gyrA mutations associated with quinolone resistance in Bacteroides fragilis group strains

Case Report: The effect of intravenous and oral antibiotics on the gut microbiome and breath volatile organic compounds over one year

BACKGROUND

Antimicrobial resistance (AMR) is a global concern and better understanding of the gut microbiome, a known 'amplifier' of AMR, may allow future clinicians to tailor therapy to minimise this risk and offer a personalised medicine approach. To examine the gut microbiome, patients are required to provide faecal samples; more convenient and cheaper solutions need to be found.

METHODS

As part of a pilot study looking at how routes of administration affect the gut microbiome in NHS patients undergoing routine clinical management for infections, we hypothesised that effects on the gut microbiome varied with the route and metabolism of antibiotic used, and these changes may be reflected in breath metabolites. We present a case report of a patient with an unusual clinical history, alongside breath metabolite and gut microbiome data taken before, during and after antibiotic therapy over a period of one year.

RESULTS

We noted a shift in the dominant Bacteroides strain in the patient's gut microbiome between pre- and post-therapy samples, along with an alteration in the composition of breath metabolites.

CONCLUSIONS

This study provides a framework for similar future work and highlights the need for further research on the relationships between changes in microbial gut communities and antimicrobial exposure, patient clinical status, and the metabolites of human breath.

Antibiotic Resistance and Mechanisms of Pathogenic Bacteria in Tubo-Ovarian Abscess

A tubo-ovarian abscess (TOA) is a common type of inflammatory lump in clinical practice. TOA is an important, life-threatening disease, and it has become more common in recent years, posing a major health risk to women. Broad-spectrum antimicrobial agents are necessary to cover the most likely pathogens because the pathogens that cause TOA are polymicrobial. However, the response rate of antibiotic treatment is about 70%, whereas one-third of patients have poor clinical consequences and they require drainage or surgery. Rising antimicrobial resistance serves as a significant reason for the unsatisfactory medical outcomes. It is important to study the antibiotic resistance mechanism of TOA pathogens in solving the problems of multi-drug resistant strains. This paper focuses on the most common pathogenic bacteria isolated from TOA specimens and discusses the emerging trends and epidemiology of resistant Escherichia coli, Bacteroides fragilis, and gram-positive anaerobic cocci. Besides that, new methods that aim to solve the antibiotic resistance of related pathogens are discussed, such as CRISPR, nanoparticles, bacteriophages, antimicrobial peptides, and pathogen-specific monoclonal antibodies. Through this review, we hope to reveal the current situation of antibiotic resistance of common TOA pathogens, relevant mechanisms, and possible antibacterial strategies, providing references for the clinical treatment of drug-resistant pathogens.

Case Report: The effect of intravenous and oral antibiotics on the gut microbiome and breath volatile organic compounds over one year

BACKGROUND

Antimicrobial resistance (AMR) is a global concern and better understanding of the gut microbiome, a known 'amplifier' of AMR, may allow future clinicians to tailor therapy to minimise this risk and offer a personalised medicine approach. To examine the gut microbiome, patients are required to provide faecal samples; more convenient and cheaper solutions need to be found.

METHODS

As part of a pilot study looking at how routes of administration affect the gut microbiome in NHS patients undergoing routine clinical management for infections, we hypothesised that effects on the gut microbiome varied with the route and metabolism of antibiotic used, and these changes may be reflected in breath metabolites. We present a case report of a patient with an unusual clinical history, alongside breath metabolite and gut microbiome data taken before, during and after antibiotic therapy over a period of one year.

RESULTS

We noted a shift in the dominant Bacteroides strain in the patient's gut microbiome between pre- and post-therapy samples, along with an alteration in the composition of breath metabolites.

CONCLUSIONS

This study provides a framework for similar future work and highlights the need for further research on the relationships between changes in microbial gut communities and antimicrobial exposure, patient clinical status, and the metabolites of human breath.

