Mutant prevention concentrations of fluoroquinolones for Enterobacteriaceae expressing the plasmid-carried quinolone resistance determinant qnrA1

Impact of low-level resistance to fluoroquinolones due to qnrA1 and qnrS1 genes or a gyrA mutation on ciprofloxacin bactericidal activity in a murine model of Escherichia coli urinary tract infection

We investigated the impact of low-level resistance to fluoroquinolones on the bactericidal activity of ciprofloxacin in a murine model of urinary tract infection. The susceptible Escherichia coli strain CFT073 (ciprofloxacin MIC [CIP MIC] of 0.008 microg/ml) was compared to its transconjugants harboring qnrA1 or qnrS1 and to an S83L gyrA mutant. The three derivatives showed similar low-level resistance to fluoroquinolones (CIP MICs, 0.25 to 0.5 microg/ml). Bactericidal activity measured in vitro after 1, 3, and 6 h of exposure to 0.5 microg/ml of ciprofloxacin was significantly lower for the derivative strains (P < 0.01). In the murine model of urinary tract infection (at least 45 mice inoculated per strain), mice were treated with a ciprofloxacin regimen of 2.5 mg/kg, given subcutaneously twice daily for 2 days. In mice infected with the susceptible strain, ciprofloxacin significantly decreased viable bacterial counts (log10 CFU/g of tissue) in the bladder (4.2 +/- 0.5 versus 5.5 +/- 1.3; P = 0.001) and in the kidney (3.6 +/- 0.8 versus 5.0 +/- 1.1; P = 0.003) compared with those of untreated mice. In contrast, no significant decrease in viable bacterial counts was observed with any of the three derivative strains. The area under the concentration-time curve from 0 to 24 h/MIC and the maximum concentration of drug in serum/MIC ratios measured in plasma were indeed equal to 827 and 147, respectively, for the parental strain, and only 12.4 to 24.8 and 2.2 to 4.4, respectively, for the derivative strains. In conclusion, low-level resistance to fluoroquinolones conferred by a qnr gene is associated with decreased bactericidal activity of ciprofloxacin, similar to that obtained with a gyrA mutation.

Drug interactions and the evolution of antibiotic resistance

Large-scale, systems biology approaches now allow us to systematically map synergistic and antagonistic interactions between drugs. Consequently, drug antagonism is emerging as a powerful tool to study biological function and relatedness between cellular components as well as to uncover mechanisms of drug action. Furthermore, theoretical models and new experiments suggest that antagonistic interactions between antibiotics can counteract the evolution of drug resistance.

Prevalence of plasmid-mediated quinolone resistance determinants over a 9-year period

Recently, several plasmid-mediated quinolone resistance (PMQR) genes conferring low levels of quinolone resistance have been discovered. To evaluate the temporal change in the prevalence of PMQR genes over a decade in a tertiary hospital in the Republic of Korea, we selected every fifth isolate of Escherichia coli and Klebsiella pneumoniae and every third isolate of Enterobacter cloacae between 1998 and 2001 and between 2005 and 2006 from a collection of blood isolates. Six PMQR genes [qnrA, qnrB, qnrC, qnrS, aac(6')-Ib-cr, and qepA] were screened by multiplex PCR and then confirmed by direct sequencing, and the aac(6')-Ib-positive PCR products were digested with BtsCI to identify the aac(6')-Ib-cr variant. Of 461 isolates, 37 (8%) had one of the six PMQR genes; 13 (5%) of 261 E. coli strains, 13 (10%) of 135 K. pneumoniae strains, and 11 (17%) of 65 E. cloacae strains. qnrB was the most common PMQR gene and was found as early as 1998, whereas qnrS, aac(6')-Ib-cr, and qepA emerged after 2000. None of the isolates carried qnrA or qnrC. Ciprofloxacin resistance increased over time (P < 0.001), and the overall prevalence of PMQR genes tended to increase (P = 0.20). PMQR-positive isolates had significantly higher ciprofloxacin resistance and multidrug resistance rates (P = 0.005 and P < 0.001, respectively). The increasing frequency of ciprofloxacin resistance in Enterobacteriaceae was associated with an increasing prevalence of PMQR genes, and this change involved an increase in the diversity of the PMQR genes and also an increase in the prevalence of the mutations in gyrA, parC, or both in PMQR-positive strains but not PMQR-negative strains.

