Highly conserved composite transposon harbouring aerobactin iuc3 in Klebsiella pneumoniae from pigs.

is an important opportunistic pathogen associated with severe invasive disease in humans. Hypervirulent , which are with several acquired virulence determinants such as the siderophore aerobactin and others, are more prominent in countries in South and South-East Asia compared to European countries. This pathotype is capable of causing liver abscesses in immunocompetent persons in the community. has not been extensively studied in non-human niches. In the present study, isolated from caecal samples (=299) from healthy fattening pigs in Norway were characterized with regard to population structure and virulence determinants. These data were compared to data from a previous study on from healthy pigs in Thailand. Lastly, an in-depth plasmid study on with aerobactin was performed. Culturing and whole-genome sequencing was applied to detect, confirm and characterize isolates. Phylogenetic analysis described the evolutionary relationship and diversity of the isolates, while virulence determinants and sequence types were detected with Kleborate. Long-read sequencing was applied to obtain the complete sequence of virulence plasmids harbouring aerobactin. A total of 48.8 % of the investigated Norwegian pig caecal samples (=299) were positive for . Acquired virulence determinants were detected in 72.6 % of the isolates, the most prominent being aerobactin (69.2 %), all of which were . In contrast, only 4.6 % of the isolates from Thailand harboured aerobactin. The aerobactin operon was located on potentially conjugative IncFIB/FII plasmids of varying sizes in isolates from both countries. A putative, highly conserved composite transposon with a mean length of 16.2 kb flanked by truncated IS-family IS-group insertion sequences was detected on these plasmids, harbouring the aerobactin operon as well as several genes that may confer increased fitness in mammalian hosts. This putative composite transposon was also detected in plasmids harboured by from several countries and sources, such as human clinical samples. The high occurrence of harbouring aerobactin in Norwegian pigs, taken together with international data, suggest that pigs are a reservoir for with . Truncation of the flanking ISKpn78-element suggest that the putative composite transposon has been permanently integrated into the plasmid, and that it is no longer mobilizable.