Mechanisms of Quinolone resistance

A New Calcium(II)-Based Substitute for Enrofloxacin with Improved Medicinal Potential

Enrofloxacin (EFX) reacting with Ca(II) afforded a new complex, [Ca(EFX)₂(H₂O)₄] (EFX-Ca), which was structurally characterized both in solid and solution chemistry. E. coli and S. typhi were tested to be the most sensitive strains for EFX-Ca. The LD₅₀ value of EFX-Ca in mice was 7736 mg/kg, implying the coordination of EFX to Ca(II) effectively reduced its acute toxicity. EFX-Ca also decreased the plasma-binding rate and enhanced the drug distribution in rats along with longer elimination half-life. EFX-Ca also showed similar low in vivo acute toxicity and higher anti-inflammation induced by H₂O₂ or CuSO₄ in zebrafish, with reactive oxygen species (ROS)-related elimination. The therapeutic effects of EFX-Ca on two types (AA and 817) of E. coli-infected broilers were also better than those of EFX, with cure rates of 78% and 88%, respectively. EFX-Ca showed promise as a bio-safe metal-based veterinary drug with good efficacy and lower toxicity.

Contribution of biofilm formation genetic locus, pgaABCD, to antibiotic resistance development in gut microbiome

The human gut microbiome is the presumed site in which the emergence and evolution of antibiotic-resistant organisms constantly take place. To delineate the genetic basis of resistance formation in gut microbiome strains, we investigated the changes in the subpopulation structure of Escherichia coli in rat intestine before and after antimicrobial treatment. We observed that antibiotic treatment was selected for an originally minor subpopulation E. coli carrying the biofilm-forming genetic locus pgaABCD and the toxin-antitoxin system HipAB. Such strains possessed dramatically enhanced ability to withstand the detrimental effects of antibiotics, becoming a dominant subspecies upon antibiotic treatment and eventually evolving into resistant mutants. In contrast, E. coli strains that did not carry pgaABCD and HipAB were eradicated upon antibiotic treatment. Our findings, therefore, suggested that genes encoding biofilm-forming ability played an important role in conferring specific gut E. coli strains the ability to evolve into resistant strains upon a prolonged antibiotic treatment, and that such strains may therefore be considered bacterial antibiotic resistance progenitor cells in the gut microbiome.

Analytical profiling of mutations in quinolone resistance determining region of gyrA gene among UPEC

Mutations in gyrA are the primary cause of quinolone resistance encountered in gram-negative clinical isolates. The prospect of this work was to analyze the role of gyrA mutations in eliciting high quinolone resistance in uropathogenic E.coli (UPEC) through molecular docking studies. Quinolone susceptibility testing of 18 E.coli strains isolated from UTI patients revealed unusually high resistance level to all the quinolones used; especially norfloxacin and ciprofloxacin. The QRDR of gyrA was amplified and sequenced. Mutations identified in gyrA of E.coli included Ser83Leu, Asp87Asn and Ala93Gly/Glu. Contrasting previous reports, we found Ser83Leu substitution in sensitive strains. Strains with S83L, D87N and A93E (A15 and A26) demonstrated norfloxacin MICs ≥1024mg/L which could be proof that Asp87Asn is necessary for resistance phenotype. Resistance to levofloxacin was comparatively lower in all the isolates. Docking of 4 quinolones (ciprofloxacin, ofloxacin, levofloxacin and norfloxacin) to normal and mutated E.coli gyrase A protein demonstrated lower binding energies for the latter, with significant displacement of norfloxacin in the mutated GyrA complex and least displacement in case of levofloxacin.

Molecular characterisation of quinolone-resistant Shigella strains isolated in Tehran, Iran

Over the past few years, the number of Shigella strains resistant to nalidixic acid has increased and has made the selection of effective antimicrobial therapy more difficult. The purpose of this study was to investigate the molecular mechanism of quinolone resistance in Shigella strains. Shigella strains isolated from 1100 diarrhoeal patients in Tehran, Iran, were assessed for their susceptibility to nalidixic acid prior to PCR-RFLP and sequence analysis of their quinolone resistance genes. Among 73 Shigella strains isolated, 23 (31.5%) were resistant to nalidixic acid. The most common Shigella spp. was Shigella sonnei (54; 74.0%). Of the 23 quinolone-resistant isolates, 4 (17.4%) (including 2 Shigella flexneri, 1 S. sonnei and 1 Shigella boydii) contained the qnrS gene. However, none of the isolates harboured qnrA or qnrB genes. PCR-RFLP analysis of gyrA showed a mutation profile in two nalidixic acid-resistant strains, including one S. sonnei and one S. flexneri. Sequencing of mutant gyrA genes revealed a point mutation at position 83, resulting in the replacement of serine by leucine. In conclusion, molecular mechanisms of resistance to quinolones were identified in 6 of 23 Shigella isolates. Other possible mechanisms of resistance should also be investigated for better characterisation of quinolone-resistant Shigella isolates.

