Bacteroides fragilis resistant to metronidazole after long-term therapy

Amidinoquinoxaline N-oxides: synthesis and activity against anaerobic bacteria

We present herein an in-depth study on the activity of amidinoquinoxaline N-oxides 1 against Gram-positive and Gram-negative anaerobic bacteria. Based on 5-phenyl-2,3-dihydropyrimidoquinoxaline N-oxide 1a, the selected structural variations included in our study comprise the substituents α- to the N-oxide function, the benzofused ring, substitution and quaternization of the amidine moiety, and the amidine ring size. Compounds 1 showed good to excellent antianaerobic activity, evaluated as the corresponding CIM50 and CIM90 values, and an antimicrobial spectrum similar to metronidazole. Six out of 13 compounds 1 had CIM90 values significantly lower than the reference drug. Among them, imidazoline derivatives 1i-l were the most active structures. Such compounds were synthesized by base-promoted ring closure of the corresponding amidines. The N-oxides under study showed no significant cytotoxicity against RAW 264.7 cells, with high selectivity indexes. Their calculated ADME properties indicate that the compounds are potentially good oral drug candidates. The antianaerobic activity correlated satisfactorily with the electron affinity of the compounds, suggesting that they may undergo bioreductive activation before exerting their antibacterial activity.

Recent Trends in Antimicrobial Resistance among Anaerobic Clinical Isolates

Anaerobic bacteria are normal inhabitants of the human commensal microbiota and play an important role in various human infections. Tedious and time-consuming, antibiotic susceptibility testing is not routinely performed in all clinical microbiology laboratories, despite the increase in antibiotic resistance among clinically relevant anaerobes since the 1990s. β-lactam and metronidazole are the key molecules in the management of anaerobic infections, to the detriment of clindamycin. β-lactam resistance is usually mediated by the production of β-lactamases. Metronidazole resistance remains uncommon, complex, and not fully elucidated, while metronidazole inactivation appears to be a key mechanism. The use of clindamycin, a broad-spectrum anti-anaerobic agent, is becoming problematic due to the increase in resistance rate in all anaerobic bacteria, mainly mediated by Erm-type rRNA methylases. Second-line anti-anaerobes are fluoroquinolones, tetracyclines, chloramphenicol, and linezolid. This review aims to describe the up-to-date evolution of antibiotic resistance, give an overview, and understand the main mechanisms of resistance in a wide range of anaerobes.

Phenotypic and Genotypic Correlation of Antimicrobial Susceptibility of Bacteroides fragilis: Lessons Learnt

Background Bacteroides fragilis is an opportunistic pathogen causing severe infections, including bacteremia. There have been increased reports of antimicrobial resistance in B. fragilis. However, phenotypic testing of susceptibility is time consuming and not cost effective for anaerobes. The present study investigates the correlation of phenotypic susceptibility with genotypic markers; to determine if these could be considered for deciding empirical therapy for B. fragilis. Material and methods Bacteroides fragilis isolates from various clinical samples including exudates, tissue, and body fluids were collected between November 2018 and January 2020 in the Department of Clinical Microbiology, Christian Medical College (CMC) Vellore. Species identification was done by Matrix Assisted Laser Desorption Ionization time of flight mass spectrometry (MALDI TOF) according to the manufacturer's instructions. A total number of 51 B. fragilis isolates were tested against metronidazole, clindamycin, piperacillin/tazobactam, and meropenem phenotypically by agar dilution method using Clinical & Laboratory Standards Institute (CLSI) 2019 guidelines and minimum inhibitory concentrations (MIC) were interpretated. The genotypic markers for antimicrobial resistance genes (nim, emrF, and cfiA) were studied by polymerase chain reaction (PCR) assay as per the standard protocol on all isolates to detect resistance genes. Results B. fragilis isolates in this study expressed 45%, 41%, and 16% phenotypic resistance to clindamycin, metronidazole, and meropenem, respectively, with least resistance to piperacillin/tazobactam (6%). Among the metronidazole resistant isolates, 52% harbored nim gene. Nim gene was also present in 76% (23/30) of the metronidazole susceptible isolates. Similarly, cfiA was present in all eight meropenem resistant isolates in addition to 22% (9/41) of the susceptible isolates. All cfiA negative isolates were phenotypically susceptible. Interestingly, 74% (17/23) of the clindamycin resistant isolates were positive for ermF. Conclusions Detection of a limited set of genes does not always correlate with phenotypic resistance to metronidazole and clindamycin due to the reported influence of insertion sequence (IS) elements, efflux, and other genetic determinants. Certainly, the absence of the cfiA gene can be employed to rule out meropenem resistance. However, redundant use of antibiotics such as meropenem along with metronidazole could be avoided for B. fragilis, which might otherwise elevate meropenem resistance. Recommendation of metronidazole requires prior phenotypic testing due to the reported 41% resistance.

