Metronidazole Susceptibility Research Articles

Metronidazole- and clarithromycin-resistant Helicobacter pylori in dyspeptic patients in western Sydney as determined by testing multiple isolates from different gastric sites.

It is unknown whether antibiotic susceptibility testing of antral isolates alone is representative of Helicobacter pylori susceptibility. We aimed to determine: (i) the prevalence of metronidazole- and clarithromycin-resistant strains in infected dyspeptic patients; and (ii) whether there is consistency in the susceptibility to metronidazole and clarithromycin among isolates cultured from different gastric sites. Antral, body and fundus biopsies were taken from 242 consecutive patients and cultured on blood agar under micro-aerophilic conditions for 5-7 days. Isolates from 66 patients (13 had one, 15 had two and 38 had three isolates) were tested for susceptibility to metronidazole and clarithromycin using previously validated disc diffusion tests. Of the 66 patients, 42 (64%) had strains resistant to metronidazole while four (6.1%) had clarithromycin-resistant strains. The prevalence of metronidazole resistance was not significantly different between men and women (65% vs 60%) or across different age groups. In five (9.4%) of the 53 patients with multiple isolates, discrepant results for metronidazole susceptibility were observed: susceptible antral and body isolates but resistant fundus isolates in two cases and susceptible antral isolates but resistant body and fundus isolates in the others. Clarithromycin susceptibilities were consistent among the isolates cultured from different gastric sites in all patients. It is concluded that metronidazole-resistant strains of H. pylori are common while clarithromycin-resistant strains are rare. Metronidazole susceptibility testing of antral isolates does not appear to be representative of isolates from the body and fundus in a subset of patients.

Simplified 10-day bismuth triple therapy for cure of Helicobacter pylori infection: experience from clinical practice in a population with a high frequency of metronidazole resistance.

To evaluate the cure rate of Helicobacter pylori infection, including the impact of in vitro metronidazole resistance (M-R), and the side effects of a simplified 10-day bismuth triple therapy in routine clinical practice. From September 1995 to March 1996, 248 consecutive H. pylori-positive patients received 10 days of bismuth subnitrate 150 mg, oxytetracycline 500 mg, and metronidazole 400 mg, all t.id. Before treatment, upper endoscopy, including biopsy specimens for microbiological analysis and IgG serology were performed. M-R was found in 45% of females and 36% of males. At least 2 months after treatment, H. pylori status was assessed by the 14C urea breath test (n = 131), endoscopy (n = 37), urea breath test and endoscopy (n = 63), or solely by IgG serology (n = 7). Ten patients withdrew. IgG serology was performed again after 1 yr. H. pylori infection was cured in 205 patients: 86% by all-patients-treated analysis and 83% by intention-to-treat analysis. When patients were classified according to pretreatment metronidazole susceptibility, cure of infection was achieved in 76% of females harboring M-R strains versus 96% of those with sensitive strains (p = 0.002) and in 81% versus 88% (p = 0.34) of males with M-R versus sensitive strains, respectively. Twelve patients (5 %) had to stop treatment prematurely because of severe side effects, but eight of them were treated successfully. One case of H. pylori infection (0.6 %) was detected at 1-yr follow-up. Ten-day bismuth triple therapy t.i.d. was effective in curing H. pylori infection in the context of routine clinical practice. The efficacy was reduced in females harboring M-R strains.

Characterisation of a Transposon-induced Pleiotropic Mutant ofClostridium acetobutylicumP262

Transposon-induced metronidazole resistance was used as a selection system for the isolation ofClostridium acetobutylicumP262 mutants with altered electron transport pathways. The metronidazole resistant transconjugant of interest, mutant 3R, displayed resistance to DNA damaging agents, UV and bleomycin, and harboured a single transposon insertion within a structural gene, designatedsum(susceptibility tometronidazole). Thesumgene encoded a 334 amino-acid protein, with 36% identity and 57–58% similarity at the amino acid level to two archaebacterial protein sequences which appear to represent a class of uncharacterised reductase enzymes. Physiological studies of mutant 3R revealed a number of pleiotropic characteristics which included enhanced autolysin activity, increased motility, impaired clostridial cell formation, and resistance to the toxic tripeptide analogue, bialaphos. The introduction of thesumgene in multiple copies on a plasmid vector into the related strainClostridium beijerinckiiNCIMB 8052, resulted in inhibition of cell division, motility and autolysin activity. Thesumgene appears to be a member of a new subgroup of activases with reducing activity, which may control a regulon affecting different stationary phase processes such as clostridial differentiation and sporulation inC. acetobutylicumP262. The metronidazole resistant phenotype of thesummutant can be attributed to an increased capacity for DNA repair.

