Genetic analysis and plasmid-mediated bla_CMY-2 in Salmonella and Shigella and the Ceftriaxone Susceptibility regulated by the ISEcp-1 tnpA-bla_CMY-2-blc-sugE

Background: Nontyphoid Salmonella and Shigella can cause gastroenteritis in humans. Ceftriaxone (CRO) has been used to treat their infection, however, development of CRO resistance are often associated with plasmid-mediated bla_CMY. Here, we investigated the presence of plasmid-mediated ISEcp-1 tnpA-bla_CMY-2-blc-sugE and the role of these genes in regulation of CRO susceptibility in different hosts.

Methods: 194 strains of Salmonella serovars and Shigella were tested for CRO susceptibility. Non-susceptibility strains were examined for plasmid-mediated ISEcp-1 tnpA-bla_CMY-2-blc-sugE by PCR amplification, Southern blot, and DNA sequencing. The plasmid profiles were determined by HindIII-digested restriction fragment length polymorphism (RFLP). Four recombinant plasmids with different genes from ISEcp-1 tnpA-bla_CMY-2-blc-sugE were constructed and then were transferred into Escherichia coli and different Salmonella serovars to evaluate the CRO susceptibility.

Results: Among 20 CRO-nonsusceptible isolates of Salmonella Choleraesuis (5), S. Typhimurium (4), S. Mons (1), S. Stanley (2) and Shigella sonnei (8) with plasmid-mediated bla_CMY-2, 19 isolates carried the ISEcp-1 tnpA-bla_CMY-2-blc-sugE and only one isolate with tnpA-bla_CMY-2. Transformation of these plasmids into E. coli pir116 produced multidrug resistance. Furthermore, PCR-RFLP analysis determined 5 different plasmid profiles and identical RFLP pattern between S. Typhimurium and S. sonnei. Transformation of the recombinant plasmids into E. coli and different Salmonella serovars resulted in phenotypes ranging from susceptible to resistant (especially inducible resistance) to CRO that were dependent on the genes, and host.

Conclusion: The CRO susceptibility associated with the ISEcp-1 tnpA-bla_CMY-2-blc-sugE element is regulated positively by ISEcp-1 tnpA and SugE and negatively regulated by Blc and unknown species-dependent host factor(s).