Molecular study on metronidazole resistance in Bacteroides fragilis group isolates from a South Indian tertiary care center

OBJECTIVE

Bacteroides species are an important part of human intestinal microbiota. They can cause infections of significant mortality and morbidity when moved out of their niche in the gut. The cornerstone drug for prophylaxis and therapy, metronidazole, is exhibiting signs of resistance, which are frequently attributed to nitroimidazole (nim) resistance genes. The aim of this study was to use Epsilometer test (E-test) to assess the metronidazole susceptibility and conventional PCR methodology to map the distribution of nim genes in Bacteroides fragilis group (BFG) isolates.

METHODS

MALDI-TOF MS was used to identify BFG isolates. Using the E-test methodology, metronidazole minimum inhibitory concentrations (MICs) were determined. The presence of nim genes in these isolates were checked by conventional PCR methodology. Sequencing was done on selected amplicons for determining the nim gene types.

RESULTS

Bacteroides fragilis accounted for 55.3% of the total 273 BFG members identified. Of these, 196 (71.8%) were susceptible, 43 (15.8%) intermediate and 34 (12.5%) resistant to metronidazole as determined by the E-test. nim gene was present in 101 (37%) of the total 273 isolates. Out of the 34 phenotypically resistant isolates (MIC ≥32 μg/ml), 29 harboured nim gene (Chi-square test, p < 0.0000001) but nim gene was absent in 5 (14.7%) isolates. Also, nim gene was detected in 72 (30.1%) of the 239 isolates with susceptible and intermediate metronidazole MIC. Sequencing of 20 amplicons gave a nimE gene type.

CONCLUSIONS

In view of the rising metronidazole resistance among BFG and its close association with nim genes, there is a need for implementing routine metronidazole susceptibility testing and more researches are needed to find the molecular basis of these nim genes.

Derivation of a Precise and Consistent Timeline for Antibiotic Development

Antibiotic resistance is a global health crisis. New classes of antibiotics that can treat drug-resistant infections are urgently needed. To communicate this message, researchers have used antibiotic development timelines, but these are often contradictory or imprecise. We conducted a systematic literature review to produce an antibiotic timeline that incorporates the dates of discovery, first use, and initial reports of the emergence of resistance for the 38 classes of clinically used antibiotics. From our timeline, we derive lessons for identifying new antibiotics that are less prone to resistance. These include a required focus on molecules that exhibit multiple modes of action, possess unusually long 'resistance windows', or those that engage cellular targets whose molecular architectures are at least in part decoupled from evolutionary pressures. Our analysis also further highlights the importance of safeguarding antibiotics as a mechanism for mitigating the development of resistance. We have made our data and sources freely available so that the research community can adapt them to their own needs.

Genotyping of multi drug resistant Bacteroides fragilis group of clinical isolates from mangalore, south India

INTRODUCTION

Bacteroides fragilis group, the most encountered anaerobic bacterium is emerging with resistance to antibiotics. This study explores the antibiogram and occurrence of resistance genes in isolates of B fragilis group from clinical samples.

METHOD

In this study the antimicrobial susceptibility test was done using commercially available E strip test and the results were recorded according to CLSI guidelines. Genotypic investigations were performed by conventional PCR to detect the target resistant genes.

RESULTS

Ceftriaxone, cefoxitin, clindamycin and imipenem were found to be the most resistant antimicrobials in E test method. Metronidazole has shown resistance in 7 strains in vitro while resistance nim genes were detected in 12 strains from 62 randomly selected isolates. Other resistance genes (cfiA, ermF and cepA) were expressed at 58%, 62.9% and 48.3% respectively, among these strains.