Multiplex Real-Time Polymerase Chain Reaction-Based Method for the Rapid Detection of gyrA and parC Mutations in Quinolone-Resistant Escherichia coli and Shigella spp

Two real-time polymerase chain reaction assays were developed to detect mutations in codons 83 and 87 in gyrA and in codons 80 and 91 in parC, the main sites that causes quinolone resistance in pathogenic Escherichia coli and Shigella spp. isolates. These assays can be employed as a useful method for controlling infections caused by quinolone-resistant E coli and Shigella isolates.

Reduced expression of the vca0421 gene of Vibrio cholerae O1 results in innate resistance to ciprofloxacin

A mini-Tn5 insertion into a ciprofloxacin (CIP)-resistant mutant of Vibrio cholerae O1 revealed that overexpression of the vca0421 gene, which encodes a hypothetical protein, in the CIP-resistant mutant carrying a mutation in the quinolone resistance-determining region (QRDR) of the gyrA gene causes sensitization to CIP. We propose a new intrinsic mechanism of resistance to fluoroquinolones due to the inherently reduced expression of the vca0421 gene in V. cholerae O1.

Antibiotic susceptibility of Waddlia chondrophila in Acanthamoeba castellanii amoebae

Waddlia chondrophila is an emerging cause of miscarriage in bovines and humans. Given the strict intracellular growth of this Chlamydia-like organism, its antibiotic susceptibility was tested by amoebal coculture, cell culture, and real-time PCR. W. chondrophila was susceptible to doxycycline and azithromycin but resistant to beta-lactams and fluoroquinolones.

Gemifloxacin: a new fluoroquinolone approved for treatment of respiratory infections

OBJECTIVE

To evaluate the microbiology, pharmacokinetic parameters, drug interactions, and results of the available clinical trials of gemifloxacin for the treatment of community-acquired pneumonia (CAP) and acute exacerbation of chronic bronchitis (AECB).

DATA SOURCES

MEDLINE (1966-September 2003) was searched for primary and review articles. Data from the manufacturer were also included. Key words included adverse effects, clinical trials, drug interactions, gemifloxacin, and pharmacokinetic parameters.

STUDY SELECTION AND DATA EXTRACTION

All articles and product labeling concerning gemifloxacin, a fluoroquinolone antibiotic recently approved by the Food and Drug Administration for treatment of CAP and AECB, were included for review.

DATA SYNTHESIS

Compared with currently available fluoroquinolones, gemifloxacin demonstrated improved in vitro activity against Streptococcus pneumoniae (minimum inhibitory concentration for 90% eradication 0.03 microg/mL) and similar activity against gram-negative respiratory pathogens (Haemophilus influenzae, Moraxella catarrhalis) and atypical pathogens such as Chlamydia pneumoniae, Legionella pneumophila, and Mycoplasma pneumoniae. Gemifloxacin, consistent with other available fluoroquinolones, has insufficient activity against methicillin-resistant Staphylococcus aureus to allow clinical use for such infections. Gemifloxacin has adequate bioavailability and a favorable drug interaction profile. Gemifloxacin was comparable to commonly employed nonfluoroquinolone regimens for treatment of CAP and AECB, although the studies were designed to demonstrate equivalence. Gemifloxacin once daily for 5-7 days was well tolerated in controlled and uncontrolled clinical studies. Available clinical data, however, are insufficient to draw clinical or toxicologic distinctions between gemifloxacin and other fluoroquinolones.

CONCLUSIONS

Gemifloxacin may be a suitable choice for empiric treatment of CAP or AECB. However, due to the significant history of fluoroquinolone-induced hepatic failure and dermatologic complications, the use of this drug should be closely monitored.

