A novel, selective synthetic acetamide containing culture medium for isolating Pseudomonas aeruginosa from milk

A selective synthetic medium has been developed both in liquid (Z-broth) and solid (Z-agar) forms for selective isolation of Pseudomonas aeruginosa from foods. The simple, easy to prepare peptone-free synthetic medium contained acetamide that is metabolized to ammonia and acetic acid providing nitrogen and carbon supply. The medium contained no inhibitors. Selectivity of the liquid medium was tested by inoculation of pure cultures of different bacteria belonging to the groups Bacillus, Pseudomonas, Enterobacteriaceae and Staphylococcus. It was found that the selectivity of the medium was complete for the examined range of bacteria. However, a similar result was obtained when nitrofurantoin broth was used. Applicability of the synthetic agar medium was also tested by a nation-wide inter-laboratory test using two milk samples containing 10(3)/ml (sample I) and 10(5)/ml (sample II) Pseudomonas aeruginosa. According to this test, no microbiologically relevant differences were found between the results obtained by Z-agar and cetrimide-agar a frequently used selective agar in case of sample II. However, a relevant and statistically significant difference was found in the results of sample I in favour of the Z-agar, that could indicate the presence of a low number of bacteria. Concerning repeatability and reproducibility, Z-agar proved to be superior to cetrimide agar.

Asymptomatic bacteriuria in pregnancy: epidemiological, clinical and microbiological approach

Asymptomatic urinary tract infection is a risk factor for fetal and maternal morbidity including development of pyelonephritis, premature labor and impaired intra-uterine development. In this study, 326 pregnant and 100 non-pregnant control women were screened for significant asymptomatic bacteriuria from April 8 to July 25, 1997 to gain insight into the prevalence rate, clinical characteristics of the disease and microbiological assessments of the causative agents. All the subjects were clinically identified to have no signs and symptoms of urinary tract infection (UTI). The age ranges of the study and control groups were between 15-40 years for both groups with mean of 25.1 and 25.3 years, respectively. Bacteriological screening of mid-stream urine (MSU) revealed that 24/326 (7%) and 3/100 (3%) were positive for asymptomatic bacteriuria in the study group and controls, respectively (P < 0.05). Further biochemical species identification showed that Escherichia coli was found in 11/24 (46%) followed by coagulase-negative staphylococci (CNS) in 8/24 (33%) and Citrobacter freundii in 2/24 (8%). Others found in smaller number included Staphylococcus aureus, Enterobacter cloacae and Proteus rettgeri in 1/24 (4%) each. Antimicrobial susceptibility test revealed that 10/11 (91%) of the E. coli isolates were resistant to ampicillin and amoxicillin and 10/11 (91%) of them sensitive to nitrofurantoin.

The labile iron pool in hepatocytes: prooxidant-induced increase in free iron precedes oxidative cell injury

The labile iron pool (LIP) represents the nonferritin-bound, redox-active iron that has been implicated in oxidative stress and cell injury. Here we examined whether alterations in LIP can be detected in cultured murine hepatocytes and whether increases in LIP are related to the oxidative damage inflicted by the redox cycling drug nitrofurantoin (NFT). Early changes in LIP were monitored with the metal-sensitive fluorescent probe calcein (CA), the fluorescence of which is quenched on binding to iron. Short-term exposure (<1 h) to NFT reduced the CA fluorescence signal by 30%, indicating that the amount of LIP-associated iron had increased. Prolonged exposure (2 h) to NFT caused oxidative cell injury. The addition of the cell-permeable ferrous iron chelator 2,2'-bipyridyl not only prevented the quenching of CA fluorescence but also partially protected from NFT toxicity. It is concluded that reductive stress-induced increase in LIP is an essential event that precedes oxidative cell damage in intact hepatocytes.

