Mechanisms accounting for fluoroquinolone resistance in Escherichia coli clinical isolates

Expression of multidrug efflux pump genes acrAB-tolC, mdfA, and norE in Escherichia coli clinical isolates as a function of fluoroquinolone and multidrug resistance

In a single quantitative study, we measured acrA, acrB, tolC, mdfA, and norE expression in Escherichia coli clinical isolates by using real-time PCR. acrA and acrB overexpression strongly correlated with fluoroquinolone and multidrug resistance; tolC, mdfA, and norE expression did not. The order of abundance of efflux pump transcripts in all fluoroquinolone-susceptible isolates was tolC (highest), then acrA and acrB, and then mdfA and norE. Our findings suggest acrAB overexpression is an indicator of multidrug resistance.

Prevalence and dissemination of oqxAB in Escherichia coli isolates from animals, farmworkers, and the environment

OqxAB has recently been identified as one of the mechanisms of plasmid-mediated quinolone resistance (PMQR). Compared to what is observed for other PMQR determinants, there is a paucity of data with regard to the prevalence and epidemiology of OqxAB and its contribution to resistance to different antimicrobials. In this study, the prevalence and dissemination of oqxAB and other PMQR genes in Escherichia coli isolates from animals, farmworkers, and the environment in 2002 in China were investigated. Of the 172 E. coli isolates, 39.0% carried oqxA, while only 4.1%, 2.9%, and 0.6% carried qnr (1 qnrB6 isolate, 5 qnrS1 isolates, and 1 qnrD isolate), qepA, and aac(6')-Ib-cr, respectively. Among the 33 isolates from farmworkers, 10 (30.3%) were positive for oqxA. oqxAB was associated with IS26 and was carried on the 43- to 115-kb IncF transferable plasmid. Transconjugants carrying oqxAB showed 4- to 16-fold increases in the MICs of quinolones, 16- to 64-fold increases in the MICs of quinoxalines, 8- to 32-fold increases in the MICs of chloramphenicol and trimethoprim-sulfamethoxazole, and 4- to 8-fold increases in the MICs of florfenicol compared to the levels for the recipient. The pulsed-field gel electrophoresis (PFGE) analysis showed that the high levels of prevalence and dissemination of oqxAB in E. coli in animal farms were primarily due to the transmission of plasmids carrying oqxAB, although clonal transmission between human and swine E. coli isolates was observed. It is concluded that oqxAB was widespread in animal farms in China, which may be due to the overuse of quinoxalines in animals. This study warrants the prudent use of quinoxalines in food animals.

Impact of recA on levofloxacin exposure-related resistance development

Genetic mutations are one of the major mechanisms by which bacteria acquire drug resistance. One of the known mechanisms for inducing mutations is the SOS response system. We investigated the effect of disrupting recA, an inducer of the SOS response, on resistance development using an in vitro hollow-fiber infection model. A clinical Staphylococcus aureus isolate and a laboratory wild-type strain of Escherichia coli were compared to their respective recA-deleted isogenic daughter isolates. Approximately 2 × 10(5) CFU/ml of bacteria were subjected to escalating levofloxacin exposures for up to 120 h. Serial samples were obtained to ascertain simulated drug exposures and total and resistant bacterial burdens. Quinolone resistance determining regions of gyrA and grlA (parC for E. coli) in levofloxacin-resistant isolates were sequenced to confirm the mechanism of resistance. The preexposure MICs of the recA-deleted isolates were 4-fold lower than those of their respective parents. In S. aureus, a lower area under the concentration-time curve over 24 h at steady state divided by the MIC (AUC/MIC) was required to suppress resistance development in the recA-deleted mutant (an AUC/MIC of >23 versus an AUC/MIC of >32 was necessary in the mutant versus the parent isolate, respectively), and a prominent difference in the total bacterial burden was observed at 72 h. Using an AUC/MIC of approximately 30, E. coli resistance emergence was delayed by 24 h in the recA-deleted mutant. Diverse mutations in gyrA were found in levofloxacin-resistant isolates recovered. Disruption of recA provided additional benefits apart from MIC reduction, attesting to its potential role for pharmacologic intervention. The clinical relevance of our findings warrants further investigations.