Exploring the Genomic Diversity and Antimicrobial Susceptibility of Bifidobacterium pseudocatenulatum in a Vietnamese Population

Bifidobacterium pseudocatenulatum is a member of the human gut microbiota, and specific variants of B. pseudocatenulatum have been associated with health benefits such as improving gut integrity and reducing inflammatory responses. Here, we aimed to assess the genomic diversity and predicted metabolic profiles of B. pseudocatenulatum cells found colonizing the gut of healthy Vietnamese adults and children. We found that the population of B. pseudocatenulatum from each individual was distinct and highly diverse, with intraclonal variation attributed largely to a gain or loss of carbohydrate-utilizing enzymes. The B. pseudocatenulatum genomes were enriched with glycosyl hydrolases predicted to target plant-based nondigestible carbohydrates (GH13, GH43) but not host-derived glycans. Notably, the exopolysaccharide biosynthesis region from organisms isolated from healthy children showed extensive genetic diversity and was subject to a high degree of genetic modification. Antimicrobial susceptibility profiling revealed that the Vietnamese B. pseudocatenulatum cells were uniformly susceptible to beta-lactams but exhibited variable resistance to azithromycin, tetracycline, ciprofloxacin, and metronidazole. The genomic presence of ermX and tet variants conferred resistance against azithromycin and tetracycline, respectively; ciprofloxacin resistance was associated with a mutation(s) in the quinolone resistance-determining region (GyrA, S115, and/or D119). Our work provides the first detailed genomic and antimicrobial resistance characterization of B. pseudocatenulatum found in the Vietnamese population, which can be exploited for the rational design of probiotics. IMPORTANCE Bifidobacterium pseudocatenulatum is a beneficial member of the human gut microbiota. The organism can modulate inflammation and has probiotic potential, but its characteristics are largely strain dependent and associated with distinct genomic and biochemical features. Population-specific beneficial microbes represent a promising avenue for the development of potential probiotics, as they may exhibit a more suitable profile in the target population. This study investigates the underexplored diversity of B. pseudocatenulatum in Vietnam and provides more understanding of its genomic diversity, metabolic potential, and antimicrobial susceptibility. Such data from indigenous populations are essential for selecting probiotic candidates that can be accelerated into further preclinical and clinical investigations.

Effects of Yeast Mannan Which Promotes Beneficial Bacteroides on the Intestinal Environment and Skin Condition: A Randomized, Double-Blind, Placebo-Controlled Study

Yeast mannan (YM) is an indigestible water-soluble polysaccharide of the yeast cell wall. In vitro fecal fermentation studies showed that YM could exhibit a notable prebiotic effect. The aim of this randomized, double-blind, placebo-controlled study was to assess the efficacy of YM intake on the intestinal environment and skin condition. One hundred and ten healthy female subjects aged 30–49 years were supplemented with YM or placebo for eight weeks. Skin dryness was set as the primary endpoint. No side effects were observed during the study. Microbiota analyses revealed that YM intake selectively increased the relative abundance of Bacteroides thetaiotaomicron and Bacteroides ovatus compared to that by placebo. Feces and urine analyses showed that YM intake lowered the concentration of fecal p-cresol, indole, and skatole, and elevated urinal equol levels compared to those in placebo. Furthermore, YM supplementation ameliorated subjective skin dryness. This study suggests that YM intake could promote beneficial Bacteroides and improve the intestinal environment and skin condition.

Antimicrobial susceptibility and prevalence of resistance genes in Bacteroides fragilis isolated from blood culture bottles in two tertiary care hospitals in Japan

