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

Evaluating LC-MS/MS To Measure Accumulation of Compounds within Bacteria

A general method for determining bacterial uptake of compounds independent of antibacterial activity would be a valuable tool in antibacterial drug discovery. LC-MS/MS assays have been described, but it has not been shown whether the data can be used directly to inform medicinal chemistry. We describe the evaluation of an LC-MS/MS assay measuring association of compounds with bacteria, using a set of over a hundred compounds (inhibitors of NAD-dependent DNA ligase, LigA) for which in vitro potency and antibacterial activity had been determined. All compounds were active against an efflux-deficient strain of Escherichia coli with reduced LigA activity ( E. coli ligA251 Δ tolC). Testing a single compound concentration and incubation time, we found that, for equipotent compounds, LC-MS/MS values were not predictive of antibacterial activity. This indicates that measured bacteria-associated compound was not necessarily exposed to the target enzyme. Our data suggest that, while exclusion from bacteria is a major reason for poor antibacterial activity of potent compounds, the distribution of compound within the bacterial cell may also be a problem. The relative importance of these factors is likely to vary from one chemical series to another. Our observations provide directions for further study of this difficult issue.

Identification of inhibitors for single-stranded DNA-binding proteins in eubacteria

OBJECTIVES

The increasing threat of drug-resistant bacteria establishes a continuing need for the development of new strategies to fight infection. We examine the inhibition of the essential single-stranded DNA-binding proteins (SSBs) SSBA and SSBB as a potential antimicrobial therapy due to their importance in DNA replication, activating the SOS response and promoting competence-based mechanisms of resistance by incorporating new DNA.

METHODS

Purified recombinant SSBs from Gram-positive (Staphylococcus aureus and Bacillus anthracis) and Gram-negative (Escherichia coli and Francisella tularensis) bacteria were assessed in a high-throughput screen for inhibition of duplex DNA unwinding by small molecule inhibitors. Secondary electrophoretic mobility shift assays further validated the top hits that were then tested for MICs using in vitro assays.

RESULTS

We have identified compounds that show cross-reactivity in vitro, as well as inhibition of both F. tularensis and B. anthracis SSBA. Five compounds were moderately toxic to at least two of the four bacterial strains in vivo, including two compounds that were selectively non-toxic to human cells, 9-hydroxyphenylfluoron and purpurogallin. Three of the SSBA inhibitors also inhibited S. aureus SSBB in Gram-positive bacteria.

CONCLUSIONS

Results from our study support the potential for SSB inhibitors as broad-spectrum antibacterial agents, with dual targeting capabilities against Gram-positive bacteria.

The impact of ofloxacin on rat testicular DNA: application of image analysis

Ofloxacin induces its antibacterial action mainly by inhibition of DNA gyrase which is equivalent to topoisomerase II in mammalian cells. The present study was focused on the impact of ofloxacin on rat testicular DNA ploidy in a dose-response relationship using an image analysis technique on testicular sections following Fuelgen DNA staining. Sperm count and motility as well as sperm head abnormality tests were also investigated. Ofloxacin was given p.o. in doses of 36, 72 and 360 mg kg(-1) day(-1) for 15 consecutive days. The animals were then left to live safely to day 60 from starting the experiment to give them a chance to complete the cycle of spermatogenesis. Results revealed that ofloxacin significantly decreased the percentage of haploid cells in a dose-dependent manner with concomitant increase in the percentage of diploid cells. Sperm count and motility was also markedly decreased in a dose-dependent fashion. Sperm head abnormality showed no marked change following ofloxacin treatment. In conclusion, ofloxacin induced a marked disturbance in rat testicular DNA ploidy, which may be explained on the basis of cross-reactivity to topoisomerase II.

Comparison of recalcitrance to ciprofloxacin and levofloxacin exhibited by Pseudomonas aeruginosa bofilms displaying rapid-transport characteristics

Attenuated total reflection Fourier transform infrared spectroscopy was used to measure transport of the fluoroquinolones (FQs) ciprofloxacin and levofloxacin into Pseudomonas aeruginosa biofilms. Biofilms were exposed to each FQ at dose levels of 100, 250, and 500 microg/ml for 30 min. A mathematical transport model was used to extract the diffusion coefficient, binding site density, and adsorption and desorption rates for each experiment. Recalcitrance of the biofilms toward each FQ was evaluated by comparison of efficacies with planktonic bacteria. By this criterion, biofilms were found to exhibit more recalcitrance toward levofloxacin than ciprofloxacin under the experimental conditions. These results cannot be explained by the more hindered transport of levofloxacin, implicating the domination of physiological factors.