Uncommon occurrence of fluoroquinolone resistance-associated alterations in GyrA and ParC in clinical strains of Chlamydia trachomatis

Twenty-three clinical strains of Chlamydia trachomatis were isolated from men with chlamydial nongonococcal urethritis and examined for the presence of fluoroquinolone resistance-associated alterations in GyrA and ParC. The minimum inhibitory concentrations (MICs) of fluoroquinolones, erythromycin, and tetracycline were determined for 6 of the 23 isolates. In 12 of the 23 isolates, a single amino-acid change was found in GyrA, and in 1 isolate, two amino acids were changed. In all 23 isolates, an Arg-83-to-Gly substitution was observed in ParC, and in 3 isolates, an additional amino-acid change was found. Some changes occurred within the quinolone resistance-determining regions (QRDRs) of GyrA and ParC, but not at positions critical for fluoroquinolone resistance. Of the 6 isolates for which MICs of the agents were determined, 1 isolate had a Cys-66 --> Arg substitution in GyrA, and all had the Arg-83 --> Gly substitution in ParC. However, all 6 isolates were susceptible to fluoroquinolones. First-pass urine specimens were obtained from two men who were positive for C. trachomatis after levofloxacin treatment, and the gyrA and parC genes of C. trachomatis were amplified by polymerase chain reaction (PCR) and examined for fluoroquinolone resistance-associated mutations. Pre- and post-treatment C. trachomatis persisting in each of them had identical amino-acid sequences in the QRDR of GyrA and ParC. Further, the substitutions found in GyrA and ParC were not located at positions critical for fluoroquinolone resistance. The present study suggests that fluoroquinolone resistance-associated alterations in GyrA and ParC may be uncommon in clinical strains of C. trachomatis.

Fluoroquinolone-resistant Streptococcus pneumoniae strains occur frequently in elderly patients in Japan

We identified and genetically characterized seven fluoroquinolone-resistant Streptococcus pneumoniae strains among 293 clinical strains isolated from 1999 to 2001 in Japan. The resistant strains were isolated only from adults, and 7 of 31 isolates (22.6%) were from patients more than 20 years old. Resistant strains were not found in 262 isolates from children under age 10.

Quinolone resistance mutations in Streptococcus pneumoniae GyrA and ParC proteins: mechanistic insights into quinolone action from enzymatic analysis, intracellular levels, and phenotypes of wild-type and mutant proteins

Mutations in DNA gyrase and/or topoisomerase IV genes are frequently encountered in quinolone-resistant mutants of Streptococcus pneumoniae. To investigate the mechanism of their effects at the molecular and cellular levels, we have used an Escherichia coli system to overexpress S. pneumoniae gyrase gyrA and topoisomerase IV parC genes encoding respective Ser81Phe and Ser79Phe mutations, two changes widely associated with quinolone resistance. Nickel chelate chromatography yielded highly purified mutant His-tagged proteins that, in the presence of the corresponding GyrB and ParE subunits, reconstituted gyrase and topoisomerase IV complexes with wild-type specific activities. In enzyme inhibition or DNA cleavage assays, these mutant enzyme complexes were at least 8- to 16-fold less responsive to both sparfloxacin and ciprofloxacin. The ciprofloxacin-resistant (Cip(r)) phenotype was silent in a sparfloxacin-resistant (Spx(r)) S. pneumoniae gyrA (Ser81Phe) strain expressing a demonstrably wild-type topoisomerase IV, whereas Spx(r) was silent in a Cip(r) parC (Ser79Phe) strain. These epistatic effects provide strong support for a model in which quinolones kill S. pneumoniae by acting not as enzyme inhibitors but as cellular poisons, with sparfloxacin killing preferentially through gyrase and ciprofloxacin through topoisomerase IV. By immunoblotting using subunit-specific antisera, intracellular GyrA/GyrB levels were a modest threefold higher than those of ParC/ParE, most likely insufficient to allow selective drug action by counterbalancing the 20- to 40-fold preference for cleavable-complex formation through topoisomerase IV observed in vitro. To reconcile these results, we suggest that drug-dependent differences in the efficiency by which ternary complexes are formed, processed, or repaired in S. pneumoniae may be key factors determining the killing pathway.

Mutation in the DNA gyrase A Gene of Escherichia coli that expands the quinolone resistance-determining region

In three Escherichia coli mutants, a change (Ala-51 to Val) in the gyrase A protein outside the standard quinolone resistance-determining region (QRDR) lowered the level of quinolone susceptibility more than changes at amino acids 67, 82, 84, and 106 did. Revision of the QRDR to include amino acid 51 is indicated.

In vivo selection of Pseudomonas Aeruginosa with decreased susceptibilities to fluoroquinolones during fluoroquinolone treatment of urinary tract infection

OBJECTIVES

To present a case of fluoroquinolone treatment failure in urinary tract infection caused by Pseudomonas aeruginosa, accompanied by in vivo selection of the post-treatment isolate that showed decreased susceptibilities to fluoroquinolones, and to report fluoroquinolone resistance mechanisms in the post-treatment isolate.