CONCLUSION

B fragilis group harbouring the resistant genes may not be fully expressed phenotypically. Hence, detection of these genes by PCR might be necessary for a pertinent conclusion.

Metronidazole: an update on metabolism, structure-cytotoxicity and resistance mechanisms

Metronidazole, a nitroimidazole, remains a front-line choice for treatment of infections related to inflammatory disorders of the gastrointestinal tract including colitis linked to Clostridium difficile. Despite >60 years of research, the metabolism of metronidazole and associated cytotoxicity is not definitively characterized. Nitroimidazoles are prodrugs that are reductively activated (the nitro group is reduced) under low oxygen tension, leading to imidazole fragmentation and cytotoxicity. It remains unclear if nitroimidazole reduction (activation) contributes to the cytotoxicity profile, or whether subsequent fragmentation of the imidazole ring and formed metabolites alone mediate cytotoxicity. A molecular mechanism underpinning high level (>256 mg/L) bacterial resistance to metronidazole also remains elusive. Considering the widespread use of metronidazole and other nitroimidazoles, this review was undertaken to emphasize the structure-cytotoxicity profile of the numerous metabolites of metronidazole in human and murine models and to examine conflicting reports regarding metabolite-DNA interactions. An alternative hypothesis, that DNA synthesis and repair of existing DNA is indirectly inhibited by metronidazole is proposed. Prokaryotic metabolism of metronidazole is detailed to discuss new resistance mechanisms. Additionally, the review contextualizes the history and current use of metronidazole, rates of metronidazole resistance including metronidazole MDR as well as the biosynthesis of azomycin, the natural precursor of metronidazole. Changes in the gastrointestinal microbiome and the host after metronidazole administration are also reviewed. Finally, novel nitroimidazoles and new antibiotic strategies are discussed.

Antimicrobials: a global alliance for optimizing their rational use in intra-abdominal infections (AGORA)

Intra-abdominal infections (IAI) are an important cause of morbidity and are frequently associated with poor prognosis, particularly in high-risk patients. The cornerstones in the management of complicated IAIs are timely effective source control with appropriate antimicrobial therapy. Empiric antimicrobial therapy is important in the management of intra-abdominal infections and must be broad enough to cover all likely organisms because inappropriate initial antimicrobial therapy is associated with poor patient outcomes and the development of bacterial resistance. The overuse of antimicrobials is widely accepted as a major driver of some emerging infections (such as C. difficile), the selection of resistant pathogens in individual patients, and for the continued development of antimicrobial resistance globally. The growing emergence of multi-drug resistant organisms and the limited development of new agents available to counteract them have caused an impending crisis with alarming implications, especially with regards to Gram-negative bacteria. An international task force from 79 different countries has joined this project by sharing a document on the rational use of antimicrobials for patients with IAIs. The project has been termed AGORA (Antimicrobials: A Global Alliance for Optimizing their Rational Use in Intra-Abdominal Infections). The authors hope that AGORA, involving many of the world's leading experts, can actively raise awareness in health workers and can improve prescribing behavior in treating IAIs.

Metronidazole and hydroxymetronidazole central nervous system distribution: 1. microdialysis assessment of brain extracellular fluid concentrations in patients with acute brain injury

The distribution of metronidazole in the central nervous system has only been described based on cerebrospinal fluid data. However, extracellular fluid (ECF) concentrations may better predict its antimicrobial effect and/or side effects. We sought to explore by microdialysis brain ECF metronidazole distribution in patients with acute brain injury. Four brain-injured patients monitored by cerebral microdialysis received 500 mg of metronidazole over 0.5 h every 8 h. Brain dialysates and blood samples were collected at steady state over 8 h. Probe recoveries were evaluated by in vivo retrodialysis in each patient for metronidazole. Metronidazole and OH-metronidazole were assayed by high-pressure liquid chromatography, and a noncompartmental pharmacokinetic analysis was performed. Probe recovery was equal to 78.8% ± 1.3% for metronidazole in patients. Unbound brain metronidazole concentration-time curves were delayed compared to unbound plasma concentration-time curves but with a mean metronidazole unbound brain/plasma AUC0-τ ratio equal to 102% ± 19% (ranging from 87 to 124%). The unbound plasma concentration-time profiles for OH-metronidazole were flat, with mean average steady-state concentrations equal to 4.0 ± 0.7 μg ml(-1). This microdialysis study describes the steady-state brain distribution of metronidazole in patients and confirms its extensive distribution.

