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

Indolin-2-one Nitroimidazole Antibiotics Exhibit an Unexpected Dual Mode of Action

Nitroimidazoles such as metronidazole are used as anti-infective drugs against anaerobic bacteria. Upon in vivo reduction of the nitro group, reactive radicals damage DNA and proteins in the absence of oxygen. Unexpectedly, a recent study of nitroimidazoles linked to an indolin-2-one substituent revealed potent activities against aerobic bacteria. This suggests a different, yet undiscovered mode of action (MoA). To decipher this MoA, we first performed whole proteome analysis of compound-treated cells, revealing an upregulation of bacteriophage-associated proteins, indicative of DNA damage. Since DNA binding of the compound was not observed, we applied activity-based protein profiling (ABPP) for direct target discovery. Labeling studies revealed topoisomerase IV, an essential enzyme for DNA replication, as the most enriched hit in pathogenic Staphylococcus aureus cells. Subsequent topoisomerase assays confirmed the inhibition of DNA decatenation in the presence of indolin-2-one nitroimidazole with an activity comparable to ciprofloxacin, a known inhibitor of this enzyme. Furthermore, we determined significantly increased redox potentials of indolin-2-one nitroimidazoles compared to classic 5-nitroimidazoles such as metronidazole, which facilitates in vivo reduction. Overall, this study unravels that indolin-2-one-functionalized nitroimidazoles feature an unexpected dual MoA: first, the direct inhibition of the topoisomerase IV and second the classic nitroimidazole MoA of reductive bioactivation leading to damaging reactive species. Importantly, this dual MoA impairs resistance development. Given the clinical application of this compound class, the new mechanism could be a starting point to mitigate resistance.

Gene expression analysis in the potent bactericidal activity of sitafloxacin against Streptococcus pneumoniae

We investigated the degree of expression of Streptococcus pneumoniae genes associated with bacteriolysis and cell death in relation to the rapid bactericidal activity of sitafloxacin. S. pneumoniae ATCC 49619 was added to brain heart infusion containing sitafloxacin and garenoxacin concentrations equivalent to the Cmax achieved with the usual single dose and 4 h post-Cmax concentration. RNA was extracted and cDNA was prepared using reverse transcriptase. Following RNA extraction and cDNA synthesis, quantitative PCR was performed to determine the amount of gene expression for 13 genes associated with cell death. Of the 13 genes analyzed, S. pneumoniae exposed for 10 min to a sitafloxacin concentration of 4 h post-Cmax showed 3.9 times increased expression of lytA compared to the control strain. Furthermore, we observed a slightly increased expression for cibA encoding a competence induced bacteriocin. Our study suggests that the induction of a lytic enzyme and bacteriocin may reflect gene expression in response to sitafloxacin accounting for part of its rapid bactericidal activity against S. pneumoniae.

Determination of the stereoisomeric impurities of sitafloxacin by capillary electrophoresis with dual chiral additives

Because of the bioactivity against the human topoisomerase II, the stereoisomeric impurities of sitafloxacin should be controlled in the process of manufactory. In the present work, a capillary electrophoresis (CE) method was developed and validated for simultaneous determination of three stereoisomeric impurities of sitafloxacin. The separation with high resolution not only for the separation of enantiomers, but also for the separation of diastereoisomers was achieved by using a background electrolyte composed of dual chiral selectors, namely γ-cyclodextrin (γ-CD) and Cu²⁺-d-phenylalanine (D-Phe) complex. The combination of two chiral selectors is indispensable to gain a high separation selectivity due to the cooperativity effect of different chiral discrimination modes: inclusion complexation (γ-CD) and ligand exchange (Cu²⁺-d-Phe). The concentrations of γ-CD, Cu²⁺ and D-Phe were found to be critical to the separation. Because two chiral selectors were involved in the enantiomer separation system, multiple factors and their interaction should be simultaneously optimized by using the response surface methodology (RSM) with a face centred central composite design (FCCD). The obtained optimized separation conditions were as follows: 15mmol/L dipotassium hydrogenphosphate solution (pH 4.5) containing 15mmol/L D-Phe, 20mmol/L CuSO₄ and 20mmol/L γ-CD, separation voltage 15kV. The method was then validated and the robustness of the method was tested. Under the optimized conditions, as low as 0.1% (m/m) stereoisomeric impurities of sitafloxacin can be determined by the method.

Bacterial Infections in the Elderly Patient: Focus on Sitafloxacin

Sitafloxacin (DU-6859a) is a new-generation oral fluoroquinolone with in vitro activity against a broad range of Gram-positive and -negative bacteria, including anaerobic bacteria, as well as against atypical bacterial pathogens. Particularly in Japan this antibiotic was approved in 2008 for treatment of a number of bacterial infections caused by Gram-positive cocci and Gram-negative cocci and rods, including anaerobia atypical bacterial pathogens. As compared to oral levofloxacin sitafloxacin was non-inferior in the treatment of community-acquired pneumonia and non-inferior in the treatment of complicated urinary tract infections, according to the results of randomized, double-blind, multicentre, non-inferiority trials. Non-comparative studies demonstrated the efficacy of oral sitafloxacin in otorhinolaryngological infections, urethritis in men, cervicitis in women and odontogenic infections. Most common adverse reactions were gastrointestinal disorders and laboratory abnormalities in patients receiving oral sitafloxacin; diarrhea and liver enzyme elevations were among the common. In the Japanese population sitafloxacin covers broad spectrum of bacteria as compared to carbapenems, whereas in the Caucasians its use is currently limited due to the potential for ultraviolet A phototoxicity. Sitafloxacin is a promising therapeutic agent which merits further investigation in randomized clinical trials of elderly patients.

