Selection Pressure Required for Long-Term Persistence ofblaCMY-2-Positive IncA/C Plasmids
ABSTRACTMultidrug resistanceblaCMY-2plasmids that confer resistance to expanded-spectrum cephalosporins have been found in multiple bacterial species collected from different hosts worldwide. The widespread distribution ofblaCMY-2plasmids may be driven by antibiotic use that selects for the dissemination and persistence of these plasmids. Alternatively, these plasmids may persist and spread in bacterial populations in the absence of selection pressure if a balance exists among conjugative transfer, segregation loss during cell division, and fitness cost to the host. We conducted a series of experiments (bothin vivoandin vitro) to study these mechanisms for threeblaCMY-2plasmids, peH4H, pAR060302, and pAM04528. Results of filter mating experiments showed that the conjugation efficiency ofblaCMY-2plasmids is variable, from <10−7for pAM04528 and peH4H to ∼10−3for pAR060302. Neither peH4H nor pAM04528 was transferred fromEscherichia colistrain DH10B, but peH4H was apparently mobilized by the coresident trimethoprim resistance-encoding plasmid pTmpR. Competition studies showed that carriage ofblaCMY-2plasmids imposed a measurable fitness cost on the host bacteria bothin vitro(0.095 to 0.25) andin vivo(dairy calf model). Long-term passage experiments in the absence of antibiotics demonstrated that plasmids with limited antibiotic resistance phenotypes arose, but eventually drug-sensitive, plasmid-free clones dominated the populations. Given that plasmid decay or loss is inevitable, we infer that some level of selection is required for the long-term persistence ofblaCMY-2plasmids in bacterial populations.