Inhibition by quinolones of DNA gyrase from Staphylococcus aureus

In vivo and in vitro patterns of the activity of simocyclinone D8, an angucyclinone antibiotic from Streptomyces antibioticus

Simocyclinone D8 (SD8) exhibits antibiotic activity against gram-positive bacteria but not against gram-negative bacteria. The molecular basis of the cytotoxicity of SD8 is not fully understood, although SD8 has been shown to inhibit the supercoiling activity of Escherichia coli gyrase. To understand the mechanism of SD8, we have employed biochemical assays to directly measure the sensitivities of E. coli and Staphylococcus aureus type II topoisomerases to SD8 and microarray analysis to monitor the cellular responses to SD8 treatment. SD8 is a potent inhibitor of either E. coli or S. aureus gyrase. In contrast, SD8 exhibits only a moderate inhibitory effect on S. aureus topoisomerase IV, and E. coli topoisomerase IV is virtually insensitive to SD8. The antimicrobial effect of SD8 against E. coli has become evident in the absence of the AcrB multidrug efflux pump. As expected, SD8 treatment exhibits the signature responses to the loss of supercoiling activity in E. coli: upregulation of gyrase genes and downregulation of the topoisomerase I gene. Unlike quinolone treatment, however, SD8 treatment does not induce the SOS response. These results suggest that DNA gyrase is the target of SD8 in both gram-positive and gram-negative bacteria and that the lack of the antibacterial effect against gram-negative bacteria is due, in part, to the activity of the AcrB efflux pump.

Method To Detect Only Live Bacteria during PCR Amplification

Ethidium monoazide (EMA) is a DNA cross-linking agent and eukaryotic topoisomerase II poison. We previously reported that the treatment of EMA with visible light irradiation (EMA + Light) directly cleaved chromosomal DNA of Escherichia coli (T. Soejima, K. Iida, T. Qin, H. Taniai, M. Seki, A. Takade, and S. Yoshida, Microbiol. Immunol. 51:763-775, 2007). Herein, we report that EMA + Light randomly cleaved chromosomal DNA of heat-treated, but not live, Listeria monocytogenes cells within 10 min of treatment. When PCR amplified DNA that was 894 bp in size, PCR final products from 10(8) heat-treated L. monocytogenes were completely suppressed by EMA + Light. When target DNA was short (113 bp), like the hly gene of L. monocytogenes, DNA amplification was not completely suppressed by EMA + Light only. Thus, we used DNA gyrase/topoisomerase IV and mammalian topoisomerase poisons (here abbreviated as T-poisons) together with EMA + Light. T-poisons could penetrate heat-treated, but not live, L. monocytogenes cells within 30 min to cleave chromosomal DNA by poisoning activity. The PCR product of the hly gene from 10(8) heat-treated L. monocytogenes cells was inhibited by a combination of EMA + Light and T-poisons (EMA + Light + T-poisons), but those from live bacteria were not suppressed. As a model for clinical application to bacteremia, we tried to discriminate live and antibiotic-treated L. monocytogenes cells present in human blood. EMA + Light + T-poisons completely suppressed the PCR product from 10(3) to 10(7) antibiotic-treated L. monocytogenes cells but could detect 10(2) live bacteria. Considering the prevention and control of food poisoning, this method was applied to discriminate live and heat-treated L. monocytogenes cells spiked into pasteurized milk. EMA + Light + T-poisons inhibited the PCR product from 10(3) to 10(7) heat-treated cells but could detect 10(1) live L. monocytogenes cells. Our method is useful in clinical as well as food hygiene tests.

Dual-targeting properties of the 3-aminopyrrolidyl quinolones, DC-159a and sitafloxacin, against DNA gyrase and topoisomerase IV: contribution to reducing in vitro emergence of quinolone-resistant Streptococcus pneumoniae

OBJECTIVES

DC-159a (a novel quinolone) and sitafloxacin (DU-6859a) are structurally related quinolones, bearing a 3-aminopyrrolidyl substitution. We investigated the relationship between the target preferences of these 3-aminopyrrolidyl quinolones, in vitro potencies and emergence of quinolone-resistant mutants in Streptococcus pneumoniae, compared with other quinolones.