The in vitro susceptibilities of Bacteroides fragilis to antimicrobial agents, especially to carbapenem, are a major concern in the treatment of patients with bloodstream infections. In this study, 50 isolates of B. fragilis were obtained from positive blood bottles from 2014 to 2019 in Saitama, Japan. Their susceptibility to ampicillin/sulbactam was reduced to 70.0% compared with a previous report, whereas they were still sufficiently susceptible to piperacillin/tazobactam (94.0%). Five cfiA-positive isolates (5/50, 10.0%) were identified that were resistant to doripenem and meropenem, and two of them carried an insertion sequence located upstream of the cfiA-coding region. In particular, imipenem should be considered as a first-line carbapenem for the empirical treatment of B. fragilis infection because only insertion sequence and cfiA double-positive strains showed resistance to imipenem. Thirty-six percent of the isolates had a reduced minimum inhibitory concentration for moxifloxacin. In addition, metronidazole should still be considered as an active agent for B. fragilis because all isolates were susceptible to this antibiotic and the prevalence of the nim gene was low in Japan.

Antimicrobial Resistance in Clostridium and Brachyspira spp. and Other Anaerobes

This article describes the antimicrobial resistance to date of the most frequently encountered anaerobic bacterial pathogens of animals. The different sections show that antimicrobial resistance can vary depending on the antimicrobial, the anaerobe, and the resistance mechanism. The variability in antimicrobial resistance patterns is also associated with other factors such as geographic region and local antimicrobial usage. On occasion, the same resistance gene was observed in many anaerobes, whereas some were limited to certain anaerobes. This article focuses on antimicrobial resistance data of veterinary origin.

Identification and Antimicrobial Susceptibility Testing of Anaerobic Bacteria: Rubik's Cube of Clinical Microbiology?

Anaerobic bacteria have pivotal roles in the microbiota of humans and they are significant infectious agents involved in many pathological processes, both in immunocompetent and immunocompromised individuals. Their isolation, cultivation and correct identification differs significantly from the workup of aerobic species, although the use of new technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, whole genome sequencing) changed anaerobic diagnostics dramatically. In the past, antimicrobial susceptibility of these microorganisms showed predictable patterns and empirical therapy could be safely administered but recently a steady and clear increase in the resistance for several important drugs (β-lactams, clindamycin) has been observed worldwide. For this reason, antimicrobial susceptibility testing of anaerobic isolates for surveillance purposes or otherwise is of paramount importance but the availability of these testing methods is usually limited. In this present review, our aim was to give an overview of the methods currently available for the identification (using phenotypic characteristics, biochemical testing, gas-liquid chromatography, MALDI-TOF MS and WGS) and antimicrobial susceptibility testing (agar dilution, broth microdilution, disk diffusion, gradient tests, automated systems, phenotypic and molecular resistance detection techniques) of anaerobes, when should these methods be used and what are the recent developments in resistance patterns of anaerobic bacteria.

Antimicrobial susceptibility profiles of oral Treponema species

Treponemes occur in the microflora of the dental plaque. Certain Treponema species that are frequently isolated from chronic periodontitis lesions are involved in its initiation and progression. In addition to mechanical instrumentation, antimicrobial agents are used as an adjunctive treatment modality for periodontitis. Despite its importance for successful antimicrobial treatment, information about susceptibility is limited for Treponema species. The aim of this study was to assess the susceptibility of Treponema denticola strains, Treponema socranskii, and Treponema vincentii to eleven antimicrobial agents. The minimum inhibitory and minimum bactericidal concentrations of these antimicrobial agents revealed strain-specific variation. Doxycycline, minocycline, azithromycin, and erythromycin were effective against all Treponema species tested in this study, whereas fluoroquinolones only exhibited an equivalent effectiveness on T. socranskii. The susceptibility of one T. denticola strain, T. socranskii, and T. vincentii to kanamycin was influenced by prior exposure to aerobic conditions. The susceptibility to quinolone drugs varied among strains of T. denticola, although they share an amino acid sequence identity of greater than 99% for DNA gyrase (type II topoisomerase) subunit A. In addition, an ATP-binding cassette (ABC) transporter inhibitor assay for T. denticola indicated that the transport of quinolone drugs is partially related to this transporter, although there may be parallel transport mechanisms. Our results provide important insights into antimicrobial agent-Treponema dynamics and establish a basis for developing an appropriate adjunctive therapy for periodontal disease.