Inhibitory activity of garenoxacin against DNA gyrase of Mycoplasma pneumoniae

OBJECTIVES

Garenoxacin, a des-fluoro(6)-quinolone, exhibits potent activity against Mycoplasma pneumoniae, including macrolide-resistant strains. There has been no report on the inhibitory activity of garenoxacin against the target enzyme of M. pneumoniae.

METHODS

Subunits of DNA gyrase (GyrA and GyrB) proteins of M. pneumoniae FH were separately expressed as His-tagged proteins in Escherichia coli Chaperone Competent Cell BL21 by IPTG induction of plasmids containing the respective gyrA and gyrB genes. The inhibitory activities of garenoxacin, moxifloxacin, gatifloxacin and levofloxacin against DNA gyrase were evaluated by the inhibition of supercoiling activity (n = 3).

RESULTS

Against M. pneumoniae FH, garenoxacin showed 2- to 16-fold more potent activity than the other quinolones. The mean IC(50) of garenoxacin for DNA gyrase of M. pneumoniae was 2.5 mg/L. Garenoxacin showed the most potent inhibitory activity against M. pneumoniae DNA gyrase among the quinolones tested. The IC(50) values of the quinolones for DNA gyrase roughly correlated with each MIC value.

CONCLUSIONS

The antimycoplasmal activity of the quinolones was almost certainly due to inhibition of the supercoiling activity of DNA gyrase. Garenoxacin was considered a valuable quinolone in the treatment of infectious diseases caused by M. pneumoniae.

Curing bacteria of antibiotic resistance: reverse antibiotics, a novel class of antibiotics in nature

By screening cultures of soil bacteria, we re-discovered an old antibiotic (nybomycin) as an antibiotic with a novel feature. Nybomycin is active against quinolone-resistant Staphylococcus aureus strains with mutated gyrA genes but not against those with intact gyrA genes against which quinolone antibiotics are effective. Nybomycin-resistant mutant strains were generated from a quinolone-resistant, nybomycin-susceptible, vancomycin-intermediate S. aureus (VISA) strain Mu 50. The mutants, occurring at an extremely low rate (<1 × 10(-11)/generation), were found to have their gyrA genes back-mutated and to have lost quinolone resistance. Here we describe nybomycin as the first member of a novel class of antibiotics designated 'reverse antibiotics'.

Susceptibility of Propionibacterium acnes isolated from patients with acne vulgaris to zinc ascorbate and antibiotics

Purpose:

The in vitro antimicrobial activity of ascorbic acid derivatives against Propionibacterium acnes was tested either alone or in combination with a variety of antimicrobial agents, and their fractional inhibitory concentration index was determined using checkerboard tests. The antimicrobial effectiveness of zinc ascorbate in the treatment of acne vulgaris, either alone or in combination with antibiotics such as clindamycin that are commonly used in Japan for the treatment of acne vulgaris, was therefore examined.

Materials and methods:

The antimicrobial susceptibility of 41 strains of clindamycin-sensitive and/or clindamycin-resistant P. acnes isolated from acne vulgaris patients was tested, in comparison with a type strain of P. acnes.

Results:

Zinc ascorbate showed antimicrobial activity against a type strain of P. acnes and its concentration (0.064%) was sufficiently lower than the normal dose (5%) of other ascorbic acid derivatives. Combinations of zinc ascorbate with clindamycin, erythromycin, and chloramphenicol showed an additive effect, and zinc ascorbate alone effectively inhibited the growth of all P. acnes including clindamycin-resistant strains.

Conclusion:

The results provide novel evidence that the combination of zinc ascorbate and clindamycin is effective for acne vulgaris treatment.

Molecular characterization of 8-methoxyfluoroquinolone resistance in a clinical isolate of methicillin-resistant Staphylococcus aureus

BACKGROUND

Activity of gatifloxacin against clinical isolates of fluoroquinolone-resistant Staphylococcus aureus is more potent than that of other fluoroquinolones such as norfloxacin and levofloxacin. To date, few reports have described high-level resistance to gatifloxacin in clinical isolates of S. aureus, although in vitro studies have shown that mutations in both DNA gyrase and topoisomerase IV were required for gatifloxacin resistance in S. aureus.

METHODS

Minimum inhibitory concentrations were determined for fluoroquinolones and other antimicrobials in a methicillin-resistant S. aureus isolate that was cultured from blood of a patient with septicemia. Fluoroquinolone resistance was characterized by DNA sequencing and microbiologic assay.

RESULTS

The isolate showed high-level resistance to fluoroquinolones including an 8-methoxyfluoroquinolone, gatifloxacin (minimum inhibitory concentration 64 microg/ml). Amino acid mutations of Ser80Tyr and Glu84Lys in GrlA and Ser84Leu and Ser85Pro in GyrA were possibly related to this resistance in methicillin-resistant S. aureus HU2000-062, although efflux may play a minor role in resistance as well.

CONCLUSION

GyrA and GrlA mutations mainly conferred to 8-methoxyfluoroquinolone resistance in this isolate.