METHODS

A patient with urinary tract infection was treated with a suboptimal dose of a fluorinated quinolone, gatifloxacin. P. aeruginosa strains were isolated before and after fluoroquinolone treatment. The pretreatment and post-treatment isolates were examined for relatedness by arbitrarily primed polymerase chain reaction. For these isolates, the minimum inhibitory concentration of antimicrobial agents was determined and mutations in the target genes (gyrA and parC) and regulatory genes (mexR and nfxB) for drug efflux pumps were analyzed.

RESULTS

Failure of fluoroquinolone treatment of urinary tract infection was observed. The post-treatment isolate, which was assumed to be isogenic to the pretreatment isolate, exhibited fourfold to 16-fold increases in the MIC of fluoroquinolones. In this isolate, a new mutation, not observed in the pretreatment isolate, was found only in the gyrA gene, resulting in an amino acid change of aspartic acid to asparagine in codon 87 of GyrA.

CONCLUSIONS

The P. aeruginosa isolate that was initially susceptible to fluoroquinolones showed decreased susceptibility to fluoroquinolones after treatment with a suboptimal dose of one fluoroquinolone. In the post-treatment isolate, the alteration of GyrA would be responsible for the decreased susceptibility to fluoroquinolones. We should be aware that inappropriate use of fluoroquinolones could select such a strain harboring a quinolone resistance-associated alteration of DNA gyrase.

New quinolones and the impact on resistance

The changes in quinolone research have been fast and exciting over the past 5-7 years with the discovery and development of several new 8-methoxy quinolones. An additional factor is the design of the so-called 4th-generation quinolones that lack the C-6 fluorine, which might impact the development of quinolone resistance. The science behind the quinolone susceptibility and resistance patterns is fascinating, but has not yet been clearly delineated in discussions of the advantages of quinolone usage in the clinic.

Mutant prevention concentrations of fluoroquinolones for clinical isolates of Streptococcus pneumoniae

The mutant prevention concentration (MPC) represents a threshold above which the selective proliferation of resistant mutants is expected to occur only rarely. A provisional MPC (MPC(pr)) was defined and measured for five fluoroquinolones with clinical isolates of Streptococcus pneumoniae. Based on their potential for restricting the selection of resistant mutants, the five fluoroquinolones, in descending order, were found to be moxifloxacin > trovafloxacin > gatifloxacin > grepafloxacin > levofloxacin. For several compounds, 90% of about 90 clinical isolates that lacked a known resistance mutation had a value of MPC(pr) that was close to or below the serum levels that could be attained with a dosing regimen recommended by the manufacturers. Since MPC(pr) overestimates MPC, these data identify moxifloxacin and gatifloxacin as good candidates for determining whether MPC(pr) can be used as a guide for choosing and eventually administering fluoroquinolones to significantly reduce the development of resistance.

Potent antipneumococcal activity of gemifloxacin is associated with dual targeting of gyrase and topoisomerase IV, an in vivo target preference for gyrase, and enhanced stabilization of cleavable complexes in vitro

We investigated the roles of DNA gyrase and topoisomerase IV in determining the susceptibility of Streptococcus pneumoniae to gemifloxacin, a novel fluoroquinolone which is under development as an antipneumococcal drug. Gemifloxacin displayed potent activity against S. pneumoniae 7785 (MIC, 0.06 microgram/ml) compared with ciprofloxacin (MIC, 1 to 2 microgram/ml). Complementary genetic and biochemical approaches revealed the following. (i) The gemifloxacin MICs for isogenic 7785 mutants bearing either parC or gyrA quinolone resistance mutations were marginally higher than wild type at 0.12 to 0.25 microgram/ml, whereas the presence of both mutations increased the MIC to 0.5 to 1 microgram/ml. These data suggest that both gyrase and topoisomerase IV contribute significantly as gemifloxacin targets in vivo. (ii) Gemifloxacin selected first-step gyrA mutants of S. pneumoniae 7785 (gemifloxacin MICs, 0.25 microgram/ml) encoding Ser-81 to Phe or Tyr, or Glu-85 to Lys mutations. These mutants were cross resistant to sparfloxacin (which targets gyrase) but not to ciprofloxacin (which targets topoisomerase IV). Second-step mutants (gemifloxacin MICs, 1 microgram/ml) exhibited an alteration in parC resulting in changes of ParC hot spot Ser-79 to Phe or Tyr. Thus, gyrase appears to be the preferential in vivo target. (iii) Gemifloxacin was at least 10- to 20-fold more effective than ciprofloxacin in stabilizing a cleavable complex (the cytotoxic lesion) with either S. pneumoniae gyrase or topoisomerase IV enzyme in vitro. These data suggest that gemifloxacin is an enhanced affinity fluoroquinolone that acts against gyrase and topoisomerase IV in S. pneumoniae, with gyrase the preferred in vivo target. The marked potency of gemifloxacin against wild type and quinolone-resistant mutants may accrue from greater stabilization of cleavable complexes with the target enzymes.

