DNA gyrase of Staphylococcus aureus and inhibitory effect of quinolones on its activity

Effect of genetic depletion of MMP-9 on neurological manifestations of hypertension-induced intracerebral hemorrhages in aged mice

Clinical and experimental studies show that hypertension induces intracerebral hemorrhages (ICH), including cerebral microhemorrhages in the aged brain, which contribute to the pathogenesis of vascular cognitive impairment (VCI). Previous studies showed that aging increased oxidative stress-mediated activation of matrix metalloproteinases (MMPs) that importantly contributes to the pathogenesis of ICHs. In particular, oxidative stress has been implicated in activation of MMP-9, which is known to be involved in the degradation of the extracellular matrix and cleavage of collagen IV, a key constituent of the basal membrane of cerebral vessels. To determine the role of MMP-9 activation in the genesis of ICHs, we induced hypertension in 20-month-old MMP-9 null and age-matched control mice by angiotensin II and L-NAME treatment. Contrary to our hypothesis, MMP-9 deficiency did not delay the onset or incidence of neurological consequences of hypertension-induced ICHs. Our results indicate that MMP-9 activation does not play a role in the age-related exacerbation of hypertension-induced ICH.

A New-Class Antibacterial-Almost. Lessons in Drug Discovery and Development: A Critical Analysis of More than 50 Years of Effort toward ATPase Inhibitors of DNA Gyrase and Topoisomerase IV

The introduction into clinical practice of an ATPase inhibitor of bacterial DNA gyrase and topoisomerase IV (topo IV) would represent a new-class agent for the treatment of resistant bacterial infections. Novobiocin, the only historical member of this class, established the clinical proof of concept for this novel mechanism during the late 1950s, but its use declined rapidly and it was eventually withdrawn from the market. Despite significant and prolonged effort across the biopharmaceutical industry to develop other agents of this class, novobiocin remains the only ATPase inhibitor of gyrase and topo IV ever to progress beyond Phase I. In this review, we analyze the historical attempts to discover and develop agents within this class and highlight factors that might have hindered those efforts. Within the last 15 years, however, our technical understanding of the molecular details of the inhibition of the gyrase and topo IV ATPases, the factors governing resistance development to such inhibitors, and our knowledge of the physical properties required for robust clinical drug candidates have all matured to the point wherein the industry may now address this mechanism of action with greater confidence. The antibacterial spectrum within this class has recently been extended to begin to include serious Gram negative pathogens such as Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. In spite of this recent technical progress, adverse economics associated with antibacterial R&D over the last 20 years has diminished industry's ability to commit the resources and perseverance needed to bring new-class agents to launch. Consequently, a number of recent efforts in the ATPase class have been derailed by organizational rather than scientific factors. Nevertheless, within this context we discuss the unique opportunity for the development of ATPase inhibitors of gyrase and topo IV as new-class antibacterial agents with broad spectrum potential.

Assessing sensitivity to antibacterial topoisomerase II inhibitors

Both prokaryotes and eukaryotes have two major classes of topoisomerases that make transient single- or double-strand cuts in DNA. While these enzymes play critical roles in cellular processes, they are also important targets of therapeutic agents. This unit describes assays to use in characterizing topoisomerase II-targeting agents in vitro and in bacterial cells. It provides protocols for characterizing the action of small molecules against bacterial type II topoisomerases in vitro and the in vivo effects of putative topoisomerase II-targeting antibiotics, as well as for measuring trapped enzyme/DNA covalent complexes, the major cytotoxic lesion induced by fluoroquinolones.

Bismuth-norfloxacin complex: Synthesis, physicochemical and antimicrobial evaluation

Norfloxacin is a fluoroquinolone antibacterial agent which is active against various Gram-positive as well as Gram-negative microorganisms. Presence of metal ions considerably alters the activity of fluoroquinolones against potentially susceptible bacteria. As bismuth is known to possess a good antibacterial activity, bismuth complex of norfloxacin was prepared by reacting bismuth citrate with aqueous solution of norfloxacin. The structure of the bismuth-norfloxacin complex (BNC) was confirmed by spectral, chemical and elemental analysis. Antimicrobial studies were carried out using agar diffusion method against Escherichia coli (ATCC 25922), Klebsiella pneumoniae (NTCC 10320), Staphylococcus aureus (ATCC 29213), Bacillus pumilis (NTCC 8241) and Staphylococcus epidermidis (ATCC 12228). The results showed significant increase (p<0.05, Tukeys test) in antibacterial activity of BNC as compared with norfloxacin and physical mixture of norfloxacin and bismuth citrate. This increase in activity is being considered due to increased bioavailability of the metal drug complex. Thus, the use of the BNC may be preferable over norfloxacin alone.

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.

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.