Detection of live and antibiotic-killed bacteria by quantitative real-time PCR of specific fragments of rRNA

Assessing bacterial viability by molecular markers might help accelerate the measurement of antibiotic-induced killing. This study investigated whether rRNA could be suitable for this purpose. Cultures of penicillin-susceptible and penicillin-tolerant (Tol1 mutant) Streptococcus gordonii were exposed to mechanistically different penicillin and levofloxacin. Bacterial survival was assessed by viable counts and compared to quantitative real-time PCR amplification of either the 16S rRNA genes or the 16S rRNA, following reverse transcription. Penicillin-susceptible S. gordonii lost > or =4 log(10) CFU/ml of viability over 48 h of penicillin treatment. In comparison, the Tol1 mutant lost < or =1 log(10) CFU/ml. Amplification of a 427-bp fragment of 16S rRNA genes yielded amplicons that increased proportionally to viable counts during bacterial growth but did not decrease during drug-induced killing. In contrast, the same 427-bp fragment amplified from 16S rRNA paralleled both bacterial growth and drug-induced killing. It also differentiated between penicillin-induced killing of the parent and the Tol1 mutant (> or =4 log(10) CFU/ml and < or =1 log(10) CFU/ml, respectively) and detected killing by mechanistically unrelated levofloxacin. Since large fragments of polynucleotides might be degraded faster than smaller fragments, the experiments were repeated by amplifying a 119-bp region internal to the original 427-bp fragment. The amount of 119-bp amplicons increased proportionally to viability during growth but remained stable during drug treatment. Thus, 16S rRNA was a marker of antibiotic-induced killing, but the size of the amplified fragment was critical for differentiation between live and dead bacteria.

High-level ciprofloxacin resistance from point mutations in gyrA and parC confined to global hospital-adapted clonal lineage CC17 of Enterococcus faecium

To substantiate a common genetic background of ciprofloxacin-resistant Enterococcus faecium, 32 ciprofloxacin-resistant (Cip(r)) and 31 ciprofloxacin-susceptible (Cip(s)) isolates from outbreaks, clinical infections, surveillances, and animals from 10 different countries were genotyped by multilocus sequence typing. Additionally, susceptibilities to ampicillin and vancomycin and the presence of esp were determined and the quinolone resistance-determining regions of parC, gyrA, parB, and gyrE were sequenced. High-level Cip(r) (MIC > or = 64 microg/ml) due to point mutations in the quinolone resistance-determining region was unique to a distinct hospital-adapted genetic complex in E. faecium, previously designated CC17. Low-level Cip(r) (MIC = 4 microg/ml) in non-CC17 strains is not attributable to point mutations in any subunit of the topoisomerase genes, and the mechanism of resistance remains unclear. Acquisition of mutations in parC and gyrA, leading to high-level Cip(r), is, in addition to ampicillin resistance and the presence of a putative pathogenicity island, another cumulative step in hospital adaptation of CC17.

An improved expression plasmid for affinity purification of Staphylococcus aureus gyrase A subunit

Of the bacterial topoisomerases, the gyrase A subunit (GyrA) of Staphylococcus aureus is particularly difficult to purify because of its tendency to form inclusion bodies. Previous attempts at purification yielded low concentrations of protein with reduced specific activity. To overcome this problem, we modified the commercially available plasmid expression vector, pBAD/Thio-TOPO, via the addition of DNA sequences encoding a hexahistidine tag upstream and a cleavage site for tobacco etch virus protease downstream of the gene encoding thioredoxin. The resulting expression system consisting of the modified plasmid, pSAGA7, and the recommended host strain, Escherichia coli TOP 10, facilitated high level expression of soluble GyrA and its affinity purification to over 95% homogeneity. Purified GyrA had high biological activity as evidenced by a specific activity of 4.3x10(5)U/mg. The pSAGA7/TOP10 expression system also facilitated the expression and purification of a subunit of S. aureus topoisomerase IV, ParE, and a recently discovered protein unrelated to topoisomerases, QnrB, two "hard to purify" proteins. We conclude that pSAGA7 might be useful for high-level soluble expression and purification of diverse microbial proteins.

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.

Inhibitory activities of quinolones against DNA gyrase of Chlamydia pneumoniae

The gyrA and gyrB genes of Chlamydia pneumoniae TW-183 were cloned, and their proteins were purified by use of a fusion system with a maltose-binding protein. The 50% inhibitory concentrations of garenoxacin, sparfloxacin, moxifloxacin, gatifloxacin, and levofloxacin were 10.1, 47.5, 39.6, 64.2, and 156.9 microg/ml, respectively, and the MICs against C. pneumoniae TW-183 were 0.008, 0.016, 0.063, 0.125, and 0.25 microg/ml, respectively.

Novel ciprofloxacin-resistant, nalidixic acid-susceptible mutant of Staphylococcus aureus

A ciprofloxacin-resistant, nalidixic acid-susceptible mutant of Staphylococcus aureus (F145) contained no mutations within gyrA, gyrB, grlA, and grlB or within norA or its promoter region. MICs and accumulation studies suggest the role of a novel multidrug efflux pump.

Spectrum and antibiotic resistance of uropathogens from hospitalized patients with urinary tract infections: 1994-2000

During the period 1994-2000 all uropathogens cultured from urine of hospitalized urological patients were identified and susceptibility was tested against 11 antibacterials. Duplicated isolates were eliminated. There was no general trend of increased of resistance apart from E. coli to ciprofloxacin (10.4% in 2000). Vancomycin-resistant staphylococci or enterococci was not significant. The lowest overall rates of resistance were found with piperacillin/tazobactam followed by ciprofloxacin and trimethoprim/sulphamethoxazole. Ciprofloxacin was the best oral antibiotic for the empirical treatment of urinary tract infection (UTI) due to Gram-negative rods and ampicillin/sulbactam for the treatment of UTI with Gram-positive cocci.

