Identification and characterization of a spreadable IncI1 plasmid harbouring a blaCTX-M-15 gene in an Italian human isolate of Salmonella serovar Napoli

Salmonella enterica subsp. enterica serovar Napoli (S. Napoli) ranks among the top serovars causing human in-fections in Italy, although not common in other European countries. Isolates are generally pan-susceptible or resistant to aminoglycosides only, however data on antimicrobial resistance genes in strains of S. Napoli are limited. Recently an isolate encoding resistance to third generation cephalosporins was reported. This study aimed to characterize plasmid-encoded cephalosporin resistance due to the blaCTX-M-15 gene in a human S. Napoli isolate in Italy, and to investigate plasmid stability over time. S. Napoli 16/174478 was confirmed to be ESBL-producing. The bla(CTX-M-15) gene was shown to be located on an IncI1 alpha plasmid of 90,272 bp (50.03 GC%) encoding for 107 coding sequences (CDS). The plasmid was suc-cessfully transferred by conjugation to an E. coli 1816 recipient strain (conjugation frequency 3.9x 10(-2) transconjugants per donor). Transconjugants were confirmed to carry the IncI1 alpha plasmid, and to be ESBL-producing strains as well. Moreover, transconjugant colonies maintained the plasmid for up to 10 passages. The identification of S. Napoli isolates able to produce ESBLs is of great concern, as this pathogen is frequently associated with invasive infections and a higher risk of bacteraemia, and its reservoir has not yet been clearly identified.

Automated summary

The study characterized cephalosporin resistance in a human S. Napoli isolate in Italy, identifying the blaCTX-M-15 gene encoded on a plasmid. The plasmid was successfully transferred to an E. coli strain, maintaining stability for up to 10 passages, highlighting the concerning spread of ESBL-producing S. Napoli isolates in Italy.