Mutation characterization of gyrA and gyrB genes in levofloxacin-resistant Mycobacterium tuberculosis clinical isolates from Guangdong Province in China

OBJECTIVE

Fluoroquinolone (FQ)-resistant Mycobacterium tuberculosis (MTB) clinical isolates have emerged in many areas of the world. The aim of this study was to observe the molecular characterization of gyrA and gyrB genes in FQ-resistant MTB clinical isolates from Guangdong Province in China.

MATERIALS AND METHODS

A total of 62 MTB clinical strains were originally isolated from patients with pulmonary tuberculosis. The phenotype of susceptibility of each strain was determined by the absolute concentration method and the sequences of the QRDR in gyrA and gyrB genes were detected with DNA direct sequencing technique.

RESULTS

44 of 60 (73.3%) levofloxacin-resistant MTB clinical isolates, including 17 of 18 (94.4%) high-level resistant strains and 27 of 42 (64.3%) low-level resistant strains, had mutation in QRDR of gyrA gene. The mutation patterns involved six patterns of single codon mutation (H70R, A90V, S91A, D94G, D94A or D94N) and one pattern of double codons mutation (A90V with D94A). Of 60 levofloxacin-resistant MTB clinical isolates, only one (1.6%) mutated in gyrB gene (T511N).

CONCLUSIONS

These findings confirm that mutations of gyrA codons 90, 91 and 94 constitute the primary mechanism of FQ resistance in MTB. Furthermore, our findings indicate that the regional investigations are necessary for the comprehensive understanding of FQ resistance of MTB.

High level of quinolone resistance in Escherichia coli from healthy chicken broilers

The development of resistance to quinolones (nalidixic acid, ciprofloxacin and enrofloxacin) in 2006-2008 was evaluated in 317 strains of Escherichia coli isolated from healthy chicken broilers from various farms. The isolates (2006/2007/2008) showed a high resistance to nalidixic acid (87/85/67 %), ciprofloxacin (CIP) (49/54/29 %) and enrofloxacin (ENR) (52/42/22 %). Nalidixic acid-resistant isolates with low level of MIC for CIP and ENR represented a single mutation; intermediary MIC for CIP and ENR were related to two mutations and high level resistance MIC for CIP (> or =4 mg/L) and ENR (> or =16 mg/L) represented three mutations (two in gyrA and one in parC). There was a correlation between the phenotype reading of high-level resistance and mutations in gyrA (Ser83Leu, Asp87Tyr or Asp87Asn) and parC (Ser80Ile) gene. Plasmid-mediated quinolone-resistance qnrS gene was detected in one Escherichia coli strain with a high level of ciprofloxacin resistance. Our results demonstrate the increase in occurrence of multiresistant E. coli strains with a high level of chromosomal and plasmid resistance to fluoroquinolones.

The newer fluoroquinolones

Clinicians have enthusiastically used fluoroquinolones owing to their good safety profile and wide range of indications. This article reviews fluoroquinolone pharmacology, pharmacodynamic principles, and fluoroquinolone resistance mechanisms, highlighting recent trends in the epidemiology of fluoroquinolone resistance among gram-negative organisms and Streptococcus pneumonia. Important fluoroquinolone safety concerns are discussed, along with indications for the most commonly used fluoroquinolones-ciprofloxacin, levofloxacin, and moxifloxacin.