Inhibitory activities of quinolones against DNA gyrase and topoisomerase IV of Enterococcus faecalis

We have cloned the DNA gyrase and topoisomerase IV genes of Enterococcus faecalis to examine the actions of quinolones against E. faecalis genetically and enzymatically. We first generated levofloxacin-resistant mutants of E. faecalis by stepwise selection with increasing drug concentrations and analyzed the quinolone resistance-determining regions of gyrA and parC from the resistant mutants. Isogenic mutants with low-level resistance contained a mutation in gyrA, whereas those with higher levels of resistance had mutations in both gyrA and parC. These results suggested that gyrA is the primary target for levofloxacin in E. faecalis. We then purified the recombinant DNA gyrase and topoisomerase IV enzymes of E. faecalis and measured the in vitro inhibitory activities of quinolones against these enzymes. The 50% inhibitory concentrations (IC(50)s) of levofloxacin, ciprofloxacin, sparfloxacin, tosufloxacin, and gatifloxacin for DNA gyrase were found to be higher than those for topoisomerase IV. In conflict with the genetic data, these results indicated that topoisomerase IV would be the primary target for quinolones in E. faecalis. Among the quinolones tested, the IC(50) of sitafloxacin (DU-6859a), which shows the greatest potency against enterococci, for DNA gyrase was almost equal to that for topoisomerase IV; its IC(50)s were the lowest among those of all the quinolones tested. These results indicated that other factors can modulate the effect of target affinity to determine the bacterial killing pathway, but the highest inhibitory actions against both enzymes correlated with good antienterococcal activities.

A critical review of the fluoroquinolones: focus on respiratory infections

The new fluoroquinolones (clinafloxacin, gatifloxacin, gemifloxacin, grepafloxacin, levofloxacin, moxifloxacin, sitafloxacin, sparfloxacin and trovafloxacin) offer excellent activity against Gram-negative bacilli and improved Gram-positive activity (e.g. against Streptococcus pneumoniae and Staphylococcus aureus) over ciprofloxacin. Ciprofloxacin still maintains the best in vitro activity against Pseudomonas aeruginosa. Clinafloxacin, gatifloxacin, moxifloxacin, sitafloxacin, sparfloxacin and trovafloxacin display improved activity against anaerobes (e.g. Bacteroides fragilis) versus ciprofloxacin. All of the new fluoroquinolones display excellent bioavailability and have longer serum half-lives than ciprofloxacin allowing for once daily dose administration. Clinical trials comparing the new fluoroquinolones to each other or to standard therapy have demonstrated good efficacy in a variety of community-acquired respiratory infections (e.g. pneumonia, acute exacerbations of chronic bronchitis and acute sinusitis). Limited data suggest that the new fluoroquinolones as a class may lead to better outcomes in community-acquired pneumonia and acute exacerbations of chronic bronchitis versus comparators. Several of these agents have either been withdrawn from the market, had their use severely restricted because of adverse effects (clinafloxacin because of phototoxicity and hypoglycaemia; grepafloxacin because of prolongation of the QTc and resultant torsades de pointes; sparfloxacin because of phototoxicity; and trovafloxacin because of hepatotoxicity), or were discontinued during developmental phases. The remaining fluoroquinolones such as gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin have adverse effect profiles similar to ciprofloxacin. Extensive post-marketing safety surveillance data (as are available with ciprofloxacin and levofloxacin) are required for all new fluoroquinolones before safety can be definitively established. Drug interactions are limited; however, all fluoroquinolones interact with metal ion containing drugs (eg. antacids). The new fluoroquinolones (gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin) offer several advantages over ciprofloxacin and are emerging as important therapeutic agents in the treatment of community-acquired respiratory infections. Their broad spectrum of activity which includes respiratory pathogens such as penicillin and macrolide resistant S. pneumoniae, favourable pharmacokinetic parameters, good bacteriological and clinical efficacy will lead to growing use of these agents in the treatment of community-acquired pneumonia, acute exacerbations of chronic bronchitis and acute sinusitis. These agents may result in cost savings especially in situations where, because of their potent broad-spectrum activity and excellent bioavailability, they may be used orally in place of intravenous antibacterials. Prudent use of the new fluoroquinolones will be required to minimise the development of resistance to these agents.

Target preference of 15 quinolones against Staphylococcus aureus, based on antibacterial activities and target inhibition

The antibacterial activities and target inhibition of 15 quinolones against grlA and gyrA mutant strains were studied. The strains were obtained from wild-type Staphylococcus aureus MS5935 by selection with norfloxacin and nadifloxacin, respectively. The antibacterial activities of most quinolones against both mutant strains were lower than those against the wild-type strain. The ratios of MICs for the gyrA mutant strain to those for the grlA mutant strain (MIC ratio) varied from 0.125 to 4. The ratios of 50% inhibitory concentrations (IC(50)s) of quinolones against topoisomerase IV to those against DNA gyrase (IC(50) ratios) also varied, from 0.177 to 5.52. A significant correlation between the MIC ratios and the IC(50) ratios was observed (r = 0.919; P < 0.001). These results suggest that the antibacterial activities of quinolones against the wild-type strain are involved not only in topoisomerase IV inhibition but also in DNA gyrase inhibition and that the target preference in the wild-type strain can be anticipated by the MIC ratios. Based on the MIC ratios, the quinolones were classified into three categories. Type I quinolones (norfloxacin, enoxacin, fleroxacin, ciprofloxacin, lomefloxacin, trovafloxacin, grepafloxacin, ofloxacin, and levofloxacin) had MIC ratios of <1, type II quinolones (sparfloxacin and nadifloxacin) had MIC ratios of >1, and type III quinolones (gatifloxacin, pazufloxacin, moxifloxacin, and clinafloxacin) had MIC ratios of 1. Type I and type II quinolones seem to prefer topoisomerase IV and DNA gyrase, respectively. Type III quinolones seem to target both enzymes at nearly the same level in bacterial cells (a phenomenon known as the dual-targeting property), and their IC(50) ratios were approximately 2.