METHODS

MICs, resistance frequencies and mutant prevention concentrations (MPCs) were determined using quinolone-susceptible strains and first-step parC mutant strains of S. pneumoniae. Target preferences were tested by the following two methods: antibacterial activities against gyrA or parC mutants and in vitro enzyme assays for the determination of 50% inhibition (IC(50)) values.

RESULTS

DC-159a and sitafloxacin exhibited potent antibacterial activities, low frequencies of mutant selection, low MPCs and narrow mutant selection windows against both quinolone-susceptible strains and first-step parC mutants of S. pneumoniae, compared with gatifloxacin, moxifloxacin and other quinolones tested. DC-159a and sitafloxacin showed relatively low MIC ratios against single gyrA or parC mutants relative to the wild-type strain and low IC(50) ratios against DNA gyrase and topoisomerase IV.

CONCLUSIONS

DC-159a and sitafloxacin demonstrated a more balanced dual-targeting activity than gatifloxacin, moxifloxacin and other quinolones tested. In addition, DC-159a and sitafloxacin have a lower propensity for selecting first- and second-step resistant mutants.

Purification and characterization of recombinant Staphylococcus haemolyticus DNA gyrase and topoisomerase IV expressed in Escherichia coli

The subunits of DNA gyrase and topoisomerase IV from Staphylococcus haemolyticus were expressed in Escherichia coli, purified to homogeneity, and used to reconstitute active enzymes that were sensitive to known topoisomerase inhibitors. This represents the first description of a method for isolating type II topoisomerases of a coagulase-negative staphylococcal species.

Staphylococcus aureus gyrase-quinolone-DNA ternary complexes fail to arrest replication fork progression in vitro. Effects of salt on the DNA binding mode and the catalytic activity of S. aureus gyrase

Type II topoisomerases bind to DNA at the catalytic domain across the DNA gate. DNA gyrases also bind to DNA at the non-homologous C-terminal domain of the GyrA subunit, which causes the wrapping of DNA about itself. This unique mode of DNA binding allows gyrases to introduce the negative supercoils into DNA molecules. We have investigated the biochemical characteristics of Staphylococcus aureus (S. aureus) gyrase. S. aureus gyrase is known to require high concentrations of potassium glutamate (K-Glu) for its supercoiling activity. However, high concentrations of K-Glu are not required for its relaxation and decatenation activities. This is due to the requirement of high concentrations of K-Glu for S. aureus gyrase-mediated wrapping of DNA. These results suggest that S. aureus gyrase can bind to DNA at the catalytic domain independent of K-Glu concentration, but high concentrations of K-Glu are required for the binding of the C-terminal domain of GyrA to DNA and the wrapping of DNA. Thus, salt modulates the DNA binding mode and the catalytic activity of S. aureus gyrase. Quinolone drugs can stimulate the formation of covalent S. aureus gyrase-DNA complexes, but high concentrations of K-Glu inhibit the formation of S. aureus gyrase-quinolone-DNA ternary complexes. In the absence of K-Glu, ternary complexes formed with S. aureus gyrase cannot arrest replication fork progression in vitro, demonstrating that the formation of a wrapped ternary complex is required for replication fork arrest by a S. aureus gyrase-quinolone-DNA ternary complex.

Mechanism of action of the des-F(6) quinolone BMS-284756 measured by supercoiling inhibition and cleavable complex assays

BMS-284756 (T-3811ME), a novel des-F(6) quinolone, was tested in the supercoiling inhibition and cleavable complex assays against Escherichia coli DNA gyrase, a target of quinolones. The results suggest that BMS-284756 has the same mechanism of action against DNA gyrase as other quinolones and a similar level of potency.

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.

