Biological cost of fluoroquinolone resistance in Escherichia coli implicated in polyclonal infection

Plasmidic qnrA3 enhances Escherichia coli fitness in absence of antibiotic exposure

The widespread presence of plasmid-mediated quinolone resistance determinants, particularly qnr genes, has become a current issue. By protecting DNA-gyrase from quinolones, Qnr proteins confer a low level quinolone resistance that is not sufficient to explain their emergence. Since Qnr proteins were hypothesized to act as DNA-binding protein regulators, qnr genes could have emerged by providing a selective advantage other than antibiotic resistance. We investigated host fitness of Escherichia coli isogenic strains after acquisition of the qnrA3 gene, inserted either alone onto a small plasmid (pBR322), or harbored on a large conjugative native plasmid, pHe96(qnrA3) found in a clinical isolate. The isogenic strains were derived from the susceptible E. coli CFT073, a virulent B2 group strain known to infect bladder and kidneys in a mouse model of pyelonephritis. In vitro experiments included growth analysis by automatic spectrophotometry and flow cytometry, and competitions with CFU enumeration. In vivo experiments included infection with each strain and pairwise competitions in absence of antimicrobial exposure. As controls for our experiments we used mutations known to reduce fitness (rpsL K42N mutation) or to enhance fitness (tetA deletion in pBR322). E. coli CFT073 transformed with pBRAM(PBR322-qnrA3) had significantly higher maximal OD than E. coli CFT073 transformed with pBR322 or pBR322ΔtetA, and in vivo competitions were more often won by the qnrA3 carrying strain (24 victories vs. 9 loss among 42 competitions, p = 0.001). In contrast, when pHe96(qnrA3) was introduced by conjugation in E. coli CFT073, it exerted a fitness cost shown by an impaired growth observed in vitro and in vivo and a majority of lost competitions (33/35, p<0.0001). In conclusion, qnrA3 acquisition enhanced bacterial fitness, which may explain qnr emergence and suggests a regulation role of qnr. However, fitness was reduced when qnrA3 was inserted onto multidrug-resistant plasmids and this can slow down its dissemination without antibiotic exposure.

Population-based investigation of fluoroquinolones resistant tuberculosis in rural eastern China

Empirical use of fluoroquinolones (FQ) to treat a variety of bacterial infections may inadvertently select for FQ-resistant strains of Mycobacterium tuberculosis(MTB), especially in rural China where the use of FQ in treating infections has not been standardized. Here we determine the prevalence and describe the transmission of FQ-resistant MTB in two rural counties in eastern China through a combination of conventional epidemiology with IS6110-based restriction fragment length polymorphism(RFLP) analysis and DNA sequencing of drug-resistance determining regions. Phenotypic FQ resistance was detected in 31 of 351(8.8%) isolates. FQ resistance was equally distributed between patient-isolates deemed drug resistant and drug-susceptible, but mostly observed in those with treatment history of respiratory infection. Mutations in gyrA were found in 54.8% of FQ resistant isolates, and one isolate with a gyrB mutation. Despite predominating in entire bacilli population(69.2%), Beijing family strain had similar proportion of FQ resistance to the other(10.3% vs. 4.7%, p = 0.060). IS6110RFLP identified 2 clusters(4 isolates) among FQ resistant isolates and 3 clusters composed of both 4 FQ resistant isolates and 6 FQ susceptible isolates. Our results indicate that FQ-resistant MTB has emerged among the circulating bacillary population in rural eastern China. The relatively low level of clustering among FQ-resistant strains suggests most are acquired de novo, likely due to widespread FQ use.

Insights into the environmental resistance gene pool from the genome sequence of the multidrug-resistant environmental isolate Escherichia coli SMS-3-5

The increasing occurrence of multidrug-resistant pathogens of clinical and agricultural importance is a global public health concern. While antimicrobial use in human and veterinary medicine is known to contribute to the dissemination of antimicrobial resistance, the impact of microbial communities and mobile resistance genes from the environment in this process is not well understood. Isolated from an industrially polluted aquatic environment, Escherichia coli SMS-3-5 is resistant to a record number of antimicrobial compounds from all major classes, including two front-line fluoroquinolones (ciprofloxacin and moxifloxacin), and in many cases at record-high concentrations. To gain insights into antimicrobial resistance in environmental bacterial populations, the genome of E. coli SMS-3-5 was sequenced and compared to the genome sequences of other E. coli strains. In addition, selected genetic loci from E. coli SMS-3-5 predicted to be involved in antimicrobial resistance were phenotypically characterized. Using recombinant vector clones from shotgun sequencing libraries, resistance to tetracycline, streptomycin, and sulfonamide/trimethoprim was assigned to a single mosaic region on a 130-kb plasmid (pSMS35_130). The remaining plasmid backbone showed similarity to virulence plasmids from avian-pathogenic E. coli (APEC) strains. Individual resistance gene cassettes from pSMS35_130 are conserved among resistant bacterial isolates from multiple phylogenetic and geographic sources. Resistance to quinolones was assigned to several chromosomal loci, mostly encoding transport systems that are also present in susceptible E. coli isolates. Antimicrobial resistance in E. coli SMS-3-5 is therefore dependent both on determinants acquired from a mobile gene pool that is likely available to clinical and agricultural pathogens, as well, and on specifically adapted multidrug efflux systems. The association of antimicrobial resistance with APEC virulence genes on pSMS35_130 highlights the risk of promoting the spread of virulence through the extensive use of antibiotics.