Interaction of metronidazole with resistant and susceptible Bacteroides fragilis

Interaction of metronidazole with Escherichia coli deoxyribonucleic acid

To define the characteristics of the reported binding of metronidazole to DNA, we isolated the DNA from hypoxic incubation mixtures that contained both [14C]metronidazole and metronidazole-susceptible strains of Escherichia coli. Thus, either [2-14C]metronidazole or [1',2'-14C]metronidazole was incubated with either wild-type E. coli (strain AB1157) or a DNA repair mutant (strain SR58) that is highly susceptible to metronidazole. Approximately 0.02% of the radiolabel in the metronidazole was found to be associated with DNA isolated from both strains of bacteria, a percentage similar to that found to be associated with DNA from mammalian sources in a variety of in vitro and in vivo experiments performed by other investigators. The bound radioactivity was not diminished, however, when a great excess of non-radiolabeled metronidazole was included in the incubation mixture, indicating that the binding we observed was probably due to impurities in the radiolabeled metronidazole. We also examined the binding to DNA of a possible surrogate for the partially reduced form of metronidazole, 1-methyl-4-phenyl-5-nitrosoimidazole (5NO), that has been described previously. The binding of the tritiated form of 5NO to DNA was also found to be undiminished by the addition of carrier 5NO (a finding which does not refute the hypothesis that 5NO may serve as a surrogate for the study of the active form of metronidazole). These studies do not exclude the binding to DNA of either metronidazole or a possible surrogate of its active functionality, but they indicate that if such binding occurs, it must be limited to very few sites on DNA and hence will be difficult to characterize.

Effect of daptomycin, metronidazole and mezlocillin combinations on mixed bacterial cultures involving facultative and anaerobic bacteria

The in vitro activities of different daptomycin concentrations in combination with metronidazole (4 mg/l) or/and mezlocillin (8 mg/l) were investigated on mixed bacterial cultures involving gram-positive facultative cocci. Bacteroides fragilis group strains and Escherichia coli. When Streptococcus faecalis alone or together with E. coli was cultured with B. thetaiotaomicron the colony counts of the latter were 4 log units higher after incubation in the presence of daptomycin and metronidazole than when it was cultured alone. After the addition of mezlocillin, this effect disappeared and all 3 strains were killed. When the same antibiotic combinations were used in the presence of beta-lactamase producing Staphylococcus aureus, the activity of mezlocillin was decreased significantly. The colony counts of the co-cultured B. thetaiotaomicron and E. coli proved to be 2 log and 6 log higher, respectively, than those observed after they were cultured alone. The antimicrobial susceptibilities of the clinical isolates tested here in mixed cultures of up to 3 strains were modified significantly by interactions between the strains and the antibiotics used.

In vitro activity of daptomycin-metronidazole combinations against mixed bacterial cultures: reduced activity of metronidazole against Bacteroides species in the presence of Enterococcus faecalis

The in vitro activity of daptomycin-metronidazole combinations against mixed cultures of gram-positive facultative cocci and strains of the Bacteroides fragilis group was investigated. Metronidazole did not influence the high activity of daptomycin against strains of Staphylococcus aureus, Staphylococcus epidermidis and Enterococcus faecalis in the absence or presence of the co-cultured Bacteroides strains. In contrast, the Enterococcus faecalis isolates protected the co-cultured Bacteroides strains against the killing effect of metronidazole, even at a concentration four- or eightfold the MIC of metronidazole. Killing curve experiments confirmed this protective effect of different isolates of Enterococcus faecalis.

Role of hydrogenase 1 of Clostridium pasteurianum in the reduction of metronidazole

Competition studies between the phosphoroclastic reaction and the metronidazole reduction reaction using dialyzed crude cell-free extracts of Clostridium pasteurianum which were essentially devoid of Hydrogenase 1 activity demonstrated that this enzyme plays an important role in the reduction of metronidazole. To determine further the exact function for Hydrogenase 1 in the reduction of the drug, this enzyme was highly purified from C. pasteurianum. Metronidazole reduction activity copurified with Hydrogenase 1 specific activity throughout the purification procedure. Drug reduction required the presence of an electron carrier and could not be accomplished by the enzyme alone. Ferredoxin, and also the low potential electron carrier dyes, methyl and benzyl viologen, and the flavin coenzymes, FAD and flavin mononucleotide (FMN), could couple the reduction of metronidazole. Hydrogenase 1 activity and its metronidazole reduction activity were inactivated irreversibly in the presence of oxygen. Metronidazole could be reduced only by an electron carrier-Hydrogenase 1 mechanism or directly by sodium dithionite.