Plasmid-mediated quinolone resistance: a multifaceted threat

Although plasmid-mediated quinolone resistance (PMQR) was thought not to exist before its discovery in 1998, the past decade has seen an explosion of research characterizing this phenomenon. The best-described form of PMQR is determined by the qnr group of genes. These genes, likely originating in aquatic organisms, code for pentapeptide repeat proteins. These proteins reduce susceptibility to quinolones by protecting the complex of DNA and DNA gyrase or topoisomerase IV enzymes from the inhibitory effect of quinolones. Two additional PMQR mechanisms were recently described. aac(6')-Ib-cr encodes a variant aminoglycoside acetyltransferase with two amino acid alterations allowing it to inactivate ciprofloxacin through the acetylation of its piperazinyl substituent. oqxAB and qepA encode efflux pumps that extrude quinolones. All of these genes determine relatively small increases in the MICs of quinolones, but these changes are sufficient to facilitate the selection of mutants with higher levels of resistance. The contribution of these genes to the emergence of quinolone resistance is being actively investigated. Several factors suggest their importance in this process, including their increasing ubiquity, their association with other resistance elements, and their emergence simultaneous with the expansion of clinical quinolone resistance. Of concern, these genes are not yet being taken into account in resistance screening by clinical microbiology laboratories.

Pharmacodynamics of moxifloxacin against a high inoculum of Escherichia coli in an in vitro infection model

OBJECTIVES

Escherichia coli is the leading bacterial species implicated in intra-abdominal infections. In these infections a high bacterial burden with pre-existing resistant mutants are likely to be encountered and resistance could be amplified with suboptimal dosing. Our objective was to investigate the pharmacodynamics of moxifloxacin against a high inoculum of E. coli using an in vitro hollow fibre infection model (HFIM).

METHODS

Three wild-type strains of E. coli (ATCC 25922, MG1655 and EC28044) were studied by exposing approximately 2 x 10(8) cfu/mL (20 mL) to escalating dosing regimens of moxifloxacin (ranging from 30 to 400 mg, once daily). Serial samples were obtained from HFIM over 120 h to enumerate the total and resistant subpopulation. Quinolone resistance-determining regions of gyrA and parC of resistant isolates were sequenced to confirm the mechanism of resistance.

RESULTS

The pre-exposure MIC of the three wild-type strains was 0.0625 mg/L. Simulated moxifloxacin concentration profiles in HFIM were satisfactory (r(2) >or= 0.94). Placebo experiments revealed natural mutants, but no resistance amplification. Regrowth and resistance amplification was observed between 30 mg/day (AUC/MIC = 47) and 80 mg/day dose (AUC/MIC = 117). Sustained bacterial suppression was achieved at >or=120 mg/day dose (AUC/MIC = 180). Point mutations in gyrA (D87G or S83L) were detected in resistant isolates.

CONCLUSIONS

Our results suggest that suboptimal dosing may facilitate resistance amplification in a high inoculum of E. coli. The clinical dose of moxifloxacin (400 mg/day) was adequate to suppress resistance development in three wild-type strains. Clinical relevance of these findings warrants further in vivo investigation.

Evaluation of quinolones for use in detection of determinants of acquired quinolone resistance, including the new transmissible resistance mechanisms qnrA, qnrB, qnrS, and aac(6')Ib-cr, in Escherichia coli and Salmonella enterica and determinations of wild-type distributions

Fluoroquinolone resistance in members of the Enterobacteriaceae family is mostly due to mutations in the quinolone resistance-determining regions of the topoisomerase genes. However, transferable genes encoding quinolone resistance have recently been described. The current methods for susceptibility testing are not adapted to the detection of new resistance determinants, which confer low levels of resistance. The aim of this study was to compare the ability of the screening of the different quinolones by disk diffusion assays and MIC determinations to detect fluoroquinolone resistance. Sixty-nine Escherichia coli strains and 62 Salmonella strains, including strains fully susceptible to quinolones, nalidixic acid-resistant strains, strains with resistance to fluoroquinolones (resistant to nalidixic acid), and strains showing low-level resistance to fluoroquinolones conferred by transferable quinolone resistance genes, including qnrA, qnrB, qnrS, and aac(6')Ib-cr, were selected. Disk diffusion assays and MIC determinations by the agar dilution method were performed, according to CLSI standards, with nalidixic acid, flumequine, oxolinic acid, ciprofloxacin, enrofloxacin, marbofloxacin, norfloxacin, ofloxacin, and levofloxacin. The MIC of levofloxacin was determined by an Etest. The results showed a trimodal distribution of the MICs for both E. coli and Salmonella. The MIC distributions for the isolates varied with the compounds tested. Screening for nalidixic acid resistance by MIC testing or disk diffusion assay was not efficient for the detection of some of the isolates carrying qnr and aac(6')Ib-cr. Transferable resistance genes would best be detected by testing for the MIC of ciprofloxacin or norfloxacin, as testing for the MICs of the other compounds would fail to detect isolates carrying aac(6')Ib-cr because the enzyme produced is able to reduce the activities of these two compounds only due to their chemical structures. In conclusion, screening with nalidixic acid is efficient for the detection of mutants, but it is not so efficient for the detection of qnr and aac(6')Ib-cr. Detection would be maximized by screening with either ciprofloxacin or norfloxacin by both MIC determination and disk diffusion assays. Furthermore, a low concentration of ciprofloxacin (1 microg) in the disks seemed to increase the sensitivity of the disk diffusion assay.

