Characterization of pFOX-7a, a conjugative IncL/M plasmid encoding the FOX-7 AmpC-type β-lactamase, involved in a large outbreak in a neonatal intensive care unit

Objectives: FOX-type enzymes are a lineage of AmpC-type beta-lactamases from Aeromonas spp. whose genes have been mobilized to plasmids spreading among Enterobacteriaceae, where they can be responsible for resistance to extended-spectrum cephalosporins and beta-lactamase inhibitor combinations. Little is known about the genetic context and plasmid vehicles of bla(FOX) determinants. Here, we have characterized a plasmid encoding the FOX-7 beta-lactamase, which was involved in a large outbreak caused by two Klebsiella pneumoniae clones in a neonatal intensive care unit. Methods: Plasmid transferability was tested in conjugation experiments using Escherichia coli recipients. Plasmids from different strains were compared by restriction profiling and PCR mapping. The complete sequence of pFOX-7a plasmid was determined by a next-generation sequencing approach followed by gap filling using PCR and sequencing. Results: An apparently identical conjugative plasmid encoding FOX-7 was detected in representatives of the K. pneumoniae clones that caused the outbreak and in sporadic FOX-7-producing strains of other species from the same ward. The plasmid, named pFOX-7a, has an IncL/M-type backbone and two separate resistance modules including a Tn3-like transposon and a novel Tn1696 derivative, named Tn6234, which carries an integron platform, a hybrid (but still functional) mercury resistance module and a novel putative transposon of original structure, named Tn6240, associated with the bla(FOX-7) gene. Conclusions: pFOX-7a is the first completely characterized plasmid encoding a FOX-type beta-lactamase. The bla(FOX-7) gene was associated with a putative transposable element of original structure, which was likely involved in its mobilization from the Aeromonas metagenome.