Alarmingly High Segregation Frequencies of Quinolone Resistance Alleles within Human and Animal Microbiomes Are Not Explained by Direct Clinical Antibiotic Exposure

Antibiotic resistance poses a major threat to human health. It is therefore important to characterize the frequency of resistance within natural bacterial environments. Many studies have focused on characterizing the frequencies with which horizontally acquired resistance genes segregate within natural bacterial populations. Yet, very little is currently understood regarding the frequency of segregation of resistance alleles occurring within the housekeeping targets of antibiotics. We surveyed a large number of metagenomic datasets extracted from a large variety of host-associated and non host-associated environments for such alleles conferring resistance to three groups of broad spectrum antibiotics: streptomycin, rifamycins, and quinolones. We find notable segregation frequencies of resistance alleles occurring within the target genes of each of the three antibiotics, with quinolone resistance alleles being the most frequent and rifamycin resistance alleles being the least frequent. Resistance allele frequencies varied greatly between different phyla and as a function of environment. The frequency of quinolone resistance alleles was especially high within host-associated environments, where it averaged an alarming ∼40%. Within host-associated environments, resistance to quinolones was most often conferred by a specific resistance allele. High frequencies of quinolone resistance alleles were also found within hosts that were not directly treated with antibiotics. Therefore, the high segregation frequency of quinolone resistance alleles occurring within the housekeeping targets of antibiotics in host-associated environments does not seem to be the sole result of clinical antibiotic usage.

Detection of integron-associated gene cassettes and other antimicrobial resistance genes in enterotoxigenic Bacteroides fragilis

Twenty seven Enterotoxigenic Bacteroides fragilis (ETBF) strains isolated from children in Kolkata, India, were tested for their antimicrobial resistance, presence of integrons and resistance encoding genes. Almost all the strains (>90%) were resistant to two or more antimicrobials. About 59-92% of the strains were resistant to ampicillin, amoxicillin, streptomycin, tetracycline, ciprofloxacin and norfloxacin. Most of these antimicrobial agents have been used in the treatment of diarrhea and other infectious diseases. In addition, about half a number of strains (48-55%) were resistant to clindamycin, cefotaxime, ceftazidime, ampicillin/sulbactam and trimethoprim/sulfamethoxazole. Moxifloxacin and metronidazole resistance ranged from 30 to 40%. All strains however, were found to be susceptible to chloramphenicol and imipenem. Class 1 integrase (intI1) was detected in seven and class 2 integrase (intI2) in one of the twenty seven ETBF strains. Resistance gene cassettes carried by these integrons had different alleles of dfr or aad genes. Beside these integron-borne genes, other genes encoding different antimicrobial resistance were also detected. Resistance genes such as cep(A) and tet(Q) were detected in most of the ETBF strains. To the best of our knowledge, this work constituted the first extensive report from India on the detection of integrons and antimicrobial resistance genes in ETBF.

PCR-based detection of resistance genes in anaerobic bacteria isolated from intra-abdominal infections

Little information is available on the distribution of antimicrobial resistance genes in anaerobes in Japan. To understand the background of antimicrobial resistance in anaerobes involved in intra-abdominal infections, we investigated the distribution of eight antimicrobial resistance genes (cepA, cfiA, cfxA, ermF, ermB, mefA, tetQ, and nim) and a mutation in the gyrA gene in a total of 152 organisms (Bacteroides spp., Prevotella spp., Fusobacterium spp., Porphyromonas spp., Bilophila wadsworthia, Desulfovibrio desulfuricans, Veillonella spp., gram-positive cocci, and non-spore-forming gram-positive bacilli) isolated between 2003 and 2004 in Japan. The cepA gene was distributed primarily in Bacteroides fragilis. Gene cfxA was detected in about 9 % of the Bacteroides isolates and 75 % of the Prevotella spp. isolates and did not appear to contribute to cephamycin resistance. Two strains of B. fragilis contained the metallo-β-lactamase gene cfiA, but they did not produce the protein product. Gene tetQ was detected in about 81, 44, and 63 % of B. fragilis isolates, other Bacteroides spp., and Prevotella spp. isolates, respectively. The ermF gene was detected in 25, 13, 56, 64, and 16 % of Bacteroides spp., Prevotella spp., Fusobacterium spp., B. wadsworthia, and anaerobic cocci, respectively. Gene mefA was found in only 10 % of the B. fragilis strains and 3 % of the non-B. fragilis strains. Genes nim and ermB were not detected in any isolate. Substitution at position 82 (Ser to Phe) in gyrA was detected in B. fragilis isolates that were less susceptible or resistant to moxifloxacin. This study is the first report on the distribution of resistance genes in anaerobes isolated from intra-abdominal infections in Japan. We expect that the results might help in understanding the resistance mechanisms of specific anaerobes.