Molecular cloning of the gyrA and gyrB genes of Bacteroides fragilis encoding DNA gyrase

The genes encoding the DNA gyrase A and B subunits of Bacteroides fragilis were cloned and sequenced. The gyrA and gyrB genes code for proteins of 845 and 653 amino acids, respectively. These proteins were expressed in Escherichia coli, and the combination of GyrA and GyrB exhibited ATP-dependent supercoiling activity. To analyze the role of DNA gyrase in quinolone resistance of B. fragilis, we isolated mutant strains by stepwise selection for resistance to increasing concentrations of levofloxacin. We analyzed the resistant mutants and showed that Ser-82 of GyrA, equivalent to resistance hot spot Ser-83 of GyrA in E. coli, was in each case replaced with Phe. These results suggest that DNA gyrase is an important target for quinolones in B. fragilis.

DNA cleavage activities of Staphylococcus aureus gyrase and topoisomerase IV stimulated by quinolones and 2-pyridones

We have cloned Staphylococcus aureus DNA gyrase and topoisomerase IV and expressed them in Escherichia coli as polyhistidine-tagged proteins to facilitate purification and eliminate contamination by host enzymes. The enzyme preparations had specific activities similar to previously reported values. Potassium glutamate (K-Glu) stimulated the drug-induced DNA cleavage activity and was optimal between 100 and 200 mM for gyrase and peaked at 100 mM for topoisomerase IV. Higher concentrations of K-Glu inhibited the cleavage activities of both enzymes. Using a common buffer system containing 100 mM K-Glu, we tested the enzyme-mediated DNA cleavage activities of both gyrase and topoisomerase IV with oxolinic acid, norfloxacin, ciprofloxacin, trovafloxacin, clinafloxacin, and the 2-pyridone ABT-719. As expected, all drugs tested demonstrated greater potency against topoisomerase IV than against gyrase. In addition, cleavage activity was found to correlate well with antibacterial activity.

Quinolone resistance mutations in the GrlB protein of Staphylococcus aureus

Two altered GrlB proteins (one with an Asp-432-->Asn alteration and one with an Asn-470-->Asp alteration) of Staphylococcus aureus were purified as fusion proteins to maltose-binding protein. The 50% inhibitory concentrations of levofloxacin were 14 and 3.4 microg/ml against topoisomerase IV containing GrlB proteins with alterations at positions 432 and 470, respectively. These results suggest that the alteration of Asp to Asn at position 432 may be responsible for quinolone resistance.

Inhibitory activities of quinolones against DNA gyrase and topoisomerase IV purified from Staphylococcus aureus

In order to clarify the mechanism of action of quinolones against Staphylococcus aureus, GrlA and GrlB proteins of topoisomerase IV encoded by genes with or without mutations were purified separately as fusion proteins with maltose-binding protein in Escherichia coli. The reconstituted enzymes showed ATP-dependent decatenation and relaxing activities but had no supercoiling activity. The inhibitory effects of quinolones on the decatenation activity of topoisomerase IV were determined by quantitative electrophoresis with kinetoplast DNA as a substrate. The 50% inhibitory concentrations (IC50s) of levofloxacin, DR-3354, DU-6859a, DV-7751a, ciprofloxacin, sparfloxacin, and tosufloxacin against topoisomerase IV of S. aureus FDA 209-P were 2.3, 97, 0.45, 1.5, 2.5, 7.4, and 1.8 microg/ml, respectively, and were correlated well with their MICs. The IC50s of these drugs were from 2 to 20 times lower than those for the DNA gyrase. These results support genetic evidence that the primary target of new quinolones is topoisomerase IV in quinolone-susceptible strains of S. aureus. Three altered proteins of topoisomerase IV containing Ser-->Phe changes at codon 80 or Glu-->Lys changes at codon 84 of grlA, or both, were also purified. The inhibitory activities of quinolones against the topoisomerase IV which contained a single amino acid change were from 8 to 95 times weaker than those against the nonaltered enzyme. These results suggest that the mutations in the corresponding genes confer quinolone resistance.

