Dissemination of IncFII plasmids carrying fos A3 and blaCTX-M-55 in clinical isolates of Salmonella enteritidis.
Multidrug-resistant Salmonella Enteritidis (S. Enteritidis) isolates have become a significant threat to public health, and fosfomycin has been proposed as one of the therapeutic antibiotics for serious infections by resistant pathogens. In this study, a total of 501 clinical S. Enteritidis isolates were screened and 14 (2.8%) isolates exhibited resistance to fosfomycin (MIC ≥ 1,024 μg/mL) as well as ceftriaxone (MIC ≥ 128 μg/mL). The fos A3 gene was identified in these 14 isolates. The fos A3 gene that co-transferred with blaCTX-M-55 was observed on the IncFII plasmids with sizes of ~ 78 (n = 7) or ~ 111 (n = 2) kbp in 9 transconjugants. The fos A3-bearing plasmid p12367A is 111,764 bp in length and possessed a typical IncFII backbone. A 7.6-kbp multidrug resistance region (MRR) was identified in p12367A, which was comprised of fos A3 and blaCTX-M-55 genes interspersed with ΔIS Ecp1 and three copies of IS 26. Two typical antibiotic resistance determinants (IS 26-orf3-orf2-orf1-fos A3-IS 26 and IS 26-orf477-blaCTX-M-55-ΔIS Ecp1 -IS 26) shared one IS 26 in the MRR. The genetic arrangement of the MRR may have resulted from the stepwise integration of IS 26 mobile elements via homologous recombination. Horizontal transfer of IncFII plasmids might contribute to the dissemination of fos A3 and blaCTX-M-55 resistance genes in S. Enteritidis interspecies. These findings underline further challenges for the prevention and treatment of Enterobacteriaceae infections posed by epidemic IncFII plasmids bearing fos A3-blaCTX-M-55.