Formation of an amino reduction product of metronidazole in bacterial cultures: lack of bactericidal activity

To investigate whether the amino reduction product of metronidazole has antibacterial activity, 5-amino-1-beta-hydroxyethyl-2-methylimidazole (AMN) was synthesized and tested against Bacteroides fragilis and Escherichia coli strain SR58, both of which are known to be sensitive to metronidazole. Neither of these strains was found to be sensitive either to AMN or to the equivalent amine derived from dimetridazole, 5-amino-1,2-dimethylimidazole. Both of these amines are relatively stable in the presence of bacteria, making it possible to examine the bacterial reduction of radiolabeled metronidazole in the presence of AMN. This experiment indicated that at least 17% of the metronidazole that disappeared under the reducing conditions of the bacterial medium was converted to AMN. We conclude, therefore, that AMN forms during the activation of metronidazole by bacterial reduction but is not a bactericidal form of metronidazole.

Interaction of metronidazole with DNA repair mutants of Escherichia coli

It has been proposed that one of metronidazole's partially reduced intermediates interacts either with DNA to exert a bactericidal effect or with water to form acetamide. To test this hypothesis we have examined the effect of metronidazole on several mutants of Escherichia coli that are defective in DNA repair. UV-susceptible RecA- and UvrB- point mutants have an increased susceptibility to metronidazole as manifested by both a decreased minimal inhibitory concentration and a greater bactericidal response to metronidazole in resting cultures. By these criteria, however, we find that UvrB- deletion mutants, which lack the ability to reduce nitrate and chlorate, are no more susceptible to metronidazole than is the wild type. We find, however, that these deletion mutants also lack the ability to reduce metronidazole and thus possibly to form its reactive species. When metronidazole's bactericidal effect is expressed in terms of the concurrent accumulation of acetamide derived from metronidazole, then all RecA- and UvrB- mutants are killed more efficiently than their wild types. The data are consistent, therefore, with metronidazole's lethal effect being mediated by a partially reduced intermediate on the metabolic pathway between metronidazole and acetamide. Defects in other aspects of the DNA repair system do not confer this increased susceptibility to the proposed intermediate. A Tag- mutant, for example, which is defective in 3-methyl-adenine-DNA glycosylase, does not have this increased susceptibility to the presumed precursor of acetamide. Thus, these results provide further support for the hypothesis that the bactericidal effect of metronidazole is mediated by a partially reduced intermediate in the metabolic conversion of metronidazole to acetamide and suggest that this intermediate interacts with DNA to produce a lesion similar to that caused by UV light.

Role of sulfhydryl compounds in the bactericidal effect of metronidazole

The bactericidal effect of metronidazole on Escherichia coli and Bacteroides fragilis can be partially reversed by cysteamine under conditions that lead to the formation of an adduct, the thioether, 4-(2-aminoethyl)thio-2-methylimidazole-1-ethanol (4-ATME). This adduct, which is not mutagenic for the Ames histidine auxotrophs of Salmonella typhimurium, forms at a rate that is independent of live bacterial cells and, therefore, can not be shown to relate to the biological effect of cysteamine. When treated with Raney nickel, this adduct yields 2-methylimidazole-1-ethanol. To determine whether a structurally related adduct forms with bacterial protein, a culture of B. fragilis was incubated with radiolabelled metronidazole and then treated with 5% trichloroacetic acid. That the radiolabel in the precipitate did not yield 2-methylimidazole-1-ethanol when treated with Raney nickel suggests that binding of metronidazole to cellular macromolecules does not involve thioether formation.