Complexes of ciprofloxacin with metal ions contained in antacid drugs

Simultaneous administration of antacids containing magnesium or aluminium and ciprofloxacin or other quinolones decreases the gastrointestinal absorption of those antibacterial agents. Current speculation about the mechanism of this interaction has focused on drug-cation chelation. The present study was designed to detect the protonation in solutions and the formation of the complex species at the pH levels typical of the gastrointestinal tract. It involves the study of ciprofloxacin in aqueous solutions containing Al3+ and (or) Mg2+ by combining the results of potentiometric and spectroscopic (1H nuclear magnetic resonance) techniques. Calculations were only performed for data in the range 4.5 < pH < 5.5 (pH levels typical of gastrointestinal tract) and the results of both methods are made self-consistent, assuming an equilibrium model including complex species MHL, MLOH (where H2L denotes ciprofloxacin and M is Al3+ or Mg2+); their formation constants are given.

In vitro anti-Mycobacterium avium activities of quinolones: predicted active structures and mechanistic considerations

The relationship between the structures of quinolones and their anti-Mycobacterium avium activities has been previously derived by using the Multiple Computer-Automated Structure Evaluation program. A number of substructural constraints required to overcome the resistance of most of the strains have been identified. Nineteen new quinolones which qualify under these substructural requirements were identified by the program and subsequently tested. The results show that the substructural attributes identified by the program produced a successful a priori prediction of the anti-M. avium activities of the new quinolones. All 19 quinolones were found to be active, and 4 of them are as active or better than ciprofloxacin. With these new quinolones, the updated multiple computer-automated structure evaluation program structure-activity relationship analysis has helped to uncover additional information about the nature of the substituents at the C5 and C7 positions needed for optimal inhibitory activity. A possible explanation of drug resistance based on the observation of suicide inactivation of bacterial cytochrome P-450 by the cyclopropylamine moiety has also been proposed and is discussed in this report. Furthermore, we confirm the view that the amount of the uncharged form present in a neutral pH solution plays a crucial role in the drug's penetration ability.

Genetic definition of the substrate selectivity of outer membrane porin protein OprD of Pseudomonas aeruginosa

Earlier studies proved that Pseudomonas aeruginosa OprD is a specific porin for basic amino acids and imipenem. It was also considered to function as a nonspecific porin that allowed the size-dependent uptake of monosaccharides and facilitation of the uptake of quinolone and other antibiotics. In the present study, we utilized P. aeruginosa strains with genetically defined levels of OprD to characterize the in vivo substrate selectivity of this porin. An oprD::omega interposon mutant was constructed by gene replacement utilizing an in vitro mutagenized cloned oprD gene. In addition, OprD was overexpressed from the lac promoter by cloning the oprD gene into the broad-host-range plasmid pUCP19. To test the substrate selectivity, strains were grown in minimal medium with limiting concentrations of the carbon sources glucose, gluconate, or pyruvate. In minimal medium with 0.5 mM gluconate, the growth rates of the parent strain H103 and its oprD::omega mutant H729 were only 60 and 20%, respectively, of that of the OprD-overexpressing strain H103(pXH2). In contrast, no significant differences were observed in the growth rates of these three strains on glucose or pyruvate, indicating that OprD selectively facilitated the transport of gluconate. To determine the role of OprD in antibiotic uptake, nine strains representing different levels of OprD and OprF were used to determine the MICs of different antibiotics. The results clearly demonstrated that OprD could be utilized by imipenem and meropenem but that, even when substantially overexpressed, it could not be significantly utilized by other beta-lactams, quinolones, or aminoglycosides. In addition, competition experiments confirmed that imipenem had common binding sites with basic amino acids in the OprD channel, but not with gluconate or glucose.