Helicobacter pylori populations in Peruvian patients.

Helicobacter pylori is an extremely diverse species. The characterization of strains isolated from individual patients should give insights into colonization and disease mechanisms and bacterial evolution. We studied H. pylori isolates from patients in the Japanese-Peruvian Polyclinic in Lima, Peru, by determining metronidazole susceptibility or resistance and by random amplified polymorphic DNA (RAPD) fingerprinting (a measure of overall genotype). Strains isolated from several biopsy specimens from each of 24 patients were studied. Both metronidazole-susceptible and -resistant strains were isolated from 13 patients, whereas strains of more than one RAPD type were isolated from only seven patients. We propose that the homogeneity in RAPD fingerprints for strains isolated from most persons reflects selection for particular H. pylori genotypes during chronic infection in individual hosts and the human diversity in traits that are important to this pathogen. Carriage of related metronidazole-resistant and -susceptible strains could reflect frequent metronidazole use in Peru and alternating selection for resistant and susceptible phenotypes during and after metronidazole therapy.

Detection of Bifidobacteriumspecies by enzymatic methods and antimicrobial susceptibility testing

Twenty four strains of bifidobacteria and 16 strains of lactobacilli were examined for activities of fructose-6-phosphate phosphoketolase, a-galactosidase, and a-glucosidase and the resistance to metronidazole and mupirocin over 24 h. The detection of a-galactosidase, a-glucosidase, and metronidazole susceptibility gave unclear results. The mupirocin susceptibility using disc diffusion method (200 mg mupirocin per disc) had the same validity as the detection of fructose-6-phosphate phosphoketolase – the key enzyme of the bifidobacteria carbohydrate metabolism. While 24 bifidobacteria strains tested were mupirocin-resistant, 16 lactobacilli strains were susceptible. Hence, the resistance to mupirocin may serve as a simple criterion distinquishing Bifidobacterium spp. from Lactobacillus spp.

Prevalence of metronidazole resistant Helicobacter pylori strains among Chinese peptic ulcer disease patients and normal controls in Hong Kong.

A study was conducted to evaluate the prevalence of metronidazole resistant Helicobacter pylori strains among the Chinese in Hong Kong. The efficacy of the triple therapy that contains metronidazole as one of the anti-microbial agents in eradication of the metronidazole susceptible and the metronidazole resistant strains was also assessed. Culture for H pylori was attempted from antral biopsy specimens of 70 peptic ulcer and 51 control subjects. Successfully cultured H pylori strains were tested for metronidazole susceptibility. Twenty six peptic ulcer disease subjects who had received a course of triple therapy were also reassessed four to six weeks later for successful eradication of H pylori infection. H pylori was successfully cultured from antral biopsy specimens in 69 of 80 (86%) of the infected subjects. The overall metronidazole resistance rate was 53.5% (37 of 69). There was a significantly higher metronidazole resistance rate among H pylori isolates from the asymptomatic controls (20 of 25) than the peptic ulcer disease subjects (17 of 44) (p = 0.0007). Twenty three of 32 (73%) women and 14 of 37 (38%) men harboured the metronidazole resistant strains. There was no sex or age difference as far as the prevalence of metronidazole resistant strains were concerned within each study group. Pre-treatment metronidazole susceptible H pylori were significantly more likely to respond to the triple therapy used than those with the metronidazole resistant ones (14 of 15 v five of 10) (p = 0.021).

Metronidazole resistance: a predictor of failure of Helicobacter pylori eradication by triple therapy.