Mechanism of quinolone resistance in Staphylococcus aureus

The resistance mechanisms to fluoroquinolones in Staphylococcus aureus were clarified by analyzing mutations in the genes encoding target enzymes, and examining the expression of the efflux pump, and determining the inhibitory activities of fluoroquinolones against the altered enzymes. Mutations in the grlA and gyrA genes of 344 clinical strains of S. aureus isolated in 1994 in Japan were identified by combinations of methods - single-strand conformation polymorphism analysis, restriction fragment length analysis, and direct sequencing - to identify possible relationships with fluoroquinolone resistance. Five types of single-point mutations and four types of double mutations were observed in the grlA gene in 204 strains (59.3%). Four types of single-point mutations and four types of double mutations were found in the gyrA gene in 188 strains (54.7%). Among these mutations, the grlA mutation of TCC --> TTC or TAC (Ser-80 --> Phe or Tyr) and the gyrA mutation of TCA --> TTA (Ser-84 --> Leu) were the principal ones, being detected in 137 (39.8%) and 121 (35.2%) isolates, respectively. A total of 15 types of mutation combinations within both genes were related to ciprofloxacin resistance (MIC greater than or equal 3.13 microg/ml) and were present in 193 mutants (56.1%). Strains containing mutations in both genes were highly resistant to ciprofloxacin (MIC50 =50 microg/ml). Those strains with the Ser-80 --> Phe or Tyr alteration in grlA, but wild type in gyrA showed a lower level of ciprofloxacin resistance (MIC50 less than or equal 12.5 microg/ml). Levofloxacin was active against 68 of 193 isolates (35.2%) with mutations at codon 80 of grlA in the presence or absence of concomitant mutations at codons 73, 84, or 88 in gyrA (MIC less than or equal 6.25 microg/ml). Sitafloxacin (DU-6859a) showed good activity in 186 of 193 isolates (96.4%), with an MIC of less than or equal 6.25 microg/ml. The contribution of membrane-associated multidrug efflux protein (NorA) expression to fluoroquinolone resistance was clarified by the checker-board titration method for determining the MIC of norfloxacin alone and in combination with carbonyl cyanide m-chlorophenylhydrazone. Among 344 clinical isolates, 139 strains (40.4%), in which the MIC of norfloxacin varied from 1.56 to >800 microg/ml, overexpressed the NorA protein. GrlA and GrlB proteins of topoisomerase IV, and GyrA and GyrB proteins of DNA gyrase encoded by genes with or without mutations were purified separately. The inhibitory activities of fluoroquinolones against the topoisomerase IV which contained a single amino acid change (Ser --> Phe at codon 80, Glu --> Lys at codon 84 of grlA, and Asp --> Asn at codon 432 of grlB) were from 5 to 95 times weaker than the inhibitory activities against the non-altered enzyme. These results suggest that the mutations in the corresponding genes may confer quinolone resistance; the active efflux pump, NorA, was considered to be the third quinolone-resistance mechanism. The numerous and complicated mutations seen may explain the rapid and widespread development of quinolone resistance described in S. aureus. Sitafloxacin showed good antibacterial activity against ciprofloxacin- or levofloxacin-resistant mutants because of its high inhibitory activity against both topoisomerase IV and DNA gyrase.

In vitro selection of fluoroquinolone-resistant Neisseria gonorrhoeae harboring alterations in DNA gyrase and topoisomerase IV

PURPOSE

We attempted to select increasingly fluoroquinolone-resistant strains of Neisseria gonorrhoeae in vitro and to assess whether selected mutants harbored alterations in the GyrA subunit of DNA gyrase and the ParC subunit of DNA topoisomerase IV, which were analogous to those in fluoroquinolone-resistant clinical isolates.

MATERIALS AND METHODS

A fluoroquinolone-susceptible strain was exposed to norfloxacin in vitro. Selected mutants were sequentially exposed to norfloxacin, and this procedure was repeated. For 11 mutants, minimum inhibitory concentrations (MICs) of antimicrobial agents were determined, and mutations in the region corresponding to the quinolone resistance-determining region (QRDR) of the Escherichia coli gyrA gene and the analogous region of the parC gene were analyzed.