Sparfloxacin. A review of its antibacterial activity, pharmacokinetic properties, clinical efficacy and tolerability in lower respiratory tract infections

Sparfloxacin is a fluoroquinolone antibacterial agent with activity against a broad range of Gram-negative and Gram-positive organisms including Streptococcus pneumoniae, one of the main pathogens in community-acquired pneumonia. In this infection, sparfloxacin has shown efficacy similar to that of amoxicillin, erythromycin, roxithromycin, amoxicillin/clavulanic acid and amoxicillin plus ofloxacin, producing clinical cure rates of 80 to 84% assessed by intention-to-treat analyses in European or multinational trials. US studies showed similar results for sparfloxacin to those for erythromycin and cefaclor. Sparfloxacin was also as effective as all other comparator drugs in patients with other lower respiratory tract infections, usually acute exacerbations of chronic obstructive pulmonary disease (COPD). The profile of adverse effects for sparfloxacin is generally similar to that of other quinolones: gastrointestinal discomfort and CNS effects are the most common in clinical trials. Sparfloxacin causes fewer gastrointestinal disturbances than agents such as amoxicillin and erythromycin and doses not interact with theophylline, an important consideration when treating patients with respiratory disease. Its long elimination half-life permits once-daily dosage regimens. On the other hand, there are infrequent reports of prolonged QTc interval (3% increase) during sparfloxacin therapy. Photosensitivity occurs more frequently than with the other fluoroquinolones (2% of patients in an ongoing postmarketing study and 7.9% of those in US trials), and requires ongoing surveillance. In summary, the good activity of sparfloxacin against S. pneumoniae and other respiratory pathogens supports its use in lower respiratory tract infections, particularly community-acquired pneumonia. Its profile of good efficacy, once-daily dosage, good gastrointestinal tolerability and lack of interaction with theophylline are advantageous, but clinicians and patients must be alert to the possibility of photosensitivity reactions. On this basis, sparfloxacin, when appropriately prescribed, can provide the clinician with a useful alternative treatment option for these common infections.

Differential behaviors of Staphylococcus aureus and Escherichia coli type II DNA topoisomerases

Staphylococcus aureus gyrA and gyrB genes encoding DNA gyrase subunits were cloned and coexpressed in Escherichia coli under the control of the T7 promoter-T7 RNA polymerase system, leading to soluble gyrase which was purified to homogeneity. Purified gyrase was catalytically indistinguishable from the gyrase purified from S. aureus and did not contain detectable amounts of topoisomerases from the E. coli host. Topoisomerase IV subunits GrlA and GrlB from S. aureus were also expressed in E. coli and were separately purified to apparent homogeneity. Topoisomerase IV, which was reconstituted by mixing equimolar amounts of GrlA and GrlB, had both ATP-dependent decatenation and DNA relaxation activities in vitro. This enzyme was more sensitive than gyrase to inhibition by typical fluoroquinolone antimicrobial agents such as ciprofloxacin or sparfloxacin, adding strong support to genetic studies which indicate that topoisomerase IV is the primary target of fluoroquinolones in S. aureus. The results obtained with ofloxacin suggest that this fluoroquinolone could also primarily target gyrase. No cleavable complex could be detected with S. aureus gyrase upon incubation with ciprofloxacin or sparfloxacin at concentrations which fully inhibit DNA supercoiling. This suggests that these drugs do not stabilize the open DNA-gyrase complex, at least under standard in vitro incubation conditions, but are more likely to interfere primarily with the DNA breakage step, contrary to what has been reported with E. coli gyrase. Both S. aureus gyrase-catalyzed DNA supercoiling and S. aureus topoisomerase IV-catalyzed decatenation were dramatically stimulated by potassium glutamate or aspartate (500- and 50-fold by 700 and 350 mM glutamate, respectively), whereas topoisomerase IV-dependent DNA relaxation was inhibited 3-fold by 350 mM glutamate. The relevance of the effect of dicarboxylic amino acids on the activities of type II topoisomerases is discussed with regard to the intracellular osmolite composition of S. aureus.