Mechanisms of action of systemic antibiotics used in periodontal treatment and mechanisms of bacterial resistance to these drugs

Antibiotics are important adjuncts in the treatment of infectious diseases, including periodontitis. The most severe criticisms to the indiscriminate use of these drugs are their side effects and, especially, the development of bacterial resistance. The knowledge of the biological mechanisms involved with the antibiotic usage would help the medical and dental communities to overcome these two problems. Therefore, the aim of this manuscript was to review the mechanisms of action of the antibiotics most commonly used in the periodontal treatment (i.e. penicillin, tetracycline, macrolide and metronidazole) and the main mechanisms of bacterial resistance to these drugs. Antimicrobial resistance can be classified into three groups: intrinsic, mutational and acquired. Penicillin, tetracycline and erythromycin are broad-spectrum drugs, effective against gram-positive and gram-negative microorganisms. Bacterial resistance to penicillin may occur due to diminished permeability of the bacterial cell to the antibiotic; alteration of the penicillin-binding proteins, or production of β-lactamases. However, a very small proportion of the subgingival microbiota is resistant to penicillins. Bacteria become resistant to tetracyclines or macrolides by limiting their access to the cell, by altering the ribosome in order to prevent effective binding of the drug, or by producing tetracycline/macrolide-inactivating enzymes. Periodontal pathogens may become resistant to these drugs. Finally, metronidazole can be considered a prodrug in the sense that it requires metabolic activation by strict anaerobe microorganisms. Acquired resistance to this drug has rarely been reported. Due to these low rates of resistance and to its high activity against the gram-negative anaerobic bacterial species, metronidazole is a promising drug for treating periodontal infections.

Bacteremia caused by a metronidazole-resistant Prevotella sp. strain

Metronidazole resistance among Prevotella spp. is rare. We report here the first case of bacteremia due to a high-level metronidazole-resistant Prevotella sp. responsible for treatment failure.

Isolation of metronidazole-resistant Bacteroides fragilis carrying the nimA nitroreductase gene from a patient in Washington State

Members of the Bacteroides fragilis group are among the most common anaerobic bacterial isolates in clinical specimens. Metronidazole, a 5-nitroimidazole, is often used as empirical therapy for anaerobic infections. Susceptibility testing is not routinely performed because of nearly universal susceptibility of Bacteroides spp. to this agent. We report a case of metronidazole-resistant Bacteroides fragilis in the United States and demonstrate the presence of the nimA gene, encoding a nitroreductase previously shown to mediate resistance to 5-nitroimidazole antimicrobial agents in B. fragilis strains from Europe and Africa. Because clinical failures in Bacteroides infections have been associated with the use of inactive antimicrobial agents, clinicians need to be aware of the possibility of metronidazole-resistant B. fragilis strains in the United States and the importance of susceptibility testing in selected situations.

Determinants of resistance in Bacteroides fragilis strains according to recent Brazilian profiles of antimicrobial susceptibility

Susceptibility profiles of 99 Bacteroides fragilis strains for 9 antimicrobial agents were defined by using an agar dilution method. The isolates were uniformly susceptible to imipenen and metronidazole. All isolates were resistant to ampicillin. The resistance rates to amoxicillin/clavulanate, cefoxitin, cefotaxime, chloramphenicol, clindamycin and tetracycline were 3.0, 12.1, 15.1, 1.0, 18.2 and 75.7%, respectively. Sixteen strains showed reduced susceptibility to metronidazole (MIC 2-4 mg/L) but none had nim genes using PCR. All strains were also investigated for the presence of cepA, cfiA, cfxA, ermF and tetQ genes by PCR methodology and 92.9, 4.9, 24.2, 2 and 64.6% of the strains were respectively found positive. These results reflect the importance of surveys of susceptibility profiles and the relevance of detecting major genetic determinants to monitor the dissemination of these genes.