RESULTS

Mutants obtained in one step exhibited significantly increased MICs of norfloxacin, ofloxacin and ciprofloxacin and had a single amino acid change in GyrA. Two-step mutants exhibited significantly higher norfloxacin MICs. Three of four two-step selected strains had single amino acid changes in both GyrA and ParC. Three-step mutants exhibited further increases in fluoroquinolone MICs and were assigned to the ciprofloxacin-resistant category. Two had a double amino acid change in GyrA, and one had a double GyrA change and a single amino acid change in ParC.

CONCLUSION

We selected fluoroquinolone-resistant strains that carried GyrA and ParC alterations analogous to those in clinical isolates. The serial accumulation of changes in the QRDR of GyrA and the analogous region of ParC was associated with a stepwise increase in fluoroquinolone resistance, although the development of additional alterations in other regions of GyrA and ParC or other mechanisms of fluoroquinolone resistance also might contribute to the enhancement in fluoroquinolone resistance. The clinical emergence of fluoroquinolone-resistant strains may be due to in-vivo stepwise selection of strains with genetic alterations in GyrA and ParC, as observed here in the in-vitro selection of fluoroquinolone-resistant mutants.

Engineering the specificity of antibacterial fluoroquinolones: benzenesulfonamide modifications at C-7 of ciprofloxacin change its primary target in Streptococcus pneumoniae from topoisomerase IV to gyrase

We have examined the antipneumococcal mechanisms of a series of novel fluoroquinolones that are identical to ciprofloxacin except for the addition of a benzenesulfonylamido group to the C-7 piperazinyl ring. A number of these derivatives displayed enhanced activity against Streptococcus pneumoniae strain 7785, including compound NSFQ-105, bearing a 4-(4-aminophenylsulfonyl)-1-piperazinyl group at C-7, which exhibited an MIC of 0.06 to 0.125 microg/ml compared with a ciprofloxacin MIC of 1 microg/ml. Several complementary approaches established that unlike the case for ciprofloxacin (which targets topoisomerase IV), the increased potency of NSFQ-105 was associated with a target preference for gyrase: (i) parC mutants of strain 7785 that were resistant to ciprofloxacin remained susceptible to NSFQ-105, whereas by contrast, mutants bearing a quinolone resistance mutation in gyrA were four- to eightfold more resistant to NSFQ-105 (MIC of 0.5 microg/ml) but susceptible to ciprofloxacin; (ii) NSFQ-105 selected first-step gyrA mutants (MICs of 0.5 microg/ml) encoding Ser-81-to-Phe or -Tyr mutations, whereas ciprofloxacin selects parC mutants; and (iii) NSFQ-105 was at least eightfold more effective than ciprofloxacin at inhibiting DNA supercoiling by S. pneumoniae gyrase in vitro but was fourfold less active against topoisomerase IV. These data show unequivocally that the C-7 substituent determines not only the potency but also the target preference of fluoroquinolones. The importance of the C-7 substituent in drug-enzyme contacts demonstrated here supports one key postulate of the Shen model of quinolone action.

Gatifloxacin activity against quinolone-resistant gyrase: allele-specific enhancement of bacteriostatic and bactericidal activities by the C-8-methoxy group

Antibacterial activities of gatifloxacin (AM1155), a new C-8-methoxy fluoroquinolone, and two structurally related compounds, AM1121 and ciprofloxacin, were studied with an isogenic set of ten quinolone-resistant, gyrA (gyrase) mutants of Escherichia coli. To compare the effect of each mutation on resistance, the mutant responses were normalized to those of wild-type cells. Alleles exhibiting the most resistance to growth inhibition mapped in alpha-helix 4, which is thought to lie on a GyrA dimer surface that interacts with DNA. The C-8-methoxy group lowered the resistance due to these mutations more than it lowered resistance arising from several gyrA alleles located outside alpha-helix 4. These data are consistent with alpha-helix 4 being a distinct portion of the quinolone-binding site of GyrA. A helix change to proline behaved more like nonhelix alleles, indicating that helix perturbation differs from the other changes at helix residues. Addition of a parC (topoisomerase IV) resistance allele revealed that the C-8-methoxy group also facilitated attack of topoisomerase IV. When lethal effects were measured at a constant multiple of the minimum inhibitory concentration for each fluoroquinolone to normalize for differences in bacteriostatic action, gatifloxacin was more potent than the C-8-H compounds, both in the presence and absence of protein synthesis (an exception was observed when alanine was substituted for aspartic acid at position 82). Collectively, these data show that the C-8-methoxy group contributes to the enhanced activity of gatifloxacin against resistant gyrase and wild-type topoisomerase IV.