Contribution of mutations in gyrA and parC genes to fluoroquinolone resistance of mutants of Streptococcus pneumoniae obtained in vivo and in vitro

We have analyzed by gene amplification and sequencing mutations in the quinolone resistance-determining regions of the gyrA, gyrB, and parC genes of fluoroquinolone-resistant Streptococcus pneumoniae mutants obtained during therapy or in vitro. Mutations leading to substitutions in ParC were detected in the two mutants obtained in vivo, BM4203-R (substitution of a histidine for an aspartate at position 84 [Asp-84-->His]; Staphylococcus aureus coordinates) and BM4204-R (Ser-80-->Phe), and in two mutants obtained in vitro (Ser-80-->Tyr). An additional mutant obtained in vitro, BM4205-R3, displayed a higher level of fluoroquinolone resistance and had a mutation in gyrA leading to a Ser-84-->Phe change. We could not detect any mutation in the three remaining mutants obtained in vitro. Total DNA from BM4203-R, BM4204-R, and BM4205-R3 was used to transform S. pneumoniae CP1000 by selection on fluoroquinolones. For the parC mutants, transformants with phenotypes indistinguishable from those of the donors were obtained at frequencies (5 x 10(-3) to 8 x 10(-3)) compatible with monogenic transformation. By contrast, transformants were obtained at a low frequency (4 x 10(-5)), compatible with the transformation of two independent genes, for the gyrA mutant. Resistant transformants of CP1000 were also obtained with an amplified fragment of parC from BM4203-R and BM4204-R but not with a gyrA fragment from BM4205-R3. All transformants had mutations identical to those in the donors. These data strongly suggest that ParC is the primary target for fluoroquinolones in S. pneumoniae and that BM4205-R3 is resistant to higher levels of the drugs following the acquisition of two mutations, including one in gyrA.

Cloning and characterization of the parC and parE genes of Streptococcus pneumoniae encoding DNA topoisomerase IV: role in fluoroquinolone resistance

DNA topoisomerase IV mediates chromosome segregation and is a potential target for antibacterial agents including new antipneumococcal fluoroquinolones. We have used hybridization to a Staphylococcus aureus gyrB probe in concert with chromosome walking to isolate the Streptococcus pneumoniae parE-parC locus, lying downstream of a putative new insertion sequence and encoding 647-residue ParE and 823-residue ParC subunits of DNA topoisomerase IV. These proteins exhibited greatest homology respectively to the GrlB (ParE) and GrlA (ParC) subunits of S. aureus DNA topoisomerase IV. When combined, whole-cell extracts of Escherichia coli strains expressing S. pneumoniae ParC or ParE proteins reconstituted a salt-insensitive ATP-dependent decatenase activity characteristic of DNA topoisomerase IV. A second gyrB homolog isolated from S. pneumoniae encoded a 648-residue protein which we identified as GyrB through its close homology both to counterparts in S. aureus and Bacillus subtilis and to the product of the S. pneumoniae nov-1 gene that confers novobiocin resistance. gyrB was not closely linked to gyrA. To examine the role of DNA topoisomerase IV in fluoroquinolone action and resistance in S. pneumoniae, we isolated mutant strains stepwise selected for resistance to increasing concentrations of ciprofloxacin. We analysed four low-level resistant mutants and showed that Ser-79 of ParC, equivalent to resistance hotspots Ser-80 of GrlA and Ser-84 of GyrA in S. aureus, was in each case substituted with Tyr. These results suggest that DNA topoisomerase IV is an important target for fluoroquinolones in S. pneumoniae and establish this organism as a useful gram-positive system for resistance studies.