Isolation of nitrofurantoin-resistant mutants of nitroreductase-producing Clostridium sp. strains from the human intestinal tract

Five spontaneous nitrofurantoin-resistant mutants (one each of Clostridium leptum, Clostridium paraputrificum, two other Clostridium spp. strains from the human intestinal microflora, and Clostridium perfringens ATCC 3626) were selected by growth on a nitrofurantoin-containing medium. All of the Clostridium wild-type and mutant strains produced nitroreductase, as was shown by the conversion of 4-nitrobenzoic acid to 4-aminobenzoic acid. High-performance liquid chromatography (HPLC) analysis of the mutants during incubation with 50 microg of nitrofurantoin per ml showed the gradual disappearance of the nitrofurantoin peak. The nitrofurantoin peak also disappeared when cell-free supernatants instead of cultures of each of the resistant and wild-type bacteria were used, but it persisted if the cell-free supernatants had been inactivated by heat. At least two of the mutants converted nitrofurantoin to metabolites without antibacterial activity, as was shown by a bioassay with a nitrofurantoin-susceptible Bacillus sp. strain. Nitrofurantoin at a high concentration (50 microg/ml) continued to exert some toxicity, even on the resistant strains, as was evident from the longer lag phases. This study indicates that Clostridium strains can develop resistance to nitrofurantoin while retaining the ability to produce nitroreductase; the mutants metabolized nitrofurantoin to compounds without antibacterial activity.

An active efflux system for heavy metals in cisplatin-resistant human KB carcinoma cells

The mechanism for cisplatin resistance in cisplatin-resistant KCP-4 cells was studied. Although multidrug resistance-associated protein (MRP) was not detected in KCP-4 cells, the cells were more resistant to heavy metals than multidrug-resistant C-A120 cells that overexpressed MRP. KCP-4 cells expressed metallothionein, but it was scarcely involved in cisplatin resistance in these cells. KCP-4 cells did not express canalicular multispecific organic anion transporter (cMOAT). The glutathione (GSH) level was 4.7-fold higher in KCP-4 cells than in KB-3-1 cells. When the GSH level in KCP-4 cells was decreased by treating the cells with buthionine sulfoximine and nitrofurantoin, the accumulation of and sensitivity to cisplatin in the cells were increased. C-A120 cells were only 3.0-fold more resistant to cisplatin than KB-3-1 cells and this resistance was not affected by the increased glutathione level. The accumulation of platinum in C-A120 and KCP-4 cells was 68.5 and 20.4% of that in KB-3-1 cells, respectively, while the intracellular levels of antimony potassium tartrate in C-A120 and KCP-4 cells were 13.2 and 9.9% of that in KB-3-1 cells, respectively. The ATP-dependent efflux of antimony was enhanced in both C-A120 and KCP-4 cells. These results, taken together, suggest an efflux pump for heavy metals different from MRP and cMOAT is involved in cisplatin resistance in KCP-4 cells.

Susceptibility of different isolates of Vibrio harveyi to antibiotics

The susceptibility of six Vibrio harveyi strains to antibiotics was studied. Four strains originally isolated from diseased penaeids and two reference strains originally isolated from either sea water (ATCC 25919) or diseased Talorchestia sp. (ATCC 14126) were used in the present study. Results revealed that all three strains isolated in Taiwan exhibited resistance against nitrofurantoin, novobiocin and sulphonamide. The two reference strains and the strain isolated in Indonesia were susceptible to these three antibiotics.

Endonuclease V (nfi) mutant of Escherichia coli K-12

Endonuclease V (deoxyinosine 3' endonuclease), the product of the nfi gene, has a specificity that encompasses DNAs containing dIMP, abasic sites, base mismatches, uracil, and even untreated single-stranded DNA. To determine its importance in DNA repair pathways, nfi insertion mutants and overproducers (strains bearing nfi plasmids) were constructed. The mutants displayed a twofold increase in spontaneous mutations for several markers and an increased sensitivity to killing by bleomycin and nitrofurantoin. An nfi mutation increased both cellular resistance to and mutability by nitrous acid. This agent should generate potential cleavage sites for the enzyme by deaminating dAMP and dCMP in DNA to dIMP and dUMP, respectively. Relative to that of a wild-type strain, an nfi mutant displayed a 12- to 1,000-fold increase in the frequency of nitrite-induced mutations to streptomycin resistance, which are known to occur in A x T base pairs. An nfi mutation also enhanced the lethality caused by a combined deficiency of exonuclease III and dUTPase, which has been attributed to unrepaired abasic sites. However, neither the deficiency nor the overproduction of endonuclease V affected the growth of the single-stranded DNA phages M13 or phiX174 nor of Uracil-containing bacteriophage lambda. These results suggest that endonuclease V has a significant role in the repair of deaminated deoxyadenosine (deoxyinosine) and abasic sites in DNA, but there was no evidence for its cleavage in vivo of single-stranded or uracil-containing DNA.