Triple therapy (bismuth and two antibiotics) will eradicate Helicobacter pylori infection in 70-90% of subjects. Treatment failure has been attributed to patient compliance and antimicrobial drug resistance. The aim of this study was to examine factors influencing the eradication of H. pylori following triple therapy. Thirty seven subjects with H. pylori cultured from antral biopsies were treated with colloidal bismuth subcitrate (120 mg qid for 2 weeks), metronidazole (400 mg tid for 1 week) and amoxycillin (500 mg tid for 1 week). Pretreatment isolates of H. pylori were tested for metronidazole susceptibility by agar dilution according to the National Committee for Clinical Laboratory Standards guidelines. Factors including age, sex, clinical diagnosis and metronidazole resistance were evaluated in relation to H. pylori. The overall metronidazole resistance was 32%. Metronidazole resistant strains were more frequent in females, with a resistance rate of 54%. Helicobacter pylori eradication occurred in 68% of patients with a metronidazole susceptible stain and only 17% of patients with a metronidazole resistant strain (P < 0.03). Helicobacter pylori eradication is dependent upon susceptibility to metronidazole. This data would support the role for routine metronidazole susceptibility testing using appropriate standardized methods when triple therapy is to be considered.

Only four days of quadruple therapy can effectively cure Helicobacter pylori infection

To determine whether 4 days of quadruple therapy using bismuth, tetracycline and metronidazole combined with omeprazole is effective treatment for Helicobacter pylori infection. Non-ulcer dyspepsia, as well as chronic peptic ulcer patients with biopsy-proven H. pylori infection received 4 days of quadruple therapy. They were pretreated with 3 days of omeprazole. At least 5-6 weeks later, endoscopy was repeated with 10 biopsies for urease test, histology and culture to establish cure of infection. None of the 54 patients included was lost to follow-up but two had a 14C-urea breath test instead of endoscopy. Side-effects did not interfere with compliance. Forty-nine out of 54 patients (91%; 95% CI: 80-97%) were cured. Metronidazole susceptibility data were available from 43 pre-treatment isolates. Of these 38/40 (95%) with a metronidazole-sensitive strain, and one of three with a metronidazole-resistant strain were cured. Four days of quadruple therapy after omeprazole pre-treatment is a feasible, well tolerated, and effective treatment for H. pylori infection, especially in those carrying a metronidazole-sensitive strain. It seems that in quadruple therapy, cure rate and treatment duration have a non-linear relation. Our results need confirmation, but for patients suffering from side-effects with the 7-day regimen stopping treatment after 4 days is justified.

Role of the Reference Laboratory in Susceptibility Testing of Anaerobes and a Survey of Isolates Referred from Laboratories in England and Wales During 1993–1994

Methods for susceptibility testing of anaerobes and their patterns of susceptibility remain debatable. The responsibilities of the Public Health Laboratory Service Anaerobe Reference Unit include identification of referred isolates; development of methods; monitoring of the occurrence of anaerobic infections; performance of taxonomic studies; maintenance of a reference collection; and monitoring of the antibiotic susceptibility of clinical isolates. Selection of antibiotics for anaerobic infections is often based upon the findings of batch tests on referred isolates in specialized centers. If the information gathered is to be a reliable guide, there should be a structured rolling survey of clinical isolates from a wide range of hospital laboratories. Strains of Bacteroides, Prevotella, Clostridium, and Propionibacterium species and peptostreptococci referred to the Anaerobe Reference Unit during 1993-1994 were tested with the Etest to determine the minimum inhibitory concentrations of metronidazole, penicillin, amoxicillin/clavulanate, cefoxitin, imipenem, erythromycin, clindamycin, and tetracycline. Metronidazole susceptibility (generally) and penicillin resistance among Bacteroides species were consistent findings, and most isolates were susceptible to imipenem, but other results varied between and within species.

Standardization of disk diffusion test and its clinical significance for susceptibility testing of metronidazole against Helicobacter pylori

Susceptibilities of 121 clinical Helicobacter pylori strains to metronidazole were determined by both a 5-micrograms metronidazole disk diffusion test and a plate dilution method in duplicate and after different periods of incubation. The distribution of MICs of metronidazole against H. pylori among the strains was found to be bimodal. The diameters of inhibitory zones obtained by the disk diffusion test and the MICs obtained by the plate dilution method correlated well, especially after 4 days of incubation (r = 0.77). An inhibitory zone diameter of 20 mm was found to correspond to a MIC of 8 micrograms/ml and is recommended as a suitable zone for differentiating susceptibility and resistance with a 5-micrograms metronidazole disk. Three interpretive categories of susceptibility results were defined; strains with inhibitory zone diameters of more than 26 mm were defined as susceptible (MIC, < 4 micrograms/ml), strains with zone diameters of 20 to 26 mm were deemed intermediate (MIC, 4 to 8 micrograms/ml), and those with zone diameters of less than 20 mm were deemed resistant (MIC, > 8 micrograms/ml). Furthermore, 76 H. pylori-positive patients with duodenal ulcers or nonulcer dyspepsia were treated with a 1 week of triple therapy (colloidal bismuth subcitrate, metronidazole, and tetracycline). H. pylori strains were isolated before treatment from antral biopsies from those patients, and the metronidazole susceptibilities of the strains were determined by the disk diffusion test. H. pylori status was evaluated again 4 weeks after completion of treatment. The eradication rates for susceptible, intermediate, and resistant strains were 95.9% (47 of 49), 62.5% (5 of 8), and 52.6% (10 of 19), respectively. It is included that the 5-micrograms disk diffusion test is easy to perform and gives final results similar to those of the plate dilution method. The three interpretive categories of susceptibility may be of benefit for clinical choice of chemotherapy in eradicating H. pylori.

