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 β-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 β-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 β-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 β-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.