Identification of DNA gyrase inhibitor (GyrI) in Escherichia coli

DNA gyrase is an essential enzyme in DNA replication in Escherichia coli. It mediates the introduction of negative supercoils near oriC, removal of positive supercoils ahead of the growing DNA fork, and separation of the two daughter duplexes. In the course of purifying DNA gyrase from E. coli KL16, we found an 18-kDa protein that inhibited the supercoiling activity of DNA gyrase, and we coined it DNA gyrase inhibitory protein (GyrI). Its NH2-terminal amino acid sequence of 16 residues was determined to be identical to that of a putative gene product (a polypeptide of 157 amino acids) encoded by yeeB (EMBL accession no. U00009) and sbmC (Baquero, M. R., Bouzon, M., Varea, J., and Moreno, F. (1995) Mol. Microbiol. 18, 301-311) of E. coli. Assuming the identity of the gene (gyrI) encoding GyrI with the previously reported genes yeeB and sbmC, we cloned the gene after amplification by polymerase chain reaction and purified the 18-kDa protein from an E. coli strain overexpressing it. The purified 18-kDa protein was confirmed to inhibit the supercoiling activity of DNA gyrase in vitro. In vivo, both overexpression and antisense expression of the gyrI gene induced filamentous growth of cells and suppressed cell proliferation. GyrI protein is the first identified chromosomally nucleoid-encoded regulatory factor of DNA gyrase in E. coli.

Synthesis, characterization and antibacterial activity of novel Fe(III), Co(II), and Zn(II) complexes with norfloxacin

Three novel complexes of norfloxacin (abbreviated as NFL), [M(NFL)2(H2O)2]Cl3.6H2O, (M = Fe, Co), and [Zn(NFL)2]Cl2.7H2O, have been prepared. The compounds were characterized by IR, UV-Vis, NMR spectra, molar conductivity, and elemental analyses. In all of the complexes, the ligand NFL was coordinated through two carboxyl oxygen atoms. Octahedral and tetrahegon geometries have been proposed for Fe(III)-, Co(II)-complexes and Zn(II)-complex, respectively. In vitro test of susceptibility of Fe(III)- and Zn(II)-complexes showed stronger activity than that of norfloxacin against G(-) E.Coli and Bacillus dysenteriae bacteria.

DNA topoisomerases from Streptomyces noursei: influence of coumarins and quinolones on the enzymic activity

DNA topoisomerases have been purified from Streptomyces noursei. DNA gyrase and topoisomerase I show a differential sensitivity against quinolones and coumarins compared to the E. coli enzymes. Streptomyces gyrase is resistant to much higher levels of various drugs than is the E. coli enzyme. The observed differences between the gyrases from streptomycetes and E. coli are discussed in the light of present literature data.

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.

Lack of effect of carbonyl cyanide m-chlorophenylhydrazone on KB-5246 accumulation by Staphylococcus aureus

The accumulation of KB-5246 in a quinolone-susceptible strain of Staphylococcus aureus was about 70 times that of norfloxacin. Carbonyl cyanide m-chlorophenylhydrazone increased the accumulation of norfloxacin about eightfold, but it did not influence that of KB-5246. The low efflux of KB-5246 from S. aureus may contribute to its potent antibacterial activity.

Purification and properties of DNA gyrase from Vibrio cholerae

The DNA gyrase was purified from Vibrio cholerae strain 569B. It appeared to be composed of two subunits of Mr 120,000 and 97,000 and had a great deal of similarity to the Escherichia coli gyrase. Unlike the E. coli enzyme, however, it could neither relax supercoiled DNA nor induce a cleavage of double-stranded DNA, under experimental conditions in which E. coli gyrase could do so.

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.

A novel locus conferring fluoroquinolone resistance in Staphylococcus aureus

Fluoroquinolones such as ciprofloxacin and ofloxacin are potent antimicrobial agents that antagonize the A subunit of DNA gyrase. We selected and mapped a novel fluoroquinolone resistance gene on the Staphylococcus aureus chromosome. Resistant mutants were selected with ciprofloxacin or ofloxacin and were uniformly localized to the A fragment of chromosomal DNA digested with SmaI and arrayed by pulsed-field gel electrophoresis. Several mutants (cfxB, ofxC) were genetically mapped between the thr and trp loci in the A fragment. A majority of A fragment fluoroquinolone resistance mutations were associated with reduced susceptibility to novobiocin, an antagonist of the B subunit of DNA gyrase. Two genes previously associated with fluoroquinolone resistance, the gyrA gene of DNA gyrase and the norA gene (associated with decreased drug accumulation), were localized to the G and D fragments, respectively. Thus, the fluoroquinolone resistance mutations in the A fragment are distinct from previously identified fluoroquinolone resistance mutations in gyrA and norA. Whether mutations in the A fragment after a second topoisomerase or another gene controlling supercoiling or affect drug permeation is unknown.