Impact of Metronidazole Resistance on the Eradication of Helicobacter pylori

We investigated how often Helicobacter pylori was resistant to metronidazole before treatment in 283 H. pylori positive patients and whether the in vitro susceptibility to metronidazole could predict the clinical outcome of several drug regimens containing metronidazole. Metronidazole susceptibility was tested using either disc diffusion or plates containing metronidazole. Metronidazole-resistant strains were found in 41% of patients before their first anti-H. pylori treatment course. In patients with metronidazole-susceptible isolates, eradication was achieved in 74% after colloidal bismuth subcitrate (CBS) with metronidazole and in 91% after triple therapies consisting of CBS, metronidazole and amoxicillin or tetracycline. Dual therapies of 1 or 4 weeks' duration and triple therapies lasting 1 week were ineffective in patients with metronidazole-resistant strains. A 4-week course of triple therapy could still eradicate H. pylori in 68% of patients with metronidazole-resistant strains, but at the cost of significant side-effects. It is concluded that metronidazole resistance is common and is generally associated with a poor outcome of anti-H. pylori therapies containing metronidazole. Alternative drug schedules are urgently needed for patients with metronidazole-resistant H. pylori.

Metronidazole susceptibility testing for Helicobacter pylori: comparison of disk, broth, and agar dilution methods and their clinical relevance

Since the methods for metronidazole susceptibility testing of Helicobacter pylori have not been standardized or validated, we compared three methods that are used to test the metronidazole susceptibilities of 25 isolates of H. pylori. Specifically, we examined the methods of Steer's replicator agar dilution, tube broth microdilution, and modified Kirby-Bauer disk diffusion. The metronidazole disk zone sizes obtained by the disk diffusion method correlated well (r = 0.74) with the MICs obtained by the agar dilution method. Afterward, the disk diffusion method was used to characterize the metronidazole susceptibilities of 44 isolates of H. pylori. Dual therapy (bismuth and metronidazole) proved to be highly effective against metronidazole-susceptible strains (81.6% eradication rate) but fared poorly against resistant strains (16.7% eradication rate; P < 0.01). Using agar dilution testing, we validated the modified Kirby-Bauer disk diffusion method for metronidazole susceptibility testing of H. pylori and conclude that it is practical, accurate, and clinically applicable.

Biotype and molecular fingerprints of metronidazole-resistant strains of Helicobacter pylori from antral gastric mucosa.

Biotypes, ribosomal RNA gene restriction patterns (ribopatterns), whole-cell protein patterns and plasmid profiles of paired Helicobacter pylori isolates from 17 patients were examined. Each pair comprised a pre- and a post-treatment isolate; nine of the 17 post-treatment isolates were obtained after treatment with tripotassium dicitrate bismuthate (De-Nol) and metronidazole. All strains of H. pylori had identical biotypes, but exhibited diversity between pairs in their molecular fingerprints. Each of the 17 strain pairs had unique ribopatterns; the pre- and post-treatment isolates in most pairs (16 of 17) were similar or identical, irrespective of metronidazole susceptibility. DNA subtype variants were detected in three patient sets. Although nine post-treatment isolates had acquired resistance to metronidazole, most (six of nine) resembled the pre-treatment isolates in their ribopattern, protein and plasmid profiles. No significant correlation was observed between metronidazole resistance and plasmid content in these H. pylori isolates. Emergence of post-treatment metronidazole-resistant isolates of H. pylori was associated only rarely with colonisation by a novel strain or acquisition of a plasmid and, in most patients, probably resulted from spontaneous emergence of resistance in the original infecting strain.

