Abstract: Objective: In this study, the prevalence, antimicrobial susceptibility, antibiotic resistance genes and plasmid horizontal transfer of IncI1 and IncN plasmid positive Salmonella isolated from retail foods, clinical patients and food animals in the provincial-level regions of Beijing, Shanghai, Henan, Sichuan, Guangdong, Guangxi, Shaanxi and Xinjiang were investigated. Methods: The incompatible plasmids IncI1 and IncN in Salmonella were determined by a polymerase chain reaction (PCR)-based replicon typing method, the antimicrobial susceptibility was tested by the agar dilution method, and the antibiotic resistance genes were detected using PCR. The horizontal transfer of IncI1 and IncN plasmids was conducted via conjugation. Results: Forty-two IncI1 positive isolates (4.39%) and 3 IncN positive isolates (0.31%) were identified among the 956 strains of Salmonella. Of these, 26 representatives of IncI1 and/or IncN plasmid positive Salmonella showed the highest prevalence of antibiotic resistance to ceftiofur (100.0%), nalidixic acid (92.3%), ampicillin (92.3%), cefoperazone (88.5%), tetracycline (84.6%), ceftriaxone (80.8%), and sulfamethoxazole/trimethoprim (76.9%), streptomycin (76.9%) and chloramphenicol (61.5%) followed by kanamycin (26.9%), gentamicin (23.1%), polymyxin B (23.1%), ciprofloxacin (19.2%), amoxicillin-clavulanic acid (15.4%), cefoxitin (11.5%) and amikacin (3.8%). The IncI1 plasmid positive Salmonella exhibited broader antibiotic-resistance spectra and higher resistance to cephalosporins than the IncN plasmid positive one, whereas the IncN plasmid positive isolates were more resistant to aminoglycoside antibiotics. blaTEM and blaCTX-M were more frequently detected from the IncI1 positive Salmonella, while qnrA, qnrB and qnrS were more prevalent in the IncN positive isolates. The IncI1 and/or IncN plasmid in the donor strains could be transferred to the recipients Salmonella and Escherichia coli at conjugation frequencies ranging from 3.2 × 10-5 to 2.0 × 10-3 and 8.7 × 10-7 to 9.6 × 10-5 transconjugant per recipient cell, respectively. The antibiotic resistance genes qnrB, acc(6’)-Ib, blaTEM and blaCTX-M could be transferred from the donors to the recipients during conjugation to impart the recipients with antibiotic resistance phenotypes to kanamycin, ceftiofur, ceftriaxone, ampicillin, streptomycin, and gentamicin after conjugation. Conclusion: The prevalence of IncI1 and IncN plasmid positive Salmonella was relatively low, and the antibiotic resistance phenotypes of the hosts were found to be associated with the types of plasmids they carried to some extent. The antibiotic resistance genes in the plasmids could be transferred from the donor to the recipient by horizontal gene flow among different species to confer antibiotic resistance to the recipient.

Key words: Salmonella; IncN plasmid; IncI1 plasmid; antibiotic resistance genes; conjugative transfer