Fluoroquinolone supersusceptibility mediated by outer membrane protein OprH overexpression in Pseudomonas aeruginosa: evidence for involvement of a nonporin pathway

Overexpression of Pseudomonas aeruginosa outer membrane protein OprH led to an 8- to 32-fold increase in susceptibility to chloramphenicol and the quinolones nalidixic acid, norfloxacin, ciprofloxacin, and fleroxacin in comparison with the susceptibility of the wild-type strain H103 grown on Mg(2+)-sufficient medium. This was true regardless of whether OprH overexpression was induced by growth of strain H103 on Mg(2+)-deficient medium, the addition of 5 mM m-toluate to cells containing the cloned oprH gene behind the inducible tol promoter in plasmid pGB25, or mutation in the polymyxin-resistant derivative strain H181. In contrast, OprH overexpression failed to reverse the quinolone resistance phenotype of a nalB mutant. OprH was purified to homogeneity by selective detergent solubilization and fast protein liquid chromatography. The addition of OprH to the solution bathing a black lipid bilayer membrane failed to give rise to an increase in membrane conductance. This suggests that OprH is not a porin but, instead, may cause increased uptake of quinolones and chloramphenicol via a non-porin pathway.

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.

In vitro antibacterial activities of tosufloxacin against and uptake of tosufloxacin by outer membrane mutants of Escherichia coli, Proteus mirabilis, and Salmonella typhimurium

The antibacterial activities of tosufloxacin and other quinolones against and apparent uptakes of tosufloxacin and other quinolones by outer membrane mutants of Escherichia coli, Proteus mirabilis, and Salmonella typhimurium were studied. The hydrophobicity of tosufloxacin was nearly equal to that of ofloxacin or lower than those of sparfloxacin and nalidixic acid. OmpF- and OmpC-deficient E. coli and 40-kDa porin-deficient P. mirabilis mutants were twofold more susceptible to tosufloxacin and sparfloxacin but two- to fourfold less susceptible to other quinolones than their parent strains. In S. typhimurium lipopolysaccharide-deficient (rough) mutants, the differences in susceptibility to tosufloxacin were similar to those to sparfloxacin and nalidixic acid. The apparent uptake of tosufloxacin by intact cells was increased in porin-deficient mutants compared with that by their parent strain. These results suggest that the permeation route of tosufloxacin across the outer membrane is different from that of other fluoroquinolones and that tosufloxacin may permeate mainly through the nonporin pathway, presumably phospholipid bilayers. However, this characteristic is independent of the hydrophobicity of the molecule.

Quinolone accumulation in Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus

The accumulation of quinolones by Escherichia coli JF568, Pseudomonas aeruginosa PAO1, and Staphylococcus aureus ATCC 29213 was measured by a modified fluorometric assay (J. S. Chapman and N. H. Georgopapadakou, Antimicrob. Agents Chemother. 33:27-29, 1989). The quinolones examined were fleroxacin, pefloxacin, norfloxacin, difloxacin, A56620, ciprofloxacin, ofloxacin, and Ro 09-1168. In all three organisms, uptake was complete in less than 5 min and was proportional to extracellular quinolone concentrations between 2 and 50 micrograms/ml, which is consistent with simple diffusion. Washing cells with quinolone-free buffer decreased accumulation by up to 70% in E. coli and P. aeruginosa but not in S. aureus. Similarly, incubation with the uncouplers 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone increased accumulation up to fourfold in E. coli and P. aeruginosa, though not in S. aureus, suggesting endogenous, energy-dependent efflux. High quinolone hydrophobicity was generally associated with decreased accumulation in E. coli and P. aeruginosa (except in the case of pefloxacin) but was associated with increased accumulation in S. aureus (except in the case of difloxacin). Ciprofloxacin had the highest accumulation in E. coli and P. aeruginosa, while pefloxacin had the highest accumulation in S. aureus.

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.