New findings in beta-lactam and metronidazole resistant Bacteroides fragilis group

Beta-lactam antibiotics and 5-nitroimidazoles have been extensively used against anaerobic bacteria. However, antibiotic resistance is increasingly common among anaerobic Gram-negative bacilli. The classical mechanisms of resistance to beta-lactams are, (1) production of beta-lactamases; (2) alteration of penicillin-binding proteins (PBPs); and (3) changes in outer membrane permeability to beta-lactams. The 5-nitroimidazole molecule is a prodrug whose activation depends upon reduction of the nitro group in the absence of oxygen. Decreased uptake and altered reduction are believed to be responsible for metronidazole resistance. Five nim genes (A, B, C, D and E) have been identified in Bacteroides fragilis group spp. that confer resistance to 5-nitroimidazole antibiotics. Knowledge of the status and the mechanisms of resistance is critical for both the selection of antimicrobial therapy and the design of new antimicrobial agents. The purpose of this article is to review the mechanisms for and the prevalence of beta-lactam and metronidazole resistance in strains belonging to the B. fragilis group.

Prevalence and characteristics of nim genes encoding 5-nitroimidazole resistance among Bacteroides strains isolated in Morocco

We report here an evaluation of the dissemination of nim genes, encoding 5-nitroimidazoles resistance, among Bacteroides clinical strains isolated in Morocco. This study was done using a PCR method. Among 60 strains studied, nine contain a copy of a nim gene. The sequence determination of these genes showed that they are homologous to three nim genes previously characterized in strains isolated in France: nimB (five genes), nimC (three genes), and nimA (one gene). Although the nimA and nimC genes were previously identified on plasmids pIP417 and pIP419, respectively, we found here that they have a chromosomal location. The MICs of three 5-nitroimidazole antibiotics (metronidazole, ornidazole, and tinidazole) of the nim gene-containing strains were very low (0.5-2 microg/ml), indicating that the nim genes were not efficiently expressed in these clinical isolates.

Clindamycin, metronidazole, and chloramphenicol

Clindamycin, metronidazole, and chloramphenicol are three antimicrobial agents useful in the treatment of anaerobic infections. Clindamycin is effective in the treatment of most infections involving anaerobes and gram-positive cocci, but emerging resistance has become a problem in some clinical settings. Metronidazole is effective in the treatment of infections involving gram-negative anaerobes, but it is unreliable in the treatment of gram-positive anaerobic infections and is ineffective in treating aerobic infections. Additionally, metronidazole is often the drug of choice in treating infections in which Bacteroides fragilis is a serious concern. Chloramphenicol is effective in the treatment of a wide variety of bacterial infections, including serious anaerobic infections, but is rarely used in Western countries because of concerns about toxicity, including aplastic anemia and gray baby syndrome.

Vaginal carriage of enterotoxigenic Bacteroides fragilis in pregnant women

Bacteroides fragilis is an anaerobic bacterial species that is involved in gynecological infections and pathology. The incidence of vaginal carriage is largely unknown, and in order to study this, 120 pregnant women attending a general hospital for delivery were examined. Cultures were positive for eight of these women (6.6%). Interestingly, potential clonal relatedness could be demonstrated among several of the nonenterotoxigenic B. fragilis strains. Among the strains, only one produced metalloprotease enterotoxin. The presence of the gene for the metalloprotease, giving rise to the pathogenic effect on cultured eukaryotic HT29/C1 cells, was confirmed by a newly designed specific PCR assay. The enterotoxigenic B. fragilis (ETBF) strain was analyzed with the help of arbitrarily primed PCR (AP-PCR) and PCR-mediated ribotyping. The ETBF strain was shown to be genetically different compared to several other strains obtained from diverse sources. Our data indicate a relatively high vaginal B. fragilis carriage rate among pregnant women in Warsaw, Poland. Although neither ETBF nor B. fragilis colonization presented a clinical problem, the possible genetic relatedness among the colonizing B. fragilis strains indicates the need for additional research in the field of ETBF transmission and molecular epidemiology.