Inhibition by quinolones of DNA gyrase from Staphylococcus aureus

In order to clarify the mechanism of action of quinolones against Staphylococcus aureus, the subunit A and B proteins of DNA gyrase were separately purified from a crude extract of S. aureus FDA 209-P. The reconstituted enzyme exhibited ATP-dependent DNA supercoiling activity. The inhibitory effects of quinolones on the supercoiling activity of the purified enzyme were measured by the quantitative electrophoresis method (17), using plasmid DNA, pBR322 or pUB110, as substrates and expressed as the 50% inhibitory concentrations (IC50s). The IC50s of ofloxacin, DR-3355 (l-ofloxacin), ciprofloxacin, tosufloxacin, sparfloxacin, and DS-4524, a new quinolone derivative, for pBR322 were 63.0, 37.8, 30.5, 46.0, 28.5, and 3.2 micrograms/ml, respectively. These values were closely correlated with antibacterial activity (MIC), with correlation coefficients of 0.953 for pBR322 and 0.938 for pUB110. These results indicate that, in S. aureus, as in gram-negative bacteria, DNA gyrase is likely to be a major target enzyme of quinolones.

Mechanisms of clinical resistance to fluoroquinolones in Enterococcus faecalis

About 10% of 100 clinical isolates of Enterococcus faecalis were resistant to greater than or equal to 25 micrograms of norfloxacin, ofloxacin, ciprofloxacin, and temafloxacin per ml. In this study, the DNA gyrase of E. faecalis was purified from a fluoroquinolone-susceptible strain (ATCC 19433) and two resistant isolates, MS16968 and MS16996. Strains MS16968 and MS16996 were 64- to 128-fold and 16- to 32-fold less susceptible, respectively, to fluoroquinolones than was ATCC 19433; MICs of nonquinolone antibacterial agents for these strains were almost equal. The DNA gyrase from ATCC 19433 had two subunits, designated A and B, with properties similar to those of DNA gyrase from other gram-positive bacteria such as Bacillus subtilis and Micrococcus luteus. Inhibition of the supercoiling activity of the enzyme from ATCC 19433 by the fluoroquinolones correlated with their antibacterial activities. In contrast, preparations of DNA gyrase from MS16968 and MS16996 were at least 30-fold less sensitive to inhibition of supercoiling by the fluoroquinolones than the gyrase from ATCC 19433 was. Experiments that combined heterologous gyrase subunits showed that the A subunit from either of the resistant isolates conferred resistance to fluoroquinolones. These findings indicate that an alteration in the gyrase A subunit is the major contributor to fluoroquinolone resistance in E. faecalis clinical isolates. A difference in drug uptake may also contribute to the level of fluoroquinolone resistance in these isolates.

DNA gyrase gyrA mutations in ciprofloxacin-resistant strains of Staphylococcus aureus: close similarity with quinolone resistance mutations in Escherichia coli

The gyrA genes isolated from three ciprofloxacin-resistant clinical isolates of Staphylococcus aureus carried codon 84 (serine----leucine) and/or codon 85 (serine----proline) mutations that were absent in pretreatment susceptible strains. These substitutions occur in a region of the gyrase A protein wherein directly analogous mutations of serine 83----leucine and alanine 84----proline in Escherichia coli confer quinolone resistance. Thus, DNA gyrase A subunit mutations are implicated in resistance to ciprofloxacin in S. aureus.

Mycoplasma pneumoniae DNA gyrase genes

We have identified a clone from a lambda EMBL3 library containing a 19kb insert of Mycoplasma pneumoniae DNA which includes the genes that encode both subunits of DNA gyrase. The gyrB gene and the 5' end of the gyrA gene have been subcloned into M13. The gyrB gene is 1953bp in length and overlaps the gyrA gene by a single base. The nucleotide sequence of these subclones has significant homology to previously reported gyrase genes. In terms of the size of the gyrB gene and its proximity to the gyrA gene, M. pneumoniae is more similar to Bacillus subtilis than to Escherichia coli.

DNA cloning and organization of the Staphylococcus aureus gyrA and gyrB genes: close homology among gyrase proteins and implications for 4-quinolone action and resistance

Staphylococcus aureus gyrA and gyrB genes, which encode the DNA gyrase A and B proteins, have been isolated and found to map contiguously. DNA sequence analysis revealed close homology between the S. aureus gyrase subunits and their counterparts in Bacillus subtilis and Escherichia coli, including several conserved amino acid residues whose substitution in E. coli confers resistance to 4-quinolones. These results are discussed in regard to quinolone resistance mechanisms in S. aureus.