Efficacies of moxifloxacin, ciprofloxacin, and vancomycin against experimental endocarditis due to methicillin-resistant Staphylococcus aureus expressing various degrees of ciprofloxacin resistance

The new 8-methoxyquinolone moxifloxacin was tested against two ciprofloxacin-susceptible Staphylococcus aureus strains (strains P8 and COL) and two ciprofloxacin-resistant derivatives of strain P8 carrying a single grlA mutation (strain P8-4) and double grlA and gyrA mutations (strain P8-128). All strains were resistant to methicillin. The MICs of ciprofloxacin and moxifloxacin were 0.5 and 0.125 mg/liter, respectively, for P8; 0.25 and 0.125 mg/liter, respectively, for COL; 8 and 0.25 mg/liter, respectively, for P8-4; and >or=128 and 2 mg/liter, respectively, for P8-128. In vitro, the rate of spontaneous resistance of P8 and COL was 10(-7) on agar plates containing ciprofloxacin at two times the MIC, whereas it was <or=10(-10) on agar plates containing moxifloxacin at two times the MIC. Rats with experimental aortic endocarditis were treated with doses of drugs that simulate the kinetics in humans: moxifloxacin, 400 mg orally once a day; ciprofloxacin, 750 mg orally twice a day; or vancomycin, 1 g intravenously twice a day. Treatment was started either 12 or 24 h after infection and lasted for 3 days. Moxifloxacin treatment resulted in culture-negative vegetations in a total of 20 of 21 (95%) rats infected with P8, 10 of 11 (91%) rats infected with COL, and 19 of 24 (79%) rats infected with P8-4 (P < 0.05 compared to the results for the controls). In contrast, ciprofloxacin treatment sterilized zero of nine (0%) vegetations infected with first-level resistant mutant P8-4. Vancomycin sterilized only 8 of 15 (53%), 6 of 11 (54%), and 12 of 23 (52%) of the vegetations, respectively. No moxifloxacin-resistant derivative emerged among these organisms. However, moxifloxacin treatment of highly ciprofloxacin-resistant mutant P8-128 failed and selected for variants for which the MIC increased two times in 2 of 10 animals. Thus, while oral moxifloxacin might deserve consideration as treatment for staphylococcal infections in humans, caution related to its use against strains for which MICs are borderline is warranted.

Fluoroquinolones: place in ocular therapy

The fluoroquinolones have become widely used antibacterial agents in the treatment of ocular infections, with topical, intravitreal and systemic routes of administration being used. In general, fluoroquinolones (such as ciprofloxacin, ofloxacin, lomefloxacin and norfloxacin) have good activity against gram-negative and gram-positive bacteria. Therapeutic concentrations are achieved in the cornea after topical administration so that the fluoroqinolones have largely replaced combination therapy for the treatment of bacterial keratitis. However, a second line agent is needed when resistance is likely, such as in disease caused by streptococcal species. Reversal of resistance to quinolones may not occur with withdrawal of the antibacterial. This stresses the importance of prudent prescribing to reduce the occurrence of resistance to quinolones. When used in therapeutic topical dosages, corneal toxicity does not occur. Similarly, retinal toxicity is not seen when fluoroquinolones are used at therapeutic dosages, systemically or topically. Corneal precipitation occurs, particularly with ciprofloxacin and to a lesser extent norfloxacin, but does not appear to interfere with healing. In the treatment of endophthalmitis there is reasonable penetration of systemic fluoroquinolones into the vitreous but sufficiently high concentrations to reach the minimum inhibitory concentration for 90% of isolates (MIC90) of all important micro-organisms may not be guaranteed. Systemic administration may be useful for prophylaxis after ocular trauma.

Fluoroquinolones inhibit preferentially Streptococcus pneumoniae DNA topoisomerase IV than DNA gyrase native proteins

The genes encoding the subunits of DNA topoisomerase IV (parC and parE) and DNA gyrase (gyrA and gyrB) of Streptococcus pneumoniae were cloned and overproduced in Escherichia coli by using the T7promoter-T7 RNA polymerase system. The four subunits were separately purified to near homogeneity by column chromatography. Protein purification was achieved by DEAE-sepharose, heparin-agarose, and hydroxylapatite chromatography. DNA topoisomerase IV was reconstituted when ParC and ParE were combined at a 3.8-fold excess of ParE. The reconstituted topoisomerase IV showed to generate efficient ATP-dependent DNA decatenation activity. The DNA gyrase ATP-dependent supercoiling activity was reconstituted by mixing equimolar amounts of the two gyrase subunits. The inhibitory effects of four representative fluoroquinolones on the DNA decatenation activity of topoisomerase IV and DNA supercoiling of gyrase have been examined and compared. All four compounds were more active in inhibiting topoisomerase IV than gyrase. Moreover, there was a positive correlation between the inhibitory activity against topoisomerase IV decatenation and DNA gyrase supercoiling. The classification of the four fluoroquinolones, considering their inhibitory activities in decatenation, supercoiling and growth was the following: clinafloxacin > trovafloxacin > sparfloxacin > ciprofloxacin. These results suggest these drugs primarily target topoisomerase IV of S. pneumoniae, and gyrase secondarily, in agreement with genetic data.