Overexpression of the hslVU operon suppresses SOS-mediated inhibition of cell division in Escherichia coli

A multicopy clone was isolated which conferred resistance to the SOS inducer nitrofurantoin in an Escherichia coli lon mutant. Plasmid pHL1 was found to contain a 7-8 kbp HindIII DNA insert from a region of the chromosome at 88.5 minutes. Further characterisation of pHL1 revealed that resistance to nitrofurantoin was due to the overexpression of the hslV-hslU operon which encodes an ATP-dependent protease complex in E. coli. The overexpression of hslVU also conferred resistance to ultraviolet irradiation in the lon mutant. It is proposed that when overproduced, the HslV-HslU protease complex can degrade SulA which is an endogenous inhibitor of the essential cell division protein FtsZ. The ability of HslVU to degrade SulA in vivo suggests that Lon and HslVU may share a range of substrates. Furthermore, the suppression of lon could be used as a simple genetic test of proteolytic activity of cloned HslVU.

[Resistant antibiotic analysis of methicillin-resistant staphylococcus]

Methicillin-resistant staphylococcus aureus is one of the important patholonic bacteria which cause nosocomial infection. In order to investigate the resistant antibiotic circumstances of this organism and hence provide foundations of prevention and treatment, we determined the resistant rates of 88 staphylococcus strains for methicillin and for other 14 kinds of commonly used antibiotics, and we determined the engendering beta-lactamase. The results showed that the isolation rate of methicillin-resistant staphylococcus aureus (MRSA) was 56%; the rate of methicillin-resistant staphylococcus coagulase-negative (MRSCoN) was 47.6%; the resistance rate and the multi-resistant rate of methicillin-resistant staphylococcus (MRS) for the 14 kinds of antibiotics were higher as compared with those of methicillin-sensitive (MSSA). Among all the tested strains, MRSA was the highest to engender beta-lactamase (92.9%); Methicillin-sensitive staphylococcus coagulase-negative (MSSCoN) was the lowest (39.4%). MRS was sensitive to vancomycin and furantoin. These suggest that when treating this bacterium, we should select these two antibiotics first.

Transient reverse ventilation-perfusion mismatch in acute pulmonary nitrofurantoin reaction

A 67-yr-old woman with a history of myocardial infarct was admitted to emergency for marked dyspnea, nonproductive cough, nausea and fever. The thorax X-ray revealed a bilateral alveolar and interstitial infiltration pattern with basal accentuation. The cardiac examinations were normal. Technegas ventilation and Tc-99m-macroaggregated albumin (MAA) perfusion scans were performed to rule out pulmonary embolism. Bilateral multiple ventilation defects with normal perfusion was observed. The patient had been taking nitrofurantoin for four days for a bladder infection. Hypersensitivity to nitrofurantoin was suspected and the drug was discontinued. An antihistaminic and anxiolytic medication was started. The majority of the clinical symptoms disappeared within 24 hours. The control chest X-rays disclosed a marked improvement. Ventilation and perfusion scans obtained 48 hours after nitrofurantoin withdrawal were normal. The drug-related pulmonary reactions should be taken into account in patients on medication. Reversible ventilation defects can be the only lung-scintigraphic finding encountered in acute pulmonary nitrofurantoin reaction.

