Attributes of carbapenemase encoding conjugative plasmid pNDM-SAL from an extensively drug-resistant Salmonella enterica Serovar Senftenberg

A carbapenem resistant Salmonella enterica serovar Senftenberg isolate BCH 2406 was isolated from a diarrheal child attending an outpatient unit of B.C. Roy Hospital in Kolkata, India. This isolate was positive for the bla NDM-1 in the PCR assay, which was confirmed by amplicon sequencing. Except for tetracycline, this isolate was resistant to all the tested antimicrobials. The bla NDM-1 was found to be located on a 146.13-kb mega plasmid pNDM-SAL, which could be conjugally transferred into Escherichia coli and other enteric pathogens such as Vibrio cholerae O1 Ogawa and Shigella flexneri 2a. However, the expression of β-lactam resistance is not the same in different bacteria. The whole genome sequence of pNDM-SAL was determined and compared with other pNDM plasmids available in public domain. This plasmid is an IncA/C incompatibility type composed of 155 predicted coding sequences and shares homology with plasmids of E. coli pNDM-1_Dok01, Klebsiella pNDM-KN, and Citrobacter pNDM-CIT. In pNDM-SAL, gene cluster containing bla NDM-1 was located between IS26 and IS4321 elements. Between the IS26 element and the bla NDM-1, a truncated ISAba125 insertion sequence was identified. Downstream of the bla NDM-1, other genes, such as ble MBL, trpF, tat, and an ISCR1 element with class 1 integron containing aac(6')-Ib were detected. Another β-lactacamase gene, bla CMY -4 was found to be inserted in IS1 element within the type IV conjugative transfer loci of the plasmid. This gene cluster had blc and sugE downstream of the bla CMY -4. From our findings, it appears that the strain S. Senftenberg could have acquired the NDM plasmid from the other members of Enterobacteriaceae. Transfer of NDM plasmids poses a danger in the management of infectious diseases.