In vitro activities of six quinolones and mechanisms of resistance in Staphylococcus aureus and coagulase-negative staphylococci

Of 94 clinical isolates of Staphylococcus aureus (n = 51) and coagulase-negative staphylococci (CNS) (n = 43), mutations in the quinolone resistance-determining region of topoisomerases GrlA, GrlB, GyrA, and GyrB together with MICs of six quinolones were analyzed. Amino acid substitutions at identical residues (GrlA residues 80 and 84; GyrA residues 84 and 88) were found in S. aureus and CNS. Active efflux, as suggested by blocking by reserpine, contributed substantially to the resistance phenotype in some strains. Among ciprofloxacin, clinafloxacin, levofloxacin, nalidixic acid, trovafloxacin, and sparfloxacin, a 0.5-microg/ml concentration of sparfloxacin discriminated best between strains with two or three mutations and those with no mutations.

Quinolone resistance in Staphylococci: activities of new nonfluorinated quinolones against molecular targets in whole cells and clinical isolates

The activity of three new, 8-methoxy-nonfluorinated quinolones (NFQs) against multiple-drug-resistant staphylococci was investigated. First, using Staphylococcus aureus strains containing point mutations in the serine 84-80 hot spots of the target genes (gyrA and grlA), cell growth inhibition potencies of the NFQs as a result of DNA gyrase and topoisomerase IV inhibition were estimated and compared with those of known fluoroquinolones. The NFQs and clinafloxacin showed higher affinities toward both the targets than ciprofloxacin, trovafloxacin and gatifloxacin. Furthermore, the ratio of the calculated affinity parameter for DNA gyrase to that for topoisomerase IV was lower in the case of the NFQs, clinafloxacin, and gatifloxacin than in the case of ciprofloxacin and trovafloxacin. These results suggest that the former group of quinolones is better able to exploit both the targets. Next, using clinical isolates of methicillin-resistant S. aureus (MRSA; n = 34) and coagulase-negative staphylococci (CoNS; n = 24), the NFQs and clinafloxacin were shown to be more potent (MIC at which 90% of the isolates are inhibited [MIC90] = 2 microg/ml for MRSA and 0.5 microg/ml for CoNS) than ciprofloxacin, trovafloxacin, and gatifloxacin (MIC90 = 16 to >64 microg/ml for MRSA and 4 to >32 microg/ml for CoNS). Bactericidal kinetics experiments, using two MRSA isolates, showed that exposure to the NFQs at four times the MIC reduced the bacterial counts (measured in CFU per milliliter) by > or =3 log units in 2 to 4 h. Overall, the NFQs and clinafloxacin were less susceptible than the other quinolones to existing mechanisms of quinolone resistance in staphylococci.

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.

Induction of fibronectin-binding proteins and increased adhesion of quinolone-resistant Staphylococcus aureus by subinhibitory levels of ciprofloxacin

We recently reported that strain EN1252a, a fluoroquinolone-resistant derivative of Staphylococcus aureus NCTC8325 with mutations in grlA and gyrA, expressed increased levels of fibronectin-binding proteins (FnBPs) and showed a significantly higher attachment to fibronectin-coated polymer surfaces after growth in the presence of subinhibitory concentrations of ciprofloxacin. The present study evaluated the occurrence and frequency of fluoroquinolone-induced FnBP-mediated adhesion in clinical isolates of fluoroquinolone-resistant methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA). Eight of ten MRSA isolates and four of six MSSA isolates with grlA and gyrA mutations exhibited significant increases in attachment to fibronectin-coated surfaces after growth in the presence of one-quarter the MIC of ciprofloxacin. Fluoroquinolone-induced FnBP-mediated adhesion of one clinical MRSA strain and the double mutant strain EN1252a also occurred on coverslips removed from the subcutaneous space of guinea pigs. For strain EN1252a, the regulation of fnb transcription by sub-MICs of ciprofloxacin was studied on reporter plasmids carrying fnb-luxAB fusions. One-quarter of the MIC of ciprofloxacin significantly increased fnbB, but not fnbA, promoter activity of the fluoroquinolone-resistant mutant but not its fluoroquinolone-susceptible parent ISP794. This response was abolished by pretreatment with rifampin, indicating an effect at the level of transcription. Activation of the fnbB promoter was not due to an indirect effect of ciprofloxacin on growth rate and still occurred in an agr mutant of strain EN1252a. These data suggest that sub-MIC levels of ciprofloxacin activate the fnbB promoter of some laboratory and clinical isolates, thus contributing to increased production of FnBP(s) and leading to higher levels of bacterial attachment to fibronectin-coated or subcutaneously implanted coverslips.

Trends and significance of antimicrobial resistance in respiratory pathogens

Although we have witnessed a dramatic increase in the prevalence of antimicrobial resistance in respiratory pathogens over the past few years, resistance has infrequently been associated with clinical failures. However, as the prevalence and degree of resistance increases in the beta-lactams and the fluoroquinolones, newly approved for respiratory infections, failures are likely to occur.

Comparison of the fluoroquinolones based on pharmacokinetic and pharmacodynamic parameters

Assessment of pharmacodynamic activity from standard in vitro minimum inhibitory concentrations (MICs) alone is insufficient to predict in vivo potency. Achievable serum and tissue concentrations as well as pharmacokinetic characteristics must be considered. When pharmacokinetic and pharmacodynamic values are combined, the area under the inhibitory curve (AUIC) and peak concentration:MIC ratio predict clinical cure for fluoroquinolones. Clinical data and animal models indicate that a peak:MIC of 10:1 and above and an AUIC of 125 and above are predictive of a clinical cure for this class of antimicrobials against gram-negative organisms. The values may be used to compare and contrast fluoroquinolones to determine which would be best for treating a specific microorganism. Pharmacodynamic data also can be used to design regimens that minimize the risk of suboptimal drug levels. Ensuring the optimal fluoroquinolone dosage based on pharmacodynamic principles would diminish the emergence of resistant organisms and prevent treatment failures.