Differential effects of nitrofurans on the production/release of steroid hormones by porcine adrenocortical cells in vitro

Changes in the biogenesis of corticosteroids caused by nitrofurans were studied. The three nitrofurans used: furazolidone, furaltadone and nitrofurantoin, altered the steroid production/release by porcine adrenocortical cells in vitro during 1 h incubations. With pregnenolone as a substrate the nitrofurans inhibited aldosterone production/release. Although the nitrofurans differed in potency (nitrofurantoin > furazolidone > furaltadone) maximum inhibition occurred at 100 microM. In this concentration the nitrofurans changed also the release/production of other corticosteroids. The output of corticosterone and cortisol decreased by 50%. The production/release of deoxycortisol stayed the same. In contrast the output of progesterone and 17alpha-hydroxyprogesterone increased to more than 200% of control. The nitrofurans slightly reduced the output of androstenedione. No significant increases of the production/release of other steroids (testosterone, dehydroepiandrosterone, estradiol-17beta and estrone) by the cell suspension could be observed. The profile of the nitrofuran-induced changes lead to the conclusion that nitrofurans interfere with mitochondrial enzymes. These enzymes, presumably cytochrome P450(11,18) mediate the hydroxylation and the oxidation at C11 and C18, the final steps in the biogenesis of aldosterone, corticosterone and cortisol. The rapid and reversible fall in the output of these steroids occurs in vitro at concentrations which are below therapeutic blood concentrations seen in vivo. At higher concentrations the nitrofurans hinder the biogenesis of androgens. Thus nitrofurans can also affect steps in the steroid biogenesis located in the endoplasmatic reticulum.

Nitroaromatic betulin derivatives as redox cycling agents

We have synthesized nitroaromatic derivatives of triterpenoid betulin (lup-20(29)-ene-3 beta, 28-diol), betulin-(28)-5'-(aziridin-1-yl)-2',4'-dinitrobenzoate and betulin-(28)-5'-nitro-2'-furoate. These compounds were reduced in single-electron way by ferredoxin: NADP+ reductase and flavocytochrome b2 at rates comparable with their simple structure analogs. Besides, these compounds were substrates for DT-diaphorase. Their toxicity to bovine leukemia virus-transformed lamb fibroblast culture was partly prevented by antioxidant N,N'-diphenyl-p-phenylene diamine and desferrioxamine, indicating an involvement of oxidative stress in their cytotoxicity.

Nitrofurantoin-induced hepatic and pulmonary biochemical changes in mice fed different vitamin E doses

The hepatic and pulmonary effects of nitrofurantoin (40 mg/kg, intraperitoneally) were determined at 4 and 24 hr following its administration in mice fed for 10 weeks with a vitamin E sufficient, deficient or enriched diet. Liver glutathione (GSH) was reduced by nitrofurantoin at 4 hr but was unchanged 20 hr later. Nitrofurantoin did not affect liver glutathione peroxidase, glutathione reductase or superoxide dismutase activities. Liver catalase activities were decreased by nitrofurantoin at 4 hr. Lung GSH levels were increased whilst glutathione peroxidase activity was decreased at 4 and 24 hr. Lung glutathione reductase activity was reduced in certain groups. Nitrofurantoin did not affect lung superoxide dismutase, but catalase was decreased at 24 hr. Liver malondialdehyde levels were increased by nitrofurantoin in the vitamin E deficient group whilst lung malondialdehyde levels remained unchanged. Both liver and lung malondialdehyde levels were unaffected by vitamin E supplementation when compared to the vitamin E-sufficient group. These results suggest that nitrofurantoin (40 mg/kg) was deleterious to the liver and lung. Nitrofurantoin-induced lipid peroxidation was seen in vitamin E deficiency but an increase in dietary vitamin E content did not provide additional protection compared to the recommended daily allowance. The antioxidant activities of alpha-tocopherol and gamma-enriched tocotrienol were similar.

