Characteristics of two transferable aminoglycoside resistance plasmids in Escherichia coli isolated from pig and chicken manure

pRKZ3 is a non-conjugative IncQ plasmid, while pKANJ7 is a conjugative IncX plasmid. The optimal mating time of pKANJ7 varied under different conditions. Both of the two transferable ARPs had little impact on the growth of their hosts. A relatively high level of fitness cost was observed for pKANJ7. The fitness cost of ARPs depended on their hosts. Plasmid-mediated antibiotic resistance genes (ARGs) have recently become a more prominent concern in the global environment. However, the prevalence of aminoglycoside resistance plasmids in the livestock industry is under reported. In this study, two transferable aminoglycoside resistance plasmids, pRKZ3 and pKANJ7, isolated from pig and chicken manure, were characterized. Results showed that pRKZ3 (8236 bp) is a non-conjugative IncQ plasmid and contains genes encoding for plasmid replication and stabilization (repA, repB and repC), mobilization (mob), and antibiotic resistance (arr-3 and aacA). pKANJ7 (30142 bp) is a conjugative IncX plasmid which codes for a type IV secretion system (T4SS). Conjugative transfer experiments showed that the optimal mating time of pKANJ7 was 8 h under the starvation condition, but the number of tranconjugants increased with time under the nutrient condition. Statistical analysis indicated that the two plasmids had little impact on the growth of their hosts, but a relatively high level of fitness cost due to pKANJ7 was observed. We also found that the fitness cost of plasmids depended on their hosts. Compared with pKANJ7, the relative fitness cost index of pRKZ3 varied within a narrow range during the 10 days of competition. The low level of fitness cost of pRKZ3 might contribute to the persistence of the plasmid in the environment. Our study provides new information for understanding the characterizations of antibiotic resistance plasmids (ARPs) in manure sources and helps to clarify the transfer and persistence of ARPs in the environment following the application of manure.