Mechanisms of quinolone resistance in clinical strains of Pseudomonas aeruginosa

Principal mechanisms of bacterial resistance to quinolones are modification of target enzymes, DNA gyrase (gyrA) and topoisomerase IV (parC), or reduction of intracellular concentration due to mutations in the regulatory genes for efflux systems, such as mexR and nfxB. We have examined gyrA, parC, mexR, and nfxB genes from 16 quinolone-resistant clinical isolates of Pseudomonas aeruginosa to determine the relation between mutations in DNA replicating enzymes or regulatory genes for efflux systems and to correlate the mutations with minimal inhibitory concentrations (MICs). The quinolone resistance-determining regions (QRDR) of these genes were amplified by PCR and sequenced by capillary electrophoresis. Fourteen of 16 isolates had mutations in gyrA, and 13/14 strains with MIC to norfloxacin > or = 8 mg/L had threonine at position 83 changed to isoleucine. Seven of 8 strains with MIC > or = 32 mg/L had mutations in parC. One of these strains showed a parC mutation at position 74 without any mutation in gyrA. Four strains had mexR and two strains nfxB mutations. The data indicate that gyrA mutation is the most important component of quinolone resistance, and simultaneous presence of parC mutations is associated with high-level resistance. parC mutation alone may contribute to resistance, and gyrA mutation may not be a prerequisite for parC mutation to express resistance. mexR and nfxB mutations were found mostly in strains with high-level resistance.

Recent progress in novel macrolides, quinolones, and 2-pyridones to overcome bacterial resistance

Macrolides, such as clarithromycin and azithromycin, having good activity against pathogens such as Legionella, Chlamydia, Campylobacter spp, Branhamella spp, Pasteurella multocida and streptococci, have gained wide acceptance for the treatment of both upper and lower respiratory tracts, as well as cutaneous infections. Emergence of bacterial resistance, particularly in gram-positive bacteria, has been observed. Macrolide-resistant Streptococcus pneumoniae and S. pyogenes are found in France and many other countries, resulting in failure of therapy for pneumonia, pharyngitis, and skin infection. RU 004, HMR 3647, and TE 802 were reported to be active against these resistant strains. Research at Abbott produced several macrolide derivatives in the anhydrolide, tricyclic and tetracyclic ketolides as well as 6-O-alkyl ketolides series having potent activity against macrolide resistant S. pyogenes and S. pneumoniae. Research on streptogramins to overcome bacterial resistance in gram-positive bacteria has produced interesting compounds. Another class of antibacterial agent called quinolones is useful for the treatment of bacterial infections of respiratory tract, urinary tract, skin and soft tissues, as well as sexually transmitted diseases. Ciprofloxacin, the market leader, however, has low potency against anaerobes. Bacterial resistance ( such as Pseudomonas aeruginosa and methicillin- resistant Staphylococcus aureus ) to ciprofloxacin is increasing rapidly. Many quinolone compounds are being synthesized to address these drawbacks. The new quinolones currently under development are characterized by enhanced activities against streptococci, staphylococci, enterococci, and anaerobes. This presentation reviews the current research in the identification of agents to overcome the macrolide and quinolone resistance.

Dual inhibitory activity of sitafloxacin (DU-6859a) against DNA gyrase and topoisomerase IV of Streptococcus pneumoniae

The in-vitro inhibitory activities of sitafloxacin (DU-6859a) and other quinolones against Streptococcus pneumoniae DNA gyrase and topoisomerase IV were measured. IC50s of levofloxacin, ciprofloxacin, sparfloxacin and tosufloxacin against DNA gyrase were almost three to 12 times higher than those against topoisomerase IV. On the other hand, sitafloxacin showed dual inhibitory activity against both enzymes and its IC50s were the lowest among those of the quinolones tested. These results suggest that sitafloxacin is an effective agent against pneumococcal infections and that the incidence of drug-resistant mutants is low.

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.

Topoisomerase sequences of coagulase-negative staphylococcal isolates resistant to ciprofloxacin or trovafloxacin

Coagulase-negative staphylococcal isolates (n = 188) were screened for susceptibility to oxacillin, ciprofloxacin, and trovafloxacin, a new fluoroquinolone. At an oxacillin concentration of >/=4 microg/ml, 43% were methicillin resistant; of these, 70% were ciprofloxacin resistant (MIC, >/=4 microg/ml). Of the methicillin-resistant, ciprofloxacin-resistant isolates, 46% were susceptible to </=2 microg of trovafloxacin per ml and 32% were susceptible to </=1 microg of trovafloxacin per ml. Sixteen isolates, including twelve that expressed fluoroquinolone resistance, were chosen for detailed analysis. Identification of species by rRNA sequencing revealed a preponderance of Staphylococcus haemolyticus and S. hominis among fluoroquinolone-resistant strains. Segments of genes (gyrA and grlA) encoding DNA gyrase and DNA topoisomerase IV were sequenced. Considerable interspecies variation was noted, mainly involving noncoding nucleotide changes. Intraspecies variation consisted of coding changes associated with fluoroquinolone resistance. As for S. aureus, ciprofloxacin resistance (MIC, >/=8 microg/ml) and increased trovafloxacin MICs (0.25 to 2 microg/ml) could be conferred by the combined presence of single mutations in each gyrA and grlA gene. Trovafloxacin MICs of >/=8 microg/ml also occurred, but these required an additional mutation in grlA.