Inhibition kinetics of camel lens zeta-crystallin: multiple inhibition studies

The inhibition of camel lens zeta-crystallin by nitrofurantoin (NF) was uncompetitive with respect to co-factor NADPH, (Ki = 90 microM) and competitive with respect to the substrate 9,10-phenanthrenequinone (PQ), (Ki = 50 microM). Inhibition at micromolar concentrations was also observed with dicoumarol, NADP+ and cibacron blue (CB). Theorell-Yonetani double-inhibition analysis showed that NF and dicoumarol were mutually exclusive inhibitors against PQ. However, analysis of NF and NADP+ by a double-inhibition plot showed that they simultaneously bind to the enzyme molecule. These studies demonstrate that NF and dicoumarol share the same site so that both molecules are prevented from binding at the same time, while NF and NADP+ can bind simultaneously to different sites on the enzyme. Although CB was noncompetitive with respect to PQ, double inhibition analysis showed that CB and dicoumarol or NF were mutually exclusive inhibitors against PQ, implying a distinct mode of inhibition for CB.

Sensitivity of Australian isolates of Salmonella enteritidis to nitrofurantoin and furazolidone

Susceptibility of 66 and 62 Australian isolates of Salmonella enteritidis to nitrofurantoin and furazolidone, respectively, was determined. Most isolates were susceptible to both antibiotics. Cross-resistance was low among all isolates, but higher among the subset of phage type 4 isolates. These results contrast directly with those of a previous study (Rampling, A., Upson, R. and Brown, D.F.J. (1990) J. Antimicrob. Chemother., 25: 285-290). Sensitivity among Australian isolates of S. enteritidis does to some extent, support the contention that furans may have played a role in the selection and enhanced colonisation of poultry by Salmonella enteritidis in Britain. Furthermore, nitrofurantoin should not be used as a selective agent in the isolation of Salmonella enteritidis.

On the nature of the adaptive response induced by mitomycin C in Vibrio cholerae OGAWA 154 cells

The induction of an adaptive response in Vibrio cholerae OGAWA 154 cells was obtained using an alkylating agent, mitomycin C, as both stimulating and challenging agent. Cross-adaptive response was observed in V. cholerae cells when pretreated with a sublethal dose of another alkylating agent, N-methyl-N'nitro-N-nitrosoguanidine, followed by challenging treatment with mitomycin C. The dose of mitomycin C for 50% survival (D50) became almost double for mitomycin C pretreated cells and 1.5 times for N-methyl-N-'nitro-N-nitrosoguanidine pretreated cells, compared to nonpretreated cells. It was also shown that pretreatment with a sublethal dose of oxidative DNA damaging agents, viz, hydrogen peroxide or nitrofurantoin, did not show any cross-adaptive response against subsequent challenge by mitomycin C.

Differential activation of heat-shock and oxidation-specific stress genes in chemically induced oxidative stress

Post-ischaemic reperfusion increases the level of the major heat-shock (stress) protein hsp 70 and of its mRNA by transcriptional mechanisms, and activates the binding of the heat-shock factor HSF to the consensus sequence HSE. In common with CoCl2 treatment, post-ischaemic reperfusion increases the level of haem oxygenase mRNA, an indicator of oxidative stress, but CoCl2 does not seem to induce the expression of the hsp 70 gene [Tacchini, Schiaffonati, Pappalardo, Gatti and Bernelli-Zazzera (1993) Lab. Invest. 68, 465-471]. Starting from these observations, we have now studied the expression of two genes of the hsp 70 family and of other possibly related genes under conditions of oxidative stress. Three different chemicals, which cause oxidative stress by various mechanisms and induce haem oxygenase, enhance the expression of the cognate hsc 73 gene, but do not activate the inducible hsp 70 gene. Expression of the other genes that have been studied seems to vary in intensity and/or time course, in relation to the particular mechanism of action of any single agent. The pattern of induction of the early-immediate response genes c-fos and c-jun observed during oxidative stress differs from that found in post-ischaemic reperfused livers. Oxidative-stress-inducing agents do not promote the binding of HSF to its consensus sequence HSE, such as occurs in heat-shock and post-ischaemic reperfusion, and fail to activate AP-1 (activator protein 1). With the possible exception of Phorone, the oxidative stress chemically induced in rat liver activates NFkB (nuclear factor kB) and AP-2 (activator protein 2) transcription factors.