Analysis of acquired ciprofloxacin resistance in a clinical strain of Pseudomonas aeruginosa

Decreasing susceptibility to ciprofloxacin was investigated in sequential clinical isolates of Pseudomonas aeruginosa from a patient on ciprofloxacin therapy. All isolates were verified as the same strain by DNA probe. MICs of all quinolones tested were 16- to 32-fold higher for the posttherapy isolates; nonquinolone MICs were unchanged. The isolates were compared by analyses of outer membrane proteins and lipopolysaccharide composition, antimicrobial susceptibilities, measurement of accumulation of ciprofloxacin, and inhibition of DNA gyrase activity by ciprofloxacin and nalidixic acid. No significant changes in outer membrane proteins or ciprofloxacin accumulation were observed; however, both posttherapy isolates lost the long chain O-polysaccharide component of lipopolysaccharide. Preparations of DNA gyrase from the quinolone-resistant posttherapy isolates were 16- to 32-fold less sensitive to inhibition of supercoiling by ciprofloxacin and nalidixic acid than was gyrase from the pretherapy isolate. Inhibition studies on combinations of heterologous gyrase subunits showed that decreased inhibition was conferred by the resistant gyrase A subunits. Thus, acquired resistance to ciprofloxacin in this strain involved an alteration in the A subunit of DNA gyrase and was associated with changes in lipopolysaccharide.

Microbial DNA topoisomerases and their inhibition by antibiotics

Supercoiling of bacterial DNA is regulated by topoisomerases and influences most of the metabolic processes involving DNA. The present review is devoted to a brief outline of the supercoiled state of DNA in bacteria and to all microbial topoisomerases hitherto described. Recent studies on topoisomerases of archaebacteria led to the discovery of a so-called reverse gyrase, the properties of which are also discussed. Special emphasis is given to a selective treatment of the effects of those antibiotics which act as gyrase inhibitors.

The fluoroquinolone antibacterial agents

In summary, the last decade has been a highly fertile and productive period in quinolone medicinal chemistry, resulting in major improvements in potency, antibacterial spectra, oral absorption and pharmacokinetic properties as well as an increased knowledge of the molecular features important to conferring these various properties. Very recent discoveries concerning replacements for the 3-carboxylic acid moiety, previously thought to be uniquely essential for activity, to give highly potent antibacterials such as (83) illustrate the potential for new breakthroughs in this field. Among the major goals for future research remains the understanding of the potential cartilage toxicity associated with this class of agents, such that an agent useful for pediatric indications may be developed. Future studies can also be expected to further enhance and refine the level of current insight into the manner by which these agents inhibit the target enzyme on a molecular level.

Contribution of permeability and sensitivity to inhibition of DNA synthesis in determining susceptibilities of Escherichia coli, Pseudomonas aeruginosa, and Alcaligenes faecalis to ciprofloxacin

To examine the correlation between bacterial cell susceptibility to ciprofloxacin and the magnitude of uptake and cell target sensitivity, the relative contribution of ciprofloxacin accumulation in intact cells and its ability to inhibit DNA synthesis were investigated among strains of Escherichia coli, Pseudomonas aeruginosa, and Alcaligenes faecalis. Uptake studies of [14C]ciprofloxacin demonstrated diffusion kinetics for P. aeruginosa and E. coli. Ciprofloxacin was more readily removed from E. coli J53 and A. faecalis ATCC 19018 by washing than from P. aeruginosa PAO503. These results indicate that the process of cell accumulation is different for P. aeruginosa in that the drug is firmly bound at an extracellular site. Whatever the washing conditions, A. faecalis accumulated less drug than either of the other two bacteria. Magnesium chloride (10 mM) caused a substantial decrease of ciprofloxacin accumulated and an increase in the MIC, depending upon the nature of the medium. The addition of carbonyl cyanide m-chlorophenylhydrazone caused a variable increase in drug accumulated, depending on the medium and the bacterial strain. The concentration of ciprofloxacin required to obtain 50% inhibition (ID50) of DNA synthesis for P. aeruginosa PAO503 and A. faecalis ATCC 19018 did not correlate with their corresponding MICs but did for E. coli J53. Treatment with EDTA decreased the ID50 of ciprofloxacin for P. aeruginosa PAO503 and its gyrA derivative by 5- and 2-fold, respectively, and decreased the ID50 for E. coli JB5R, a strain with a known decrease in OmpF, by 1.4-fold but did not decrease the ID50 for the normally susceptible E. coli J53. The ID(50) for P. aeruginosa obtained after EDTA treatment or in ether-permeabilized cells was higher than that obtained for the other two strains. The protonophore carbonyl cyanide m-chlorophenylhydrazone prevented killing by low ciprofloxacin concentrtaions, but sodium azide did not. The latter compound did not enhance killing in association with inhibition of a previously described energy-dependent efflux of ciprofloxacin susceptibility being the susceptibility to inhibition of DNA synthesis in E. coli, poor premeability associated with the small pore size of A. faecalis, and a combination of low permeability and reduced susceptibility of DNA synthesis to inhibition for P. aeruginosa.