Effect of fluoroquinolone concentration on selection of resistant mutants of Mycobacterium bovis BCG and Staphylococcus aureus

When Mycobacterium bovis BCG and Staphylococcus aureus were plated on agar containing increasing concentrations of fluoroquinolone, colony numbers exhibited a sharp drop, followed by a plateau and a second sharp drop. The plateau region correlated with the presence of first-step resistant mutants. Mutants were not recovered at concentrations above those required for the second sharp drop, thereby defining a mutant prevention concentration (MPC). A C-8-methoxy group lowered the MPC for an N-1-cyclopropyl fluoroquinolone.

A conspicuous adaptability to antibiotics in the Escherichia coli mutator strain, dnaQ49

By repeating the cycle of mutagenesis and selection, the Escherichia coli dnaQ49 mutator acquired high level resistance to ampicillin (30,000 micrograms ml-1), streptomycin (26,000 micrograms ml-1) and ofloxacin (3000 micrograms ml-1). Under the strong pressure of ofloxacin, dnaQ49 also followed the history of mutations in the gyrase and topoisomerase i.v. genes previously observed in clinical isolates of quinolone-resistant E. coli. The results of these in vitro experiments suggest that naturally existing mutators may participate in the rapid acquisition of resistance to various antibiotics in patients. A possible mechanism for the occurrence of this adaptability is discussed with special reference to the property of mutagenesis accompanying DNA replication.

Sequence analysis of the gyrA and parC homologues of a wild-type strain of Vibrio parahaemolyticus and its fluoroquinolone-resistant mutants

Vibrio parahaemolyticus causes seafood-borne gastroenteritis in humans. It is particularly important in Japan, where raw seafood is frequently consumed. Fluoroquinolone is one of the current drugs of choice for treating patients infected by V. parahaemolyticus because resistant strains are rarely found. To study a possible fluoroquinolone resistance mechanism in this organism, nucleotide sequences that are homologous to known gyrA and parC genes have been cloned from V. parahaemolyticus AQ3815 and sequenced by amplification with degenerate primers of the quinolone resistance-determining region (QRDR), followed by cassette ligation-mediated PCR. Open reading frames encoding polypeptides of 878 and 761 amino acid residues were detected in the gyrA and parC homologues, respectively. The V. parahaemolyticus GyrA and ParC sequences were most closely related to Erwinia carotovora GyrA (76% identity) and Escherichia coli ParC (69% identity) sequences, respectively. Ciprofloxacin-resistant mutants of AQ3815 were obtained on an agar medium by multistep selection with increasing levels of the quinolone. One point mutation only in the gyrA QRDR was detected among mutants with low- to intermediate-level resistance, while point mutations in both the gyrA and parC QRDRs were detected only in strains with high-level resistance. These results strongly suggest that, as in other gram-negative bacteria, GyrA and ParC are the primary and secondary targets, respectively, of ciprofloxacin in V. parahaemolyticus.

Streptococcus pneumoniae DNA gyrase and topoisomerase IV: overexpression, purification, and differential inhibition by fluoroquinolones

Streptococcus pneumoniae gyrA and gyrB genes specifying the DNA gyrase subunits have been cloned into pET plasmid vectors under the control of an inducible T7 promoter and have been separately expressed in Escherichia coli. Soluble 97-kDa GyrA and 72-kDa GyrB proteins bearing polyhistidine tags at their respective C-terminal and N-terminal ends were purified to apparent homogeneity by one-step nickel chelate column chromatography and were free of host E. coli topoisomerase activity. Equimolar amounts of the gyrase subunits reconstituted ATP-dependent DNA supercoiling with comparable activity to gyrase of E. coli and Staphylococcus aureus. In parallel, S. pneumoniae topoisomerase IV ParC and ParE subunits were similarly expressed in E. coli, purified to near homogeneity as 93- and 73-kDa proteins, and shown to generate efficient ATP-dependent DNA relaxation and DNA decatenation activities. Using the purified enzymes, we examined the inhibitory effects of three paradigm fluoroquinolones-ciprofloxacin, sparfloxacin, and clinafloxacin-which previous genetic studies with S. pneumoniae suggested act preferentially through topoisomerase IV, through gyrase, and through both enzymes, respectively. Surprisingly, all three quinolones were more active in inhibiting purified topoisomerase IV than gyrase, with clinafloxacin showing the greatest inhibitory potency. Moreover, the tested agents were at least 25-fold more effective in stabilizing a cleavable complex (the relevant cytotoxic lesion) with topoisomerase IV than with gyrase, with clinafloxacin some 10- to 32-fold more potent against either enzyme, in line with its superior activity against S. pneumoniae. The uniform target preference of the three fluoroquinolones for topoisomerase IV in vitro is in apparent contrast to the genetic data. We interpret these results in terms of a model for bacterial killing by quinolones in which cellular factors can modulate the effects of target affinity to determine the cytotoxic pathway.