Dihydroxy-acid dehydratase, a [4Fe-4S] cluster-containing enzyme in Escherichia coli: effects of intracellular superoxide dismutase on its inactivation by oxidant stress

Dihydroxy-acid dehydratase (DHAD) has a [4Fe-4S] cluster and is reported to be facilely inactivated by oxidant stress. To directly assess the biological effects in vivo of superoxide dismutase (SOD) on the oxidant sensitivity of DHAD, we used an Escherichia coli K-12 parent strain (CGSC5073) and derived strains OB 1, OB 2, and OB 3 that lacked one of or both FeSOD and MnSOD. In the K-12 parent strain half the cellular DHAD activity was lost in 15 min at 0.8 atm oxygen, less than 10 microM aerobic nitrofurantoin, or about 5 microM aerobic paraquat (PQ) and in about 1 min at 10 microM aerobic PQ. Oxygen and metabolism were required for PQ to inactivate DHAD in cells; adding dithiothreitol to cell-free extracts did not restore DHAD activity. The Km was not appreciably changed for DHAD that was 50 and 70% inactivated in cells, respectively, by hyperbaric oxygen (HBO) and PQ, compared to cells in exponential, aerobic growth. Thus, active site oxidative impairment of individual enzyme molecules apparently was all-or-none. DHAD activity was greatly decreased when measured in extracts made from strains that lacked both SODs unless SOD was added to cell suspensions before extracts were made. DHAD was more sensitive in strains lacking both SODs than in the parent strain to inactivation by aerobic PQ and HBO. Anaerobic (compared to aerobic) growth increased DHAD specific activity by 20% or less in the parent strain and in strains OB 1 and OB 2 (lacking MnSOD and FeSOD, respectively); however, in strain OB 3 (lacking both SODs) DHAD was increased 60%. DHAD was partially inactivated by the oxidant stress of aerobic growth, but remained in a form detectable by DHAD antibody, and the ratio of active to inactive DHAD decreased greatly in cells lacking SOD. Thus, SOD helped maintain DHAD as an active holoenzyme and benefitted cells growing aerobically or when exposed to low levels of PQ.

The consequences of nitrofurantoin-induced oxidative stress in isolated rat hepatocytes: evaluation of pathobiochemical alterations

Oxidative stress was induced in isolated rat hepatocytes by incubation with nitrofurantoin in the absence and presence of the GSSG reductase inhibitor BCNU. In both cases nitrofurantoin markedly reduced glutathione but exerted cytotoxicity as measured by LDH release and loss of intracellular potassium only in BCNU pretreated cells. The onset of cytotoxicity was accompanied by an increase of lipid peroxidation. Oxidation of protein thiols, however, could not be detected in the early phase of cell damage. The cytoprotective activity of N-acetyl-cysteine > dithiothreitol = deferoxamine revealed the substantial importance of glutathione for cellular defence and the sensitivity of not yet identified thiol-dependent targets of oxidative stress.

Induction of adaptive response by nitrofurantoin against oxidative DNA damage in some bacterial cells

Pretreatment with a sublethal dose of nitrofurantoin did not give any protection to Vibrio cholerae OGAWA 154 (wild-type) cells against subsequent treatment with challenging doses of MNNG and vice versa. However, pretreatment with a sublethal dose of nitrofurantoin offered significant protection to the bacterial cells against subsequent treatment with challenging doses of H2O2 and vice versa. Further, sublethal doses of nitrofurantoin or H2O2 produced almost the same degree of protection against challenges by H2O2 or nitrofurantoin. Both the alkylating agent MNNG and the oxidative agent H2O2 induced adaptive responses in Vibrio cholerae OGAWA 154 cells against subsequent challenge by the respective agents. The experiments presented in this communication revealed that nitrofurantoin produced an adaptive response in bacterial cells against oxidative and not alkylating DNA damage.