Accelerated evolution of resistance in multidrug environments

The emergence of resistance during multidrug chemotherapy impedes the treatment of many human diseases, including malaria, TB, HIV, and cancer. Although certain combination therapies have long been known to be more effective in curing patients than single drugs, the impact of such treatments on the evolution of drug resistance is unclear. In particular, very little is known about how the evolution of resistance is affected by the nature of the interactions--synergy or antagonism--between drugs. Here we directly measure the effect of various inhibitory and subinhibitory drug combinations on the rate of adaptation. We develop an automated assay for monitoring the parallel evolution of hundreds of Escherichia coli populations in a two-dimensional grid of drug gradients over many generations. We find a correlation between synergy and the rate of adaptation, whereby evolution in more synergistic drug combinations, typically preferred in clinical settings, is faster than evolution in antagonistic combinations. We also find that resistance to some synergistic combinations evolves faster than resistance to individual drugs. The accelerated evolution may be due to a larger selective advantage for resistance mutations in synergistic treatments. We describe a simple geometric model in which mutations conferring resistance to one drug of a synergistic pair prevent not only the inhibitory effect of that drug but also its enhancing effect on the other drug. Future study of the profound impact that synergy and other drug-pair properties can have on the rate of adaptation may suggest new treatment strategies for combating the spread of antibiotic resistance.

Plasmid-mediated qnrB2 and carbapenemase gene bla(KPC-2) carried on the same plasmid in carbapenem-resistant ciprofloxacin-susceptible Enterobacter cloacae isolates

Fourteen out of 16 carbapenem-resistant quinolone-susceptible Enterobacter cloacae isolates were found to carry qnrB2 and bla(KPC-2) genes encoded on the same plasmid. One isolate also carried the aac(6')-Ib-cr gene. Coexistence of quinolone resistance determinants and bla(KPC-2) on the same plasmid in quinolone-susceptible E. cloacae isolates may have important clinical implications.

Alteration of GyrA amino acid required for ciprofloxacin resistance in Klebsiella pneumoniae isolates in China

Resistance to ciprofloxacin was detected in 111 (48.1%) isolates of Klebsiella pneumoniae from China. GyrA alterations were identified in the ciprofloxacin-resistant and ciprofloxacin-susceptible isolates. The results, including previously published data, indicate that the single substitution Ser83-->Ile and three types of double mutations at Ser83 and Asp87 were required for ciprofloxacin resistance (P < 0.05).

Prevalence of qnr genes among extended-spectrum beta-lactamase-producing enterobacterial isolates in Barcelona, Spain

OBJECTIVES

To evaluate the presence of qnr genes among enterobacterial isolates carrying extended-spectrum beta-lactamases (ESBLs) in Barcelona, Spain.

METHODS

Screening for the qnrA, qnrB and qnrS genes was carried out by PCR amplification with specific primers in 305 non-duplicate, clinically relevant ESBL-producing enterobacterial isolates obtained from February 2003 to August 2004. ESBLs from all qnr-positive isolates were characterized by isoelectric focusing, PCR amplification and DNA sequencing. Plasmid analysis was performed by S1 digestion and hybridization with specific probes for the qnr and bla genes. Plasmids containing qnr genes were transferred by conjugation or transformation. The genetic environment of qnrA1 in selected isolates was characterized by cloning experiments.

RESULTS

Fifteen isolates, each from a different individual, carried qnr. Among them, 14 had qnrA1 (6 Klebsiella pneumoniae, 6 Enterobacter cloacae and 2 Escherichia coli isolates) and 1 had qnrS1 (K. pneumoniae). None of the isolates carried qnrB. Among the qnrA1-carrying isolates, 10 possessed both bla(CTX-M-9) and bla(SHV-12), 2 had both bla(CTX-M-9) and bla(SHV-92) and 2 had bla(CTX-M-9) alone. The isolate with qnrS1 possessed bla(SHV-12). The qnrA1 and ESBL genes were located together on plasmids ranging in size from 40 to 320 kb. qnrS1 and bla(SHV-12) were not located on the same plasmid. Transfer of quinolone resistance was successfully achieved from all but three isolates. The cloned region surrounding qnrA in two K. pneumoniae isolates revealed a novel genetic organization.

CONCLUSIONS

The prevalence of qnr among enterobacterial clinical isolates carrying ESBLs between 2003 and 2004 in Barcelona was 4.9%. qnrA1 was the most prevalent, whereas only one qnrS and no qnrB were detected.