Quinolones: action and resistance updated

The quinolones trap DNA gyrase and DNA topoisomerase IV on DNA as complexes in which the DNA is broken but constrained by protein. Early studies suggested that drug binding occurs largely along helix-4 of the GyrA (gyrase) and ParC (topoisomerase IV) proteins. However, recent X-ray crystallography shows drug intercalating between the -1 and +1 nucleotides of cut DNA, with only one end of the drug extending to helix-4. These two models may reflect distinct structural steps in complex formation. A consequence of drug-enzyme-DNA complex formation is reversible inhibition of DNA replication; cell death arises from subsequent events in which bacterial chromosomes are fragmented through two poorly understood pathways. In one pathway, chromosome fragmentation stimulates excessive accumulation of highly toxic reactive oxygen species that are responsible for cell death. Quinolone resistance arises stepwise through selective amplification of mutants when drug concentrations are above the MIC and below the MPC, as observed with static agar plate assays, dynamic in vitro systems, and experimental infection of rabbits. The gap between MIC and MPC can be narrowed by compound design that should restrict the emergence of resistance. Resistance is likely to become increasingly important, since three types of plasmid-borne resistance have been reported.

Relationships among ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs for fluoroquinolone-resistant Escherichia coli clinical isolates

Fluoroquinolones are some of the most prescribed antibiotics in the United States. Previously, we and others showed that the fluoroquinolones exhibit a class effect with regard to the CLSI-established breakpoints for resistance, such that decreased susceptibility (i.e., an increased MIC) to one fluoroquinolone means a simultaneously decreased susceptibility to all. For defined strains, however, clear differences exist in the pharmacodynamic properties of each fluoroquinolone and the extent to which resistance-associated genotypes affect the MICs of each fluoroquinolone. In a pilot study of 920 clinical Escherichia coli isolates, we uncovered tremendous variation in norfloxacin MICs. The MICs for all of the fluoroquinolone-resistant isolates exceeded the resistance breakpoint, reaching 1,000 microg/ml. Approximately 25% of the isolates (n = 214), representing the full range of resistant norfloxacin MICs, were selected for the simultaneous determinations of ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs. We found that (i) great MIC variation existed for all four fluoroquinolones, (ii) the ciprofloxacin and levofloxacin MICs of >90% of the fluoroquinolone-resistant isolates were higher than the resistance breakpoints, (iii) ciprofloxacin and levofloxacin MICs were distributed into two distinct groups, (iv) the MICs of two drug pairs (ciprofloxacin and norfloxacin by Kendall's Tau-b test and gatifloxacin and levofloxacin by paired t test) were similar with statistical significance but were different from each other, and (v) approximately 2% of isolates had unprecedented fluoroquinolone MIC relationships. Thus, although the fluoroquinolones can be considered equivalent with regard to clinical susceptibility or resistance, fluoroquinolone MICs differ dramatically for fluoroquinolone-resistant clinical isolates, likely because of differences in drug structure.