In vitro susceptibility of Trichomonas vaginalis to metronidazole and treatment outcome in vaginal trichomoniasis.

We have identified Trichomonas vaginalis strains resistant in vitro to metronidazole, especially under aerobic conditions. Since little is known about the relationship of treatment outcome to metronidazole susceptibility of T. vaginalis, we studied the aerobic and anaerobic susceptibility to metronidazole of 310 clinical isolates of T. vaginalis. Of 199 patients with known outcomes after metronidazole treatment for vaginal trichomoniasis, the geometric mean minimal lethal concentration (MLC) under aerobic conditions for trichomonads associated with cases cured by a single 2-g dose was 24.1 micrograms/ml (n = 146), while that of treatment-resistant isolates (n = 53) was 195.5 micrograms/ml. The corresponding mean anaerobic MLC values were 1.6 and 5.05 micrograms/ml, respectively. The average aerobic:anaerobic MLC ratio was about twofold higher for the resistant isolates. Treatment resistance was more frequent at aerobic MLC values of greater than 25 micrograms/ml or anaerobic values of greater than 1.6 micrograms/ml. Although there was overlap of the metronidazole susceptibility distribution of susceptible and resistant isolates, significant resistance to treatment was common when isolates of T. vaginalis had aerobic MLC values of greater than 100 micrograms/ml or anaerobic MLC values of greater than 3.1 micrograms/ml.

Characterisation of anaerobic curved rods (Mobiluncus spp.) isolated from the urogenital tract.

Thirty-two strains of anaerobic curved rods isolated from vaginal secretions and one isolated from seminal fluid were examined. Growth of all strains on solid media was superior to growth in liquid media, and at 37 degrees C they grew both anaerobically and in O2 5% in N2; they also grew anaerobically at 33 degrees C but not at 42 degrees C. No growth factors were identified, but strains grew more profusely at pH values above 5 X 0. The strains were screened in 80 biochemical tests, and for their susceptibility to 30 different antimicrobial agents. Most of the tests did not differentiate between the strains, but they were divided into four groups on the basis of cell morphology, metronidazole susceptibility, beta-galactosidase activity and arginine and hippurate hydrolysis. Group 1 consisted of 19 strains conforming to the species M. curtisi; group 2 consisted of five strains conforming to the species M. mulieris; group 3 consisted of five strains that resembled M. curtisi morphologically, and group 4 consisted of four strains that resembled M. mulieris morphologically, but the strains in the latter two groups reacted differently in at least one of the three major differential biochemical tests. Of three strains of M. curtisi and three of M. mulieris chosen at random, one of M. mulieris had a SDS-PAGE and fast-protein liquid chromatography protein profile indistinguishable from that of M. curtisi. We conclude that further efforts are required to clarify the taxonomic status of the genus Mobiluncus.

In vitro drug susceptibility and doses of metronidazole required for cure in cases of refractory vaginal trichomoniasis.

There are currently no laboratory or clinical guidelines for the identification and treatment of disease caused by metronidazole-resistant strains of Trichomonas vaginalis. Fifty-three isolates of T. vaginalis from cases of refractory vaginitis in the United States (26 states) and Canada were tested for aerobic and anaerobic metronidazole susceptibility, and after various dosages of metronidazole, the therapeutic outcomes were evaluated for 31 of these cases. The mean aerobic metronidazole susceptibility of these isolates was 195.5 micrograms/ml (range, 12.5-greater than 1,000), which was about eightfold higher than that seen in isolates that were not resistant to metronidazole. The mean anaerobic susceptibility was 5 micrograms/ml (range, 1.6-25), which was about threefold higher than that of isolates from nonresistant strains. The average aerobic-to-anaerobic ratio of metronidazole susceptibility in the highly resistant isolates was more than 3.5-fold greater than that seen in the nonresistant isolates. White women accounted for 88% of the resistant infections. Of 31 cases that were re-treated and monitored, the highest average dose that failed to achieve a cure was 2.1 g of metronidazole/day given over an eight-day period; 27 (87%) of 31 cases were ultimately cured with an average dosage of 2.6 g of metronidazole/day given over a mean period of nine days. Resistance to treatment with metronidazole varied from mild to severe, and the resistance was occasionally more severe than the susceptibility values indicate.