Epidemiology and antimicrobial resistance of B. fragilis group organisms isolated from clinical specimen and human intestinal microbiota

Epidemiological aspects and the antimicrobial susceptibility profile of the Bacteroides fragilis group isolated from clinical and human intestinal specimens were examined in this study. B. fragilis group strains were isolated from 46 (37%) of 124 clinical specimens and the source of the samples was: Blood culture (3), intraabdominal infection (27), brain abscess (2), soft tissue infection (17), respiratory sinus (3), pleural aspirate (9), breast abscess (3), surgical infected wound (22), pelvic inflammatory disease (22), chronic otitis media (9) and miscellaneous (7). Intraabdominal and soft tissue infections were responsible for more than half of the clinical isolates. Susceptibility to penicillin, cefoxitin, tetracycline, metronidazole, chloramphenicol and clindamycin was examined. All isolates were susceptible to metronidazole and chloramphenicol. For clindamycin and cefoxitin the resistance rates observed were 21.7% and 10.9% respectively. Susceptibility profiles varied among the different species tested. A total of 37 species of B. fragilis group isolated from intestinal microbiota of individuals who had no antimicrobial therapy for at least 1 month before the sampling was also examined. All strains were also susceptible to chloramphenicol and metronidazole and the resistance rates to clindamycin and cefoxitin were 19.4% and 5.4% respectively. A few institutions, in Brazil, have monitored the antimicrobial susceptibility of B. fragilis group strains isolated from anaerobic infections. The resistance rates to cefoxitin and clindamycin and the variation in susceptibility patterns among the species isolated in this study emphasize the need for monitoring of susceptibility patterns of B. fragilis group organisms isolated, especially at our University Hospitals.

Antibiotic susceptibility of clinical isolates of Prevotella bivia in Lagos, Nigeria

The susceptibility of 40 clinical isolates of Prevotella bivia to 11 anti-anaerobic agents was determined by disk diffusion and agar dilution methods in accordance with the NCCLS (1990) recommended guidelines. With the disk diffusion method all the P. bivia tested were susceptible to rifampicin, cefoxitin, chloramphenicol, clindamycin and metronidazole. Twelve (30%) isolates were resistant to cefotaxime and ceftriaxone while 2 (5%) each were resistant to tetracycline and erythromycin. Worthy of note, 39 (97.5%) and all 40 (100%) isolates were resistant to ampicillin and penicillin respectively. Determination of minimum inhibitory concentrations (MIC) showed that the first set of 5 antibiotics, except rifampicin, were shown to be as active against all the strains tested as with the disk diffusion sensitivity method: cefoxitin, chloramphenicol, clindamycin and metronidazole had MIC90 values ranging from 0.5-1 microgram/ml. The MIC90s of erythromycin and tetracycline were 4 micrograms/ml each while the range of MIC of cefotaxime and ceftriaxone was 0.5-32 micrograms/ml with a MIC90 of 8 micrograms/ml, well below their breakpoints, which is somewhat at variance with disk diffusion results. The clinical significance of this, however, is not clear, especially in the absence of controlled clinical trials. Ampicillin and penicillin MIC90 values were 64 micrograms/ml and 80 U/ml respectively which essentially confirmed the resistance observed by the disk method. All the P. bivia isolates were beta-lactamase producers, hence the resistance of these isolates to penicillin and ampicillin. It is conceivable that in infections with monoculture of P. bivia, agents like tetracycline, or erythromycin may still be a useful alternative for treatment.

German multicentre study on the in vitro susceptibility of Bacteroides species. The German Bacteroides Study Group

In 1990 the first German multicentre study on the in vitro susceptibility of Bacteroides species was completed. Employing a commercially prepared microbroth dilution assay, nine participating institutions evaluated approximately 100 consecutive isolates of Bacteroides species from relevant clinical specimens. A total of 911 strains (449 Bacteroides fragilis, 201 Bacteroides thetaiotaomicron, 79 Bacteroides ovatus, 78 Bacteroides vulgatus, 77 Bacteroides distasonis, 25 Bacteroides uniformis, 2 others) were tested. Most of the isolates came from surgical patients (72%); other sources included gynaecological patients (9%) and medical patients (5%). Seventy-eight percent of the anaerobes were found in mixed culture together with at least one aerobic organism (Escherichia coli 36%, streptococci 15%, or enterococci 13%), while in 22% of the cases, the anaerobes were the only bacteria grown from the specimens. The results showed that many of the strains were potent beta-lactamase producers (as judged by resistance to amoxicillin). However, all but one of the isolates demonstrated susceptibility in vitro when clavulanic acid was added to amoxicillin or ticarcillin. At the same time, 13% of the organisms were resistant to mezlocillin, 5% to cefoxitin and 4% to clindamycin. Three strains were reported resistant to imipenem and one strain to metronidazole.