Sublethal antibiotic treatment leads to multidrug resistance via radical-induced mutagenesis

Antibiotic resistance arises through mechanisms such as selection of naturally occurring resistant mutants and horizontal gene transfer. Recently, oxidative stress has been implicated as one of the mechanisms whereby bactericidal antibiotics kill bacteria. Here, we show that sublethal levels of bactericidal antibiotics induce mutagenesis, resulting in heterogeneous increases in the minimum inhibitory concentration for a range of antibiotics, irrespective of the drug target. This increase in mutagenesis correlates with an increase in ROS and is prevented by the ROS scavenger thiourea and by anaerobic conditions, indicating that sublethal concentrations of antibiotics induce mutagenesis by stimulating the production of ROS. We demonstrate that these effects can lead to mutant strains that are sensitive to the applied antibiotic but resistant to other antibiotics. This work establishes a radical-based molecular mechanism whereby sublethal levels of antibiotics can lead to multidrug resistance, which has important implications for the widespread use and misuse of antibiotics.

Bacteroides: the good, the bad, and the nitty-gritty

SUMMARY

Bacteroides species are significant clinical pathogens and are found in most anaerobic infections, with an associated mortality of more than 19%. The bacteria maintain a complex and generally beneficial relationship with the host when retained in the gut, but when they escape this environment they can cause significant pathology, including bacteremia and abscess formation in multiple body sites. Genomic and proteomic analyses have vastly added to our understanding of the manner in which Bacteroides species adapt to, and thrive in, the human gut. A few examples are (i) complex systems to sense and adapt to nutrient availability, (ii) multiple pump systems to expel toxic substances, and (iii) the ability to influence the host immune system so that it controls other (competing) pathogens. B. fragilis, which accounts for only 0.5% of the human colonic flora, is the most commonly isolated anaerobic pathogen due, in part, to its potent virulence factors. Species of the genus Bacteroides have the most antibiotic resistance mechanisms and the highest resistance rates of all anaerobic pathogens. Clinically, Bacteroides species have exhibited increasing resistance to many antibiotics, including cefoxitin, clindamycin, metronidazole, carbapenems, and fluoroquinolones (e.g., gatifloxacin, levofloxacin, and moxifloxacin).

Substitutions of amino acids in alpha-helix-4 of gyrase A confer fluoroquinolone resistance on Clostridium perfringens

DNA gyrase, an essential enzyme that regulates DNA topology in bacteria, is the target of fluoroquinolones. Three fluoroquinolone-resistant mutants derived from one strain of Clostridium perfringens had amino acid substitutions of glycine 81 to cysteine, aspartic acid 87 to tyrosine, or both, in alpha-helix-4 of gyrase A. The gyrase mutations affected the growth kinetics of mutants differently when the mutants were exposed to increasing concentrations of gatifloxacin and ciprofloxacin. Fluoroquinolone concentration-dependent effects observed during growth in the exponential and stationary phases depended on the presence of particular gyrA mutations. Introduction of a wild-type gyrA gene into the mutants enhanced their susceptibility to fluoroquinolones and decreased their growth rates proportional to increases in fluoroquinolone concentrations. Amino acid substitutions in alpha-helix-4 of gyrase A protected C. perfringens from fluoroquinolones, and a strain with two substitutions was the most resistant.