Alteration in the GyrA subunit of DNA gyrase and the ParC subunit of DNA topoisomerase IV in quinolone-resistant clinical isolates of Staphylococcus epidermidis

We examined 22 clinical isolates of Staphylococcus epidermidis to analyze the association of alterations in GyrA and ParC with fluoroquinolone resistance. The simultaneous presence of GyrA and ParC alterations was associated with a high level of fluoroquinolone resistance in the clinical isolates of S. epidermidis.

Fluoroquinolone action against mycobacteria: effects of C-8 substituents on growth, survival, and resistance

Fluoroquinolones trap gyrase on DNA as bacteriostatic complexes from which lethal DNA breaks are released. Substituents at the C-8 position increase activities of N-1-cyclopropyl fluoroquinolones against several bacterial species. In the present study, a C-8-methoxyl group improved bacteriostatic action against gyrA (gyrase-resistant) strains of Mycobacterium tuberculosis and M. bovis BCG. It also enhanced lethal action against gyrase mutants of M. bovis BCG. When cultures of M. smegmatis, M. bovis BCG, and M. tuberculosis were challenged with a C-8-methoxyl fluoroquinolone, no resistant mutant was recovered under conditions in which more than 1, 000 mutants were obtained with a C-8-H control. A C-8-bromo substituent also increased bacteriostatic and lethal activities against a gyrA mutant of M. bovis BCG. When lethal activity was normalized to bacteriostatic activity, the C-8-methoxyl compound was more bactericidal than its C-8-H control, while the C-8-bromo fluoroquinolone was not. The C-8-methoxyl compound was also found to be more effective than the C-8-bromo fluoroquinolone at reducing selection of resistant mutants when each was compared to a C-8-H control over a broad concentration range. These data indicate that a C-8-methoxyl substituent, which facilitates attack of first-step gyrase mutants, may help make fluoroquinolones effective antituberculosis agents.

DNA gyrase and topoisomerase IV are dual targets of clinafloxacin action in Streptococcus pneumoniae

We examined the response of Streptococcus pneumoniae 7785 to clinafloxacin, a novel C-8-substituted fluoroquinolone which is being developed as an antipneumococcal agent. Clinafloxacin was highly active against S. pneumoniae 7785 (MIC, 0.125 microg/ml), and neither gyrA nor parC quinolone resistance mutations alone had much effect on this activity. A combination of both mutations was needed to register resistance, suggesting that both gyrase and topoisomerase IV are clinafloxacin targets in vivo. The sparfloxacin and ciprofloxacin MICs for the parC-gyrA mutants were 16 to 32 and 32 to 64 microg/ml, respectively, but the clinafloxacin MIC was 1 microg/ml, i.e., within clinafloxacin levels achievable in human serum. S. pneumoniae 7785 mutants could be selected stepwise with clinafloxacin at a low frequency, yielding first-, second-, third-, and fourth-step mutants for which clinafloxacin MICs were 0.25, 1, 6, and 32 to 64 microg/ml, respectively. Thus, high-level resistance to clinafloxacin required four steps. Characterization of the quinolone resistance-determining regions of the gyrA, parC, gyrB, and parE genes by PCR, HinfI restriction fragment length polymorphism, and DNA sequence analysis revealed an invariant resistance pathway involving sequential mutations in gyrA or gyrB, in parC, in gyrA, and finally in parC or parE. No evidence was found for other resistance mechanisms. The gyrA mutations in first- and third-step mutants altered GyrA hot spots Ser-83 to Phe or Tyr (Escherichia coli coordinates) and Glu-87 to Gln or Lys; second- and fourth-step parC mutations changed equivalent hot spots Ser-79 to Phe or Tyr and Asp-83 to Ala. gyrB and parE changes produced novel alterations of GyrB Glu-474 to Lys and of Pro-454 to Ser in the ParE PLRGK motif. Difficulty in selecting first-step gyrase mutants (isolated with 0.125 [but not 0.25] microg of clinafloxacin per ml at a frequency of 5.0 x 10(-10) to 8.5 x 10(-10)) accompanied by the small (twofold) MIC increase suggested only a modest drug preference for gyrase. Given the susceptibility of defined gyrA or parC mutants, the results suggested that clinafloxacin displays comparable if unequal targeting of gyrase and topoisomerase IV. Dual targeting and the intrinsic potency of clinafloxacin against S. pneumoniae and its first- and second-step mutants are desirable features in limiting the emergence of bacterial resistance.