Topoisomerase IV is a target of quinolones in Escherichia coli

We have demonstrated that, in Escherichia coli, quinolone antimicrobial agents target topoisomerase IV (topo IV). The inhibition of topo IV becomes apparent only when gyrase is mutated to quinolone resistance. In such mutants, these antibiotics caused accumulation of replication catenanes, which is diagnostic of a loss of topo IV activity. Mutant forms of topo IV provided an additional 10-fold resistance to quinolones and prevented drug-induced catenane accumulation. Drug inhibition of topo IV differs from that of gyrase. (i) Wild-type topo IV is not dominant over the resistant allele. (ii) Inhibition of topo IV leads to only a slow stop in replication. (iii) Inhibition of topo IV is primarily bacteriostatic. These differences may result from topo IV acting behind the replication fork, allowing for repair of drug-induced lesions. We suggest that this and a slightly higher intrinsic resistance of topo IV make it secondary to gyrase as a quinolone target. Our results imply that the quinolone binding pockets of gyrase and topo IV are similar and that substantial levels of drug resistance require mutations in both enzymes.

Levofloxacin. A review of its antibacterial activity, pharmacokinetics and therapeutic efficacy

Levofloxacin, an oral fluoroquinolone antibacterial agent, is the optical S-(-) isomer of ofloxacin. In vitro it is generally twice as potent as ofloxacin. Levofloxacin is active against most aerobic Gram-positive and Gram-negative organisms and demonstrates moderate activity against anaerobes. Drug penetration into body tissues and fluids is rapid and widespread after oral administration. In clinical trials conducted in Japan, oral levofloxacin has demonstrated antibacterial efficacy against a variety of infections, including upper and lower respiratory tract, genitourinary, obstetric, gynaecological and skin and soft tissues. In comparative trials with ofloxacin, levofloxacin, at half the daily dosage of ofloxacin, showed equivalent efficacy and a reduced incidence of adverse effects in the treatment of lower respiratory tract and complicated urinary tract infections. Levofloxacin has a tolerability profile similar to that of other oral fluoroquinolones, with gastrointestinal and central nervous system effects reported most commonly. Theophylline dosage adjustment does not appear to be necessary in patients receiving concomitant levofloxacin. Coadministration with antacids or with other drugs containing divalent or trivalent cations reduces levofloxacin absorption. Thus, levofloxacin has potential as a broad spectrum antibacterial drug in the treatment of a variety of infections. However, clinical trials recruiting non-Japanese patients are in progress and these results will form a basis on which future recommendations for the broader use of levofloxacin can be made.

Antimicrobial activity of DV-7751a, a new fluoroquinolone

We compared the in vitro antibacterial activity of DV-7751a against gram-positive and -negative bacteria with those of quinolones currently available. MICs for 90% of the strains tested (MIC90s) against clinical isolates of methicillin-susceptible and -resistant Staphylococcus aureus and Staphylococcus epidermidis were 0.20, 0.39, 0.20, and 0.78 micrograms/ml, respectively. Moreover, MIC50s for DV-7751a against ofloxacin-resistant methicillin-resistant S. aureus were 4-, 8-, 16-, 32-, and 64-fold lower than those for tosufloxacin and sparfloxacin, levofloxacin, ofloxacin and fleroxacin, ciprofloxacin, and lomefloxacin, respectively. DV-7751a inhibited the growth of all strains of Streptococcus pneumoniae, Streptococcus pyogenes, and Peptostreptococcus spp. at 0.39, 0.39, and 0.78 micrograms/ml, respectively, and was 4- to > 16-fold more active against enterococci at the MIC90 level than the other quinolones tested. The activity of DV-7751a against Pseudomonas aeruginosa was roughly comparable to those of levofloxacin and sparfloxacin at the MIC90 level and was two- to fourfold less than that of ciprofloxacin. DV-7751a showed activity comparable to those of levofloxacin and ciprofloxacin against the other glucose-nonfermenting bacteria Haemophilus influenzae, Neisseria gonorrhoeae, and Moraxella catarrhalis (MIC90s of 0.025, 0.20, and 0.10 micrograms/ml, respectively). DV-7751a activity was not affected by medium, inoculum size, or the addition of human serum but was decreased under acidic conditions and in human urine, as were the other quinolones tested. Time-kill curve studies demonstrated the rapid bactericidal action of DV-7751a against S. aureus, S. pneumoniae, Escherichia coli, and P. aeruginosa. The frequency of spontaneous resistance to DV-7751a was less than or equal to those of the reference drugs. DV-7751a inhibited the supercoiling activity of DNA gyrases from S. aureus, E. coli, and P. aeruginosa at concentrations comparable to those of levofloxacin and sparfloxacin.

