plasmids carrying a rare extended-spectrum β-lactamase gene (blaTEM-61) to precisely understand the spread of resistance among nosocomial Serratia marcescens populations. Twenty non-duplicate ceftazidime-resistant S. marcescens nosocomial isolates (ceftazidime MICs, 32 to >128 mg/L) collected over 1 year were pulsotyped and nucleotide sequences of the blaTEM-61 gene and its promoter region were determined. Twelve representative isolates were analysed by whole-genome sequencing. The 20 isolates comprised two distinct pulsotypes: I (14 isolates) and II (6 isolates). They all contained the blaTEM-61 gene. A polymorphism in the repeat number of a 15-nucleotide sequence (5′-ATGTCATGATAATAA-3′) was found in the promoter region of blaTEM-61; two, three and four repeat units were found in 6, 12 and 2 isolates, respectively. Single nucleotide polymorphism (SNP)-based phylogenetic analysis of 12 isolates revealed that 7 isolates of pulsotype I (12–44 SNP differences) and 5 isolates of pulsotype II (15–55 SNP differences) formed two distinct clusters of genotypes 1 and 2, respectively. All 12 isolates harboured a plasmid carrying the Tn1–blaTEM-61 element, although they were slightly different in size (78 883 bp, 78 898 bp and 78 913 bp) owing to differences in the number of 15-bp repetitive sequences. A 42 542-bp broad-host-range plasmid carrying the Tn1–blaTEM-61 element was also found in one of the isolates. We characterised a plasmid-encoded novel Tn1–blaTEM-61 element and transposon-dependent mechanisms underlying the propagation of antibiotic resistance, together with repeated new polymorphic 15-bp units in the promoter of blaTEM-61. [Display omitted]