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.

Mutations in the gyrA, parC, and parE genes associated with fluoroquinolone resistance in clinical isolates of Mycoplasma hominis

Five clinical isolates of Mycoplasma hominis from three different patients were examined for resistance to fluoroquinolones; some of these isolates were probably identical. All five isolates harbored amino acid substitutions in the quinolone resistance-determining regions of both DNA gyrase (GyrA) and topoisomerase IV (ParC or ParE). Furthermore, the novobiocin MIC for three isolates showed a significant increase. This is the first characterization of fluoroquinolone-resistant clinical mycoplasma isolates from humans.

Cloning, expression, and enzymatic characterization of Pseudomonas aeruginosa topoisomerase IV

The topoisomerase IV subunit A gene, parC homolog, has been cloned and sequenced from Pseudomonas aeruginosa PAO1, with cDNA encoding the N-terminal region of Escherichia coli parC used as a probe. The homolog and its upstream gene were presumed to be parC and parE through sequence homology with the parC and parE genes of other organisms. The deduced amino acid sequence of ParC and ParE showed 33 and 32% identity with that of the P. aeruginosa DNA gyrase subunits, GyrA and GyrB, respectively, and 69 and 75% identity with that of E. coli ParC and ParE, respectively. The putative ParC and ParE proteins were overexpressed and separately purified by use of a fusion system with a maltose-binding protein, and their enzymatic properties were examined. The reconstituted enzyme had ATP-dependent decatenation activity, which is the main catalytic activity of bacterial topoisomerase IV, and relaxing activities but had no supercoiling activity. So, the cloned genes were identified as P. aeruginosa topoisomerase IV genes. The inhibitory effects of quinolones on the activities of topoisomerase IV and DNA gyrase were compared. The 50% inhibitory concentrations of quinolones for the decatenation activity of topoisomerase IV were from five to eight times higher than those for the supercoiling activities of P. aeruginosa DNA gyrase. These results confirmed that topoisomerase IV is less sensitive to fluoroquinolones than is DNA gyrase and may be a secondary target of new quinolones in wild-type P. aeruginosa.

Psammaplin A, a natural bromotyrosine derivative from a sponge, possesses the antibacterial activity against methicillin-resistant Staphylococcus aureus and the DNA gyrase-inhibitory activity

Psammaplin A, a natural bromotyrosine derivative from an associated form of two sponges (Poecillastra sp. and Jaspis sp.) was found to possess the antimicrobial effect on the Gram-positive bacteria, especially on methicillin-resistant Staphylococcus aureus (MRSA). The minimal inhibitory concentration of psammaplin A against twenty one MRSAs ranged from 0.781 to 6.25 microg/ml, while that of ciprofloxacin was 0.391-3.125 microg/ml. Psammaplin A could not bind to penicillin binding protein, but inhibited the DNA synthesis and the DNA gyrase activity with the respective 50% (DNA synthesis) and 100% (DNA gyrase) inhibitory concentration 2.83 and 100 microg/ml. These results indicate that psammaplin A has a considerable antibacterial activity, although restricted to a somewhat narrow range of bacteria, probably by inhibiting DNA gyrase.

ParC and GyrA may be interchangeable initial targets of some fluoroquinolones in Streptococcus pneumoniae

To evaluate the role of known topoisomerase IV and gyrase mutations in the fluoroquinolone (FQ) resistance of Streptococcus pneumoniae, we transformed susceptible strain R6 with PCR-generated fragments encompassing the quinolone resistance-determining regions (QRDRs) of parC or gyrA from different recently characterized FQ-resistant mutants. Considering the MICs of FQs and the GyrA and/or ParC mutations of the individual transformants, we found three levels of resistance. The first level was obtained when a single target, ParC or GyrA, depending on the FQ, was modified. An additional mutation(s) in a second target, GyrA or ParC, led to the second level. The highest increases in resistance levels were seen for Bay y3118 and moxifloxacin with the transformant harboring a double mutation in both ParC and GyrA. When a single modified target was considered, only the ParC mutation(s) led to an increase in the MICs of pefloxacin and trovafloxacin. In contrast, the GyrA or ParC mutation(s) could lead to increases in the MICs of ciprofloxacin, sparfloxacin, grepafloxacin, Bay y3118, and moxifloxacin. These results suggest that the preferential target of trovafloxacin and pefloxacin is ParC, whereas either ParC or GyrA may both be initial targets for the remaining FQs tested. The contribution of the ParC and GyrA mutations to efflux-mediated FQ resistance was also examined. Active efflux was responsible for two- to fourfold increases in the MICs of ciprofloxacin for the transformants, regardless of the initial FQ resistance levels of the recipients.

Mechanisms of fluoroquinolone resistance

Mechanisms of bacterial resistance to fluoroquinolones fall into two principal categories, alterations in drug target enzymes and alterations that limit permeation of drug to the target, both resulting from chromosomal mutations. No specific resistance mechanisms of quinolone degradation or modification have been found. The target enzymes, DNA gyrase and topoisomerase IV are most commonly altered in domains near the enzyme active sites and in some cases reduced drug binding affinity has been demonstrated. Drug permeation is altered by mutations that increase expression of endogenous multidrug efflux pumps, alter outer membrane diffusion channels, or both. Recently a new plasmid-mediated resistance of an as yet undefined mechanism was found in clinical isolates of Klebsiella pneumoniae. Copyright 1999 Harcourt Publishers Ltd.

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.