Cloning, sequencing, and expression of the DNA gyrase genes from Staphylococcus aureus

We have isolated and cloned the gyrA and gyrB genes from Staphylococcus aureus. These adjacent genes encode the subunits of DNA gyrase. The nucleotide sequence of a 5.9-kb region which includes part of an upstream recF gene, the whole of gyrB and gyrA, and about 1 kb of unknown downstream sequence has been determined. The gyrB and gyrA gene sequences predict proteins of 886 and 644 amino acid residues, respectively, which have significant homologies with the gyrase subunits of Escherichia coli and Bacillus subtilis. Residues thought to be important to the structure and function of the subunits are conserved. These genes have been expressed separately by using a T7 promoter vector. N-terminal sequencing of the cloned gene products suggests that the mature GyrB subunit exists mainly with its initial five residues removed. Protein sequencing also supports the interpretation of our DNA sequencing data, which are inconsistent in several placed with the recently published sequence of the same genes (E. E. C. Margerrison, R. Hopewell, and L. M. Fisher, J. Bacteriol. 174:1596-1603, 1992).

Transport of pefloxacin across the bacterial cytoplasmic membrane in quinolone-susceptible Staphylococcus aureus

Binding to phospholipids, uptake by simple diffusion, and an energy-dependent, carrier-mediated efflux are thought to characterize interactions between fluoroquinolones and bacterial cytoplasmic membranes. Here, we have found that an endogenous active efflux is unlikely in quinolone-susceptible Staphylococcus aureus. The protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), increased pefloxacin uptake in different membrane systems under conditions which excluded carrier-mediated transport, i.e., in bacterial cells at 4 degrees C and in protein-free phosphatidylglycerol liposomes. When plotted as a function of outer pH, the CCCP effect, both in S. aureus cells and in phosphatidylglycerol liposomes, correlated with pefloxacin labeling of everted S. aureus membrane vesicles, with all three profiles showing maximal effect at an acidic pH. So the CCCP effect may result not from inhibition of the proton motive force, as previously thought, but rather from acidification of the intramembrane space by the protonophore, leading to enhanced binding of the positive pefloxacin species to the inner leaflet of the bilayer. Moreover, antistaphylococcal potency and uptake profiles of pefloxacin in S. aureus and phosphatidylglycerol liposomes, assayed as a function of outer pH, peaked at a neutral pH. These observations suggest that zwitterionic and positive quinolone species are responsible for diffusion through and binding to the cytoplasmic membrane, respectively.

Comparative in vitro activities of a new quinolone, OPC-17116, possessing potent activity against gram-positive bacteria

The in vitro antibacterial activity of OPC-17116, a new fluoroquinolone, against a wide variety of clinical isolates was evaluated and compared with those of ciprofloxacin, ofloxacin, and norfloxacin. OPC-17116 showed potent broad-spectrum activity against gram-positive and -negative bacteria. The activity of this compound against gram-positive bacteria was higher than those of other quinolones, and its activity against gram-negative and anaerobic bacteria was roughly comparable to those of other quinolones. OPC-17116 had potent activity against important pathogens of respiratory tract infections such as Staphylococcus aureus, Streptococcus pneumoniae, Klebsiella pneumoniae, Pseudomonas aeruginosa, Haemophilus influenzae, and Branhamella catarrhalis. The MICs of this compound against 90% of these organisms, except for methicillin-resistant S. aureus, ranged from less than or equal to 0.006 to 3.13 micrograms/ml. OPC-17116 at more than one-half the MICs was bactericidal against clinical isolates of S. aureus, Escherichia coli, K. pneumoniae, and P. aeruginosa. The activity of OPC-17116 was decreased by several culture conditions such as acidic pH, high concentration of Mg2+ ions, and inoculum size of 10(7) CFU/ml. OPC-17116 inhibited the supercoiling activity of DNA gyrases from E. coli KL-16 and S. aureus SA113 (50% inhibitory concentrations, 0.19 and 23.0 micrograms/ml, respectively). The amount of OPC-17116 accumulation was higher than that of other quinolones in S. aureus.