Fluoroquinolones and Anaerobes

The usefulness of fluoroquinolones for the treatment of mixed aerobic and anaerobic infections has been investigated since these agents started being used in clinical practice. Newer compounds have increased in vitro activity against anaerobes, but clinically relevant susceptibility breakpoints for these bacteria have not been established. Pharmacodynamic analyses and corroboration by new data from clinical trials have enhanced our knowledge concerning the use of fluoroquinolones to treat selective anaerobic pathogens. These studies suggest that newer agents could be useful in the treatment of several types of mixed aerobic and anaerobic infections, including skin and soft-tissue, intra-abdominal, and respiratory infections. The major concerns with expanding the use of fluoroquinolones to treat anaerobic infections have been reports of increasing resistance in Bacteroides group isolates and the impact of these antibiotics on the incidence of Clostridium difficile-associated disease.

Changing patterns of fluoroquinolone resistance among Bacteroides fragilis group organisms over a 6-year period (1997-2002)

The evolution of susceptibility to the newer fluoroquinolones, moxifloxacin and trovafloxacin, of Bacteroides fragilis group organisms isolated in our hospital from 1997 to 2002 was studied. A total of 927 strains were included in the study. Trovafloxacin was more active than moxifloxacin against the various species of the group. During the study period, an increase in resistance to both quinolones was observed. Rates of resistance to moxifloxacin at a breakpoint of 8 microg/mL remained stable at around 6% during the period 1997-1998 and increased to 11.4% in 2000 and to 16.5% in 2001-2002 (P<0.005). Resistance to trovafloxacin rose significantly from 0.6% in 1998 to 6.8% in 1999 (P<0.05) and did not change appreciably over the last 3 years studied. This study confirms the increasing resistance of B. fragilis group organisms to trovafloxacin and moxifloxacin and emphasizes the need to perform periodic antimicrobial susceptibility tests to guide the selection of appropriate antimicrobial therapy.

Alterations in DNA gyrase and topoisomerase IV in resistant mutants of Clostridium perfringens found after in vitro treatment with fluoroquinolones

To compare mutations in the DNA gyrase (gyrA and gyrB) and topoisomerase IV (parC and parE) genes of Clostridium perfringens, which are associated with in vitro exposure to fluoroquinolones, resistant mutants were selected from eight strains by serial passage in the presence of increasing concentrations of norfloxacin, ciprofloxacin, gatifloxacin, or trovafloxacin. The nucleotide sequences of the entire gyrA, gyrB, parC, and parE genes of 42 mutants were determined. DNA gyrase was the primary target for each fluoroquinolone, and topoisomerase IV was the secondary target. Most mutations appeared in the quinolone resistance-determining regions of gyrA (resulting in changes of Asp-87 to Tyr or Gly-81 to Cys) and parC (resulting in changes of Asp-93 or Asp-88 to Tyr or Ser-89 to Ile); only two mutations were found in gyrB, and only two mutations were found in parE. More mutants with multiple gyrA and parC mutations were produced with gatifloxacin than with the other fluoroquinolones tested. Allelic diversity was observed among the resistant mutants, for which the drug MICs increased 2- to 256-fold. Both the structures of the drugs and their concentrations influenced the selection of mutants.

Genetic determinant of intrinsic quinolone resistance in Fusobacterium canifelinum

Fourteen fluoroquinolone-resistant fusobacterial strains, originating from cats or dogs, were characterized by sequencing of the 16S-23S and 16S rRNA genes and DNA-DNA hybridization and were described as a new species, Fusobacterium canifelinum. All of the strains are intrinsically resistant (MIC, >4 g/ml) to levofloxacin and other fluoroquinolones. Compared to the quinolone resistance-determining region (gyrA) of the susceptible relative F. nucleatum, we found that Ser79 was replaced with leucine and Gly83 was replaced with arginine.