Escherichia coli resistant to cephalosporins and quinolones is still susceptible to the cephalosporin-quinolone ester Ro 23-9424

Ro 23-9424 is a broad-spectrum antibacterial agent consisting of a cephalosporin (desacetylcefotaxime) linked through an ester bond to a fluoroquinolone (fleroxacin). Its activity against mutants of Escherichia coli TE18 resistant to both antibacterial components was examined. E. coli TE18 overproduces the AmpC beta-lactamase and is resistant to several cephalosporins, including desacetylcefotaxime (MIC, 50 micrograms/ml), although it is still susceptible to Ro 23-9424 (MIC, 0.2 microgram/ml). Thirty-five spontaneous, two-step mutants of E. coli TE18 which were resistant to fleroxacin (MIC, 50 micrograms/ml) were isolated. In the mutants, replicative DNA biosynthesis (permeabilized cells) was resistant to fleroxacin, and some mutants had porin abnormalities. However, all remained susceptible to Ro 23-9424 (MIC, 0.5 microgram/ml). Examination of beta-lactamase activity in the parent strain revealed that it hydrolyzes desacetylcefotaxime more rapidly than it does Ro 23-9424. Thus, Ro 23-9424 may be less susceptible to the gram-negative, chromosomal beta-lactamases that hydrolyze several broad-spectrum cephalosporins and may be effective in cases in which neither of its two components is active.

Mode of action of the new quinolones: new data

New details of the molecular interactions of quinolones with their target DNA gyrase and DNA have come from the nucleotide sequences of the gyrA genes from resistant mutants of Escherichia coli and wild-type strains of other bacteria and studies of gyrase A tryptic fragments, all suggesting the importance of an amino-terminal domain in quinolone action. Alterations in DNA supertwisting were also associated with altered quinolone susceptibility, possibly by indirect effects on DNA gyrase expression. Specific binding of relevant concentrations of norfloxacin to a complex of DNA gyrase and DNA in the presence of ATP, the cooperativity of DNA binding, and the crystalline structure of nalidixic acid have led to a model in which quinolones bind cooperatively to a pocket of single-strand DNA created by DNA gyrase. Quinolones vary in their relative activity against DNA gyrase and its eukaryotic homolog topoisomerase II, and in some assays increased action against the eukaryotic enzyme was associated with genotoxicity. Inhibition of bacterial DNA synthesis by quinolones may correlate with MICs in some species, but comparisons of drug accumulation and inhibition of DNA synthesis in permeabilized cells among species have been difficult to interpret. The specific factors necessary for bacterial killing by quinolones in addition to interaction with DNA gyrase have remained elusive, but include oxygen and new protein synthesis. The coordinate expression of the SOS proteins appears not to be necessary for quinolone lethality. Two independent mutants with selective reduced killing by quinolones and beta-lactams indicate overlap in the pathways of bactericidal activity of these classes of agents with distinct targets.

Impermeability to quinolones in gram-positive and gram-negative bacteria

The initial step in the accumulation of fluoroquinolone antimicrobial agents is binding to cell surface components reduced by lowered pH and divalent cations. Uptake into gram-negative and gram-positive bacteria is by simple diffusion. Entry through the outer membrane occurs preferentially for most agents by the porin route but a second process using the self-promoted uptake pathway is active especially for more hydrophobic agents. Fluoroquinolones bind to vesicles of phospholipid which may be the initiating step in cross-cytoplasmic membrane diffusion. An active efflux system has been described in Escherichia coli with evidence supporting its presence in several other bacteria. Total upset is not altered by a resistant gyrase. Resistant isolates associated with reduced total quinolone accumulation due to lowered uptake have been described for laboratory mutants and clinical isolates. Most but not all of these have had alterations in outer membrane proteins. A functionally dominant resistance gene has been cloned from resistant Staphylococcus aureus and codes for a highly hydrophobic protein most likely membrane associated. This gene is expressed in Escherichia coli and specifies resistance especially to hydrophilic quinolones, possibly by altered accumulation.