Nucleotide sequence of the Staphylococcus aureus gyrB-gyrA locus encoding the DNA gyrase A and B proteins

We have determined the nucleotide sequence of a 5.3-kb segment of the Staphylococcus aureus chromosome that includes the gyrA and gyrB genes coding for both subunits of DNA gyrase, the enzyme that catalyzes ATP-dependent DNA supercoiling. The gene order at this locus, dnaA-dnaN-recF-gyrB-gyrA, is similar to that found in the Bacillus subtilis replication origin region. S. aureus recF, gyrB, and gyrA genes are closely spaced, occupy the same reading frame, and may be coordinately expressed. The S. aureus gyrB and gyrA genes encode 640- and 889-residue proteins, respectively, that share strong homology with other bacterial gyrase subunits, notably those from B. subtilis. These results are discussed in regard to the mechanism of DNA gyrase and its role as a target for the 4-quinolones and other antistaphylococcal agents.

Mechanisms of clinical resistance to fluoroquinolones in Staphylococcus aureus

Mechanisms of Staphylococcus aureus resistance to fluoroquinolones were characterized. Subunit A and B proteins of DNA gyrase were partially purified from fluoroquinolone-susceptible strain SA113 and resistant isolate MS16405, which was 250- to 1,000-fold less susceptible to fluoroquinolones such as ciprofloxacin, norfloxacin, ofloxacin, temafloxacin, and sparfloxacin than SA113 was. The supercoiling activity of the gyrase from SA113 was inhibited by the fluoroquinolones, and the 50% inhibitory concentrations of the drugs correlated well with their MICs. In contrast, the gyrase from MS16405 was insensitive to inhibition of supercoiling by all of the quinolones tested, even at 800 micrograms/ml. Combinations of heterologous gyrase subunits showed that subunit A from MS16405 conferred fluoroquinolone resistance, suggesting that an alteration in gyrase subunit A is a cause of the fluoroquinolone resistance in MS16405. Uptake of hydrophilic fluoroquinolones such as ciprofloxacin and norfloxacin by MS16405 was significantly lower than that by SA113. Furthermore, this difference was abolished by the addition of an energy inhibitor, carbonyl cyanide m-chlorophenylhydrazone, suggesting that an alteration in an energy-dependent process, such as an active efflux of hydrophilic quinolones, may lead to decreased drug uptake and hence to increased resistance to fluoroquinolones in MS16405. These findings suggest that the fluoroquinolone resistance in MS16405 is due mainly to an alteration in subunit A of DNA gyrase and may also be associated with an alteration in the drug uptake process.

Inhibitory effects of ciprofloxacin and sparfloxacin on DNA gyrase purified from fluoroquinolone-resistant strains of methicillin-resistant Staphylococcus aureus

The activities of new quinolones against 140 strains of methicillin-resistant Staphylococcus aureus were determined. From the relationship between the MICs of sparfloxacin and ciprofloxacin, fluoroquinolone-resistant S. aureus 6171 (MIC of sparfloxacin, 200 micrograms/ml; MIC of ciprofloxacin, 100 micrograms/ml) and fluoroquinolone-susceptible S. aureus FDA 209-P were selected for purification of the subunit A and B proteins of their DNA gyrases. The supercoiling activities of reconstituted ArBr (r, resistant) (from strain 6171) and ArBs (s, susceptible) gyrases were 40-fold more resistant to new quinolones than those of AsBs from FDA 209-P and AsBr gyrases. The 50% inhibitory doses of ciprofloxacin and sparfloxacin for AmBm (from mutant 19) and AmBs (m, moderately resistant) gyrases were 15- to 27-fold higher than those for AsBs and AsBm gyrases. These findings indicate that one of the resistance mechanisms of S. aureus against fluoroquinolones is a modification of the gyrase subunit A protein.