Metronidazole-resistant Bacteroides fragilis wound infection

We report the isolation of metronidazole-resistant Bacteroides fragilis from a post-operative wound abscess in a 72-year-old woman who had not been treated with metronidazole during the preceding 9 months. The case illustrates the need for caution when identifying anaerobes on the basis of metronidazole sensitivity.

Genetic and molecular analysis of pIP417 and pIP419: Bacteroides plasmids encoding 5-nitroimidazole resistance

This report describes a genetic and molecular analysis of two transferable Bacteroides plasmids, pIP417 and pIP419, which carry genetic determinants conferring low-level resistance to 5-nitroimidazoles. The restriction endonuclease cleavage sites for each plasmid were localized. The NiR genetic determinants of pIP417 and pIP419 plasmids have been cloned into the Bacteroides cloning vector pBI191 (C.J. Smith, J. Bacteriol. 164, 294-301, 1985) as PvuII and Sau3A fragments, respectively. Both inserts had different restriction sites and did not cross-hybridize by Southern blot analysis. Genetic data obtained by cloning into pBI191 clearly show that the PvuII-generated fragments A (Rep) and B (Mob) of pIP417 are involved in plasmid replication and transfer, respectively. Although encoding resistance to the same antibiotic, both plasmids appeared different with regard to the 5-nitroimidazole resistance and replication genetic determinants. However, they share a homology in a region involved, at least in one case, in plasmid transfer. Considering the spontaneous high level of resistance to 5-nitroimidazole in Escherichia coli, this work, based on direct gene cloning into Bacteroides, demonstrates the value of such an approach.

Changes in the susceptibility of Bacteroides fragilis group organisms to various antimicrobial agents 1979-1989

The in vitro activity of metronidazole, chloramphenicol, clindamycin and 11 beta-lactam antibiotics against 135 clinical isolates of the Bacteroides fragilis group was compared. In addition, changes in the resistance patterns of members of the Bacteroides fragilis group isolated at the Hospital Universitario San Carlos in Madrid, Spain, between 1979 and 1989 were documented. The most active beta-lactam drugs were imipenem and beta-lactamase inhibitor combinations. In 1989, however, two strains were found to be resistant to imipenem and to all other beta-lactam agents tested. There was no emergence of resistance to metronidazole. Chloramphenicol was very effective: only one resistant strain was detected in 1979 and no chloramphenicol-resistant isolates were found during the rest of the study period. An outbreak of clindamycin resistance was noted in 1982, and the first cefoxitin resistant strains were recovered in 1985. The changing patterns of susceptibility of anaerobic bacteria to antimicrobial agents and the emergence of Bacteroides fragilis strains resistant to new beta-lactam agents suggest that periodic antimicrobial susceptibility tests should be performed in order to guide the selection of antimicrobial agents for therapy.

Review article: metronidazole and the anaerobic gut flora

Metronidazole is a nitro-imidazole drug which was discovered nearly 30 years ago. Metronidazole has remained the mainstay of anti-anaerobic therapy following a change observation that its activity included anaerobic organisms. The predominant human reservoir of these anaerobic organisms is the gastrointestinal tract. In this review, the anaerobic flora and their pathogenic potential are described. The main characteristics of metronidazole and the role of selective decontamination of the gastrointestinal tract are also discussed.

Metronidazole resistant Bacteroides fragilis infection of a prosthetic hip joint

A case of infection involving a prosthetic joint in a patient with adult Still's Disease is described. The causative organism was a strain of Bacteroides fragilis which was resistant to metronidazole. The rarity of this occurrence is emphasised. Diagnostic difficulties which arose are described and the problems encountered with therapy discussed.

Susceptibility to five antimicrobial agents of strains of the Bacteroides fragilis group isolated in Brazil

The in vitro activity of metronidazole, chloramphenicol, clindamycin, cefoxitin, and carbenicillin was tested by an agar dilution method against 228 strains of the Bacteroides fragilis group isolated from human intestinal microbiota during 1981 and 1982. All the strains were susceptible to metronidazole. Resistance rates for chloramphenicol, clindamycin, cefoxitin, and carbenicillin were 2, 37, 21, and 13%, respectively.