Development of antibacterial resistance: the Dutch experience

An ongoing registration of resistance patterns of microorganisms in a large area in the Netherlands demonstrated a slow but steady increase of resistance to several antibacterials in the past ten years. For some bacteria and for some selected antimicrobial agents this increase was such that it might no longer be justifiable to choose them for empirical therapy. For Escherichia coli, resistance increased from 14% in 1982 to 28% in 1992 for co-trimoxazole and for trimethoprim and from 24% to 34% for amoxycillin. For nitrofurantoin the figures were 1.5% and 4.0%, respectively. Comparison with other areas in the Netherlands showed similar results, so it can be considered a nationwide problem which may have consequences for national antibiotic guidelines for the treatment of infections.

Nitrofurantoin: mechanism of action and implications for resistance development in common uropathogens

Nitrofurantoin is an effective urinary tract antibacterial to which no clinically significant resistance development has occurred. We have previously shown that nitrofurantoin susceptibility in bacteria correlates with the presence of bacterial nitroreductases which convert nitrofurantoin to highly reactive electrophilic intermediates. These intermediates were shown to attack bacterial ribosomal proteins non-specifically, causing complete inhibition of protein synthesis. In the present study, we confirm previous reports that low concentrations of nitrofurantoin specifically inhibit inducible enzyme synthesis in bacteria, and show that this inhibition occurs at levels equivalent to the MICs of nitrofurantoin for several bacterial species. Our previous studies had shown that nitrofurantoin at different concentrations interacts with bacterial ribosomal proteins in qualitatively the same fashion; we now report that quantitative differences are seen in the labelling observed at different nitrofurantoin concentrations and discuss these differences as they may relate to the inhibition of inducible enzyme synthesis. In addition, we have now demonstrated the existence of a novel mechanism of action for nitrofurantoin which does not require the production of reactive nitrofurantoin metabolites by bacterial reductases. The lack of clinically significant bacterial resistance development to nitrofurantoin is likely due to the combination of nitrofurantoin's multiple sites of attack and multiple mechanisms of action.

Inhibition studies on the involvement of flavoprotein reductases in menadione- and nitrofurantoin-stimulated oxyradical production by hepatic microsomes of flounder (Platichthys flesus)

Inhibitors of mammalian cytochrome P450 and P450 reductase were used to investigate the enzymes in flounder (Platichthys flesus) hepatic microsomes involved in the stimulation of NAD(P)H-dependent iron/EDTA-mediated 2-keto-4-methiolbutyric acid (KMBA) oxidation (hydroxyl radical production) by the redox cycling compounds menadione and nitrofurantoin. Inhibitors were first tested for their effects on flounder microsomal P450 and flavoprotein reductase activities. Ellipticine gave type II difference binding spectra (app. Ks 5.36 microM; delta A max 0.16 nmol-1 P450) and markedly inhibited NADPH-cytochrome c reductase, NADPH-cytochrome P450 reductase, and monooxygenase (benzo[a]pyrene metabolism) activities. 3-aminopyridine adenine dinucleotide phosphate (AADP; competitive inhibitor of P450 reductase) inhibited NADPH-cytochrome c but not NADH-cytochrome c or NADH-ferricyanide reductase activities. Alkaline phosphatase (inhibitor of rabbit P450 reductase) stimulated NADPH-cytochrome c reductase activity seven fold but had less effect on NADH-reductase activities. AADP inhibited nitrofurantoin- and menadione-stimulated KMBA oxidation by 45 and 17%, respectively, indicating the involvement of P450 reductase at least in the former. In contrast, ellipticine had relatively little effect, possibly because, unlike cytochrome c, the smaller xenobiotic molecules can access the hydrophilic binding site of P450 reductase. Alkaline phosphatase stimulated NAD(P)H-dependent basal and xenobiotic-stimulated KMBA oxidation, showing general consistency with the results for reductase activities. Overall, the studies indicate both similarities (ellipticine, AADP) and differences (alkaline phosphatase) between the flounder and rat hepatic microsomal enzyme systems.