Metronidazole-resistant clinical isolates of Trichomonas vaginalis have lowered oxygen affinities

Oxygen affinities of metronidazole susceptible and resistant isolates of the parasitic flagellate protozoon Trichomonas vaginalis were determined by mass spectrometric methods. Apparent O 2 K m values for the respiration of non-proliferating cell suspensions were about 10-fold higher for metronidazole resistant strains than for the susceptible strains C1-NIH or NYH-286. Simultaneous monitoring of hydrogen evolution in the presence of increasing O 2 tensions enabled apparent K i values for H 2 to be determined; and this function was independent of metronidazole susceptibility. Apparent O 2 affinities of the hydrogenosomal and non-sedimentable fractions were determined for the strains CDC 85 (metronidazole resistant) and C1-NIH, which showed the deficiency in the O 2 scavenging capacity by the resistant strain to be associated with the hydrogenosome-containing fraction.

Distribution of metronidazole susceptibility factors in obligate anaerobes.

The distribution of metronidazole susceptibility factors in clinically important bacteria was studied to determine why obligate anaerobes were susceptible to metronidazole and facultative ones were not. With obligate and facultative anaerobes, nitroreductase activity, metronidazole uptake, pyruvate:ferredoxin oxidoreductase activity and drug susceptibility were estimated. Nitroreductase activity was distributed in the obligate anaerobes and some facultative ones such as Streptococcus faecalis. Propionibacterium acnes, Bifidobacterium adolescentis, Actinomyces israelii and most of the facultative anaerobes had no such activity. Bacteria with nitroreductase activity were able to absorb the drug. Pyruvate:ferredoxin oxidoreductase activity was distributed in all of the obligate anaerobes, whereas Prop. acnes, Bif. adolescentis, A. israelii and all of the facultative anaerobes had no such activity. Distribution of metronidazole susceptibility was the same as that of pyruvate:ferredoxin oxidoreductase activity. Therefore metronidazole may selectively inhibit obligate anaerobes with pyruvate:ferredoxin oxidoreductase activity. Nitroreductase activity may be necessary for drug uptake as well.

Metabolism and metronidazole uptake in Trichomonas vaginalis isolates with different metronidazole susceptibilities.

Three Trichomonas vaginalis isolates with low in vivo susceptibilities to metronidazole (95% curative dose, greater than 3 X 100 mg kg-1 in subcutaneous infections in mice) were compared with strain ATCC 30001 and with four isolates exhibiting high in vivo susceptibilities (95% curative dose, less than 3 X 15 mg kg-1). Activity of pyruvate:ferredoxin oxidoreductase, anaerobic fermentation, and anaerobic intracellular accumulation of [14C]metronidazole label showed no significant isolate-dependent differences which could be correlated with drug susceptibility. The results suggest that processes providing electrons for metronidazole activation are not defective in the resistant strains. Aerobiosis, known to inhibit the antimicrobial action of metronidazole, inhibited accumulation of label more strongly in resistant isolates than in susceptible ones. No differences were detected, however, between resistant and susceptible isolates in respiration, aerobic fermentation, and the specific activity of NADH and NADPH oxidases, the main terminal oxidases of T. vaginalis. These findings suggest that the production of electrons is not diminished under aerobic conditions. The inhibitory effect of aerobic conditions on metronidazole activation, possibly due to competition for the electrons, is markedly enhanced in the resistant isolates compared to the susceptible ones. The mechanism of this effect, however, remains unknown.

Pyruvate dehydrogenase activity and metronidazole susceptibility in Bacteroides fragilis.

Pyruvate dehydrogenase activity was present and comparable (giving a colour change of 202-673 od units/min/g protein in our assay system) in two clinical strains of Bacteroides fragilis exhibiting reduced sensitivity to metronidazole (MIC 2-8 mg/l at 48 h) in a resistant strain (Ingham's) of Bact. fragilis (MIC 64/mg/l) and in two sensitive strains of Bact. fragilis (MIC 0.5 mg/l). Pyruvate dehydrogenase activity was not detected in a capnocytophaga strain (sensitive to 50 micrograms disc of metronidazole), in Escherichia coli or in 'Bact. fragilis' strain (Britz) AM24s (MIC greater than 64 mg/l). Further testing of AM24s strain indicated that it was not Bact. fragilis but a facultatively anaerobic Gram-negative rod. Pyruvate dehydrogenase activity therefore, is similar in sensitive and resistant strains of Bact. fragilis and is unlikely to be related to the mechanism of resistance to metronidazole.