Metronidazole: a double-blind trial in untreated human periodontal disease

The effect of metronidazole was assessed in 20 subjects with untreated periodontitis, in a double-blind trial. Subjects received a 1-week course of drug or placebo immediately after baseline examination, and were reassessed 1 month and 3 months later. A 0.25 N constant force probe and darkfield microscopy were used. At 1 month, a difference was observed between the deepest individual pockets in experimental and control groups, with regard to change in probing depth and the proportions of non-motile organisms. At 3 months, differences were observed in 1-3 mm baseline pockets, where a greater proportion of control pockets increased in probing depth, and in 4-6 mm pockets, where a greater proportion of baseline bleeding pockets had ceased to bleed in the experimental group. The differences were of limited clinical significance when compared to studies where metronidazole was used as an adjunct to other therapy, and the results were equivocal with regard to the specific plaque hypothesis, since inhibition of connective tissue attachment loss was not unambiguously demonstrable.

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.

Susceptibility of bacteroides species to metronidazole during treatment of patients with Crohn's disease and healthy individuals

Patients with Crohn's disease are usually treated with metronidazole for long periods with the risk of selecting resistant bowel bacteria. This could have serious clinical and epidemiological implications, especially with regard to Bacteroides fragilis which is the most common cause of severe anaerobic infections. An investigation was therefore undertaken with the use of an agar dilution method to examine the susceptibility to metronidazole of B. fragilis and other common Bacteroides species isolated from faeces before, during and after treatment of patients with Crohn's disease amd healthy individuals. Before treatment, or during medication with sulphasalazine, 88% of the strain tested showed minimum inhibitory concentrations (MIC values) of less than or equal to 1 mg/l and none had MIC values greater than 4 mg/l. The MIC values were significantly increased during metronidazole treatment (p less than 0.02), with a maximum value of 8 mg/l. No significant changes were noted for the MIC values of strains isolated before and 1 week to 1 month after treatment (p greater than 0.1). It is concluded that it is important to examine the MIC values of bacteroides isolates from patients with long term treatment with metronidazole, especially with regard to treatment failure.

Interaction of metronidazole with resistant and susceptible Bacteroides fragilis

The kinetics of the lethal action of metronidazole and the formation of acetamide have been studied in a strain of Bacteroides fragilis which is relatively resistant to metronidazole. As with a susceptible strain of B. fragilis, the data are consistent with a model in which a labile intermediate in metronidazole metabolism interacts either with water to form acetamide or with a bacterium to cause its death. Although the relatively resistant strain grows more slowly than the susceptible one and is killed less rapidly by metronidazole, the resistant strain displays the same relationship between the lethal action of metronidazole and metronidazole metabolism to acetamide. The relatively resistant strain, like the susceptible one, has an enhanced lethal response to metronidazole in the presence of a strain of Escherichia coli. The results suggest that the proposed labile reactive intermediate of metronidazole forms more slowly in the resistant strains.

Metronidazole (Flagyl IV, Searle)

Metronidazole is a narrow spectrum antibiotic with undoubted efficacy against common anaerobic bacteria; resistance is unusual. Therapeutic concentrations of the drug are attained throughout most body compartments after either oral or intravenous administration. The limited side effects of metronidazole are generally tolerable, transient, or reversible. Clinically, metronidazole is as effective as clindamycin and probably chloramphenicol against anaerobes. It has a definite advantage over clindamycin in CNS infections since clindamycin does not penetrate the CSF well. Metronidazole has no irreversible hematologic toxicities, nor has pseudomembranous colitis been definitely attributed to intravenous use of the drug. Metronidazole may replace chloramphenicol for use in anaerobic infections since it lacks the predictable hematologic toxicity of the latter drug. It should also be useful in patients who fail to respond to clindamycin or who develop pseudomembranous colitis while receiving clindamycin. Problems with metronidazole include a complicated preparation procedure, and the high cost of the drug. The single major drawback to the use of metronidazole is uncertainty about its carcinogenic potential in humans. Metronidazole is carcinogenic in animals and mutagenic in vitro, but has not increased the incidence of cancer in humans followed for relatively short periods. Thus, the risk appears to be small. Still, the question will not be resolved for years because of the long latency periods involved in carcinogenesis. Until that time, metronidazole should be used conservatively.