Whole genome sequencing of Salmonella enterica serovar Saintpaul for elucidating the mechanisms of resistance to third generation cephalosporins.

An increase in the number of Salmonella enterica serovar Saintpaul observed in Singapore in 2015-2016 in humans was accompanied by increased resistance to third generation cephalosporins. We aimed to understand the genetic mechanisms contributing to this resistance. Whole genome sequencing using MiSeq was performed on 49 S. Saintpaul isolates collected between 2014-2016. Nanopore sequencing was also performed in an attempt to obtain a full genome of the plasmids. All but one S. Saintpaul isolates sequenced belonged to a single sequence type based on an in silico 7-gene multi-locus sequence typing scheme suggesting a clonal lineage. In total 27/49 were resistant to third generation cephalosporins as confirmed by the broth microdilution method; the resistance was due to the presence of either blaCTX-M-55 (n=23), blaCTX-M-27 (n=1) or blaCMY-2 (n=3) carried on a plasmid. Two isolates were also found to carry the mcr-1 gene on a different plasmid. Our study showed that all S. Saintpaul isolates resistant to third generation cephalosporins carried either blaCTX-M-55, blaCTX-M-27 or blaCMY-2 on a plasmid. Continuous monitoring of Salmonella serovars is warranted to track the potential spread of these plasmids.