Resistance to pefloxacin in Pseudomonas aeruginosa

Mechanisms of resistance to pefloxacin were investigated in four isogenic Pseudomonas aeruginosa strains: S (parent isolate; MIC, 2 micrograms/ml), PT1 and PT2 (posttherapy isolates obtained in animals; MICs, 32 and 128 micrograms/ml, respectively), and PT2-r (posttherapy isolate obtained after six in vitro subpassages of PT2; MIC, 32 micrograms/ml). [2-3H]adenine incorporation (indirect evidence of DNA gyrase activity) in EDTA-permeabilized cells was less affected by pefloxacin in PT2 and PT2-r (50% inhibitory concentration, 0.27 and 0.26 microgram/ml, respectively) than it was in S and PT1 (50% inhibitory concentration, 0.04 and 0.05 microgram/ml, respectively). Reduced [14C]pefloxacin labeling of intact cells in strains PT1 and PT2 correlated with more susceptibility to EDTA and the presence of more calcium (P less than 0.05) and phosphorus in the outer membrane fractions. Outer membrane protein analysis showed reduced expression of protein D2 (47 kDa) in strains PT1 and PT2. Other proteins were apparently similar in all strains. The addition of calcium chloride (2 mM) to the sodium dodecyl sulfate-solubilized samples of outer membrane proteins, before heating and Western blotting, probed with monoclonal antibody anti-OmpF showed electrophoretic mobility changes of OmpF in strains PT1 and PT2 which were not seen in strain S. Calcium-induced changes were reversed with ethyleneglycoltetraacetate. Decreased [14C]pefloxacin labeling was further correlated with an altered lipopolysaccharide pattern and increased 3-deoxy-D-mannooctulosonic acid concentration (P less than 0.01). These findings suggested that resistance to pefloxacin is associated with altered DNA gyrase in strain PT2-r, with altered permeability in PT1, and with both mechanisms in PT2. The decreased expression of protein D2 and the higher calcium and lipopolysaccharide contents of the outer membrane could be responsible for the permeability deficiency in P. aeruginosa.

Clinical pharmacokinetics of ciprofloxacin

Compared with nalidixic acid, ciprofloxacin is representative of a newer, more potent class of quinolones, termed the fluoroquinolones. It is available in both oral and parenteral dosage forms. The primary target of quinolone activity appears to be the bacterial DNA gyrase enzyme, which is a member of the class of type II topoisomerases. Bacterial do not acquire resistance to fluoroquinolones through mechanisms that are plasmid or R-factor mediated and, additionally, the quinolones do not appear to be vulnerable to degradation by bacterial inactivating mechanisms. Rather, bacterial resistance to ciprofloxacin occurs either through chromosomal mutation in the target enzyme DNA gyrase or through mutations that alter drug permeability into the bacterial cell. Ciprofloxacin and the fluoroquinolones in general are no more likely to select resistant mutant than are aminoglycosides or beta-lactam antibiotics. Ciprofloxacin displays in vitro activity against most Gram-negative and many Gram-positive pathogenic bacteria, many of which are resistant to a wide range of antibiotics. This finding is of considerable potential clinical significance. High pressure liquid chromatography (HPLC) and microbiological agar diffusion assays have been routinely used to quantify ciprofloxacin concentrations in biological fluids. Both methods are reproducible and accurate for serum but HPLC is recommended for other specimens because of the presence of microbiologically active metabolites. Absorption after oral administration is rapid and can be satisfactorily described as a zero-order process; peak serum ciprofloxacin concentrations (Cmax) are reached in approximately 1 to 2 hours. Concomitant administration of food does not cause clinically significant impairment of absorption and may be helpful in minimising gastric distress caused by the drug. A linear relationship between serum ciprofloxacin concentrations and the dose administered either orally or intravenously has been reported. The absolute bioavailability of ciprofloxacin is approximately 70%. The volume of distribution is large with a steady-state range after oral or intravenous dosing of 1.74 to 5.0 L/kg reflecting penetration of the drug into most tissues. Nonrenal clearance accounts for approximately 33% of the elimination of ciprofloxacin; to date, 4 metabolites have been identified. A first-pass effect has been reported but is thought to be clinically unimportant. Faecal recovery of ciprofloxacin accounts for approximately 15% of an intravenous dose. Nonrenal elimination includes metabolic degradation, biliary excretion and transluminal secretion across the enteric mucosa. Glomerular filtration and tubular secretion account for approximately 66% of the total serum clearance. The terminal disposition half-life (t1/2) is about 3 to 4 hours.(ABSTRACT TRUNCATED AT 400 WORDS)