Antianaerobic activity of a novel fluoroquinolone, WCK 771, compared to those of nine other agents

Agar dilution MIC methodology was used to compare the activity of WCK 771 with those of ciprofloxacin, levofloxacin, moxifloxacin, gatifloxacin, piperacillin, piperacillin-tazobactam, imipenem, clindamycin, and metronidazole against 350 anaerobes. Overall, the MICs (in micrograms per milliliter) at which 50 and 90%, respectively, of the isolates tested were inhibited were as follows: WCK 771, 0.5 and 2.0; ciprofloxacin, 2.0 and 32.0; levofloxacin, 1.0 and 8.0; gatifloxacin, 0.5 and 4.0; moxifloxacin, 0.5 and 4.0; piperacillin, 2.0 and 64.0; piperacillin-tazobactam, < or =0.125 and 8.0; imipenem, 0.125 and 1.0; clindamycin, 0.125 and 16.0; and metronidazole, 1.0 and >16.0.

Prevalence of antibiotic resistance in anaerobic bacteria: worrisome developments

Antibiotic-resistant anaerobic bacteria have become increasingly recognized as a confounding factor in the selection of therapeutic agents. The use of potent, broad-spectrum antibiotics as empirical therapy, along with appropriate adjunctive measures, has, in some ways, masked the magnitude of the antibiotic resistance problem that parallels that observed for nonanaerobic pathogens. The use of standardized testing methods that recognize resistance and an understanding of resistance mechanisms have become essential for the treatment of patients and the development of new agents.

Mechanism of quinolone resistance in anaerobic bacteria

Several recently developed quinolones have excellent activity against a broad range of aerobic and anaerobic bacteria and are thus potential drugs for the treatment of serious anaerobic and mixed infections. Resistance to quinolones is increasing worldwide, but is still relatively infrequent among anaerobes. Two main mechanisms, alteration of target enzymes (gyrase and topoisomerase IV) caused by chromosomal mutations in encoding genes, or reduced intracellular accumulation due to increased efflux of the drug, are associated with quinolone resistance. These mechanisms have also been found in anaerobic species. High-level resistance to the newer broad-spectrum quinolones often requires stepwise mutations in target genes. The increasing emergence of resistance among anaerobes may be a consequence of previous widespread use of quinolones, which may have enriched first-step mutants in the intestinal tract. Quinolone resistance in the Bacteroides fragilis group strains is strongly correlated with amino acid substitutions at positions 82 and 86 in GyrA (equivalent to positions 83 and 87 of Escherichia coli). Several studies have indicated that B. fragilis group strains possess efflux pump systems that actively expel quinolones, leading to resistance. DNA gyrase seems also to be the primary target for quinolones in Clostridium difficile, since amino acid substitutions in GyrA and GyrB have been detected in resistant strains. To what extent other mechanisms, such as mutational events in other target genes or alterations in outer-membrane proteins, contribute to resistance among anaerobes needs to be further investigated.

Antimicrobial susceptibility of Bacteroides fragilis group isolates in Europe

OBJECTIVE

To evaluate the activity of old and newer antianaerobic drugs against clinical isolates of Bacteroides fragilis group strains from different parts of Europe.

METHODS

Bacteroides fragilis group isolates from 37 laboratories in 19 countries were biochemically characterized. The MICs of seven antimicrobial agents were determined by the agar dilution method as recommended by the NCCLS. Production of beta-lactamase was detected by nitrocefin.

RESULTS

There were 1284 B. fragilis group isolates included in the study. Abdominal infections and wounds were the most common sources of isolation and B. fragilis was the dominating species. Ninety-nine percent of the strains were resistant to ampicillin (breakpoint 2 mg/L), 6% to cefoxitin (64 mg/L), 15% to clindamycin (8 mg/L) and 9% to moxifloxacin (8 mg/L). Less than 1% were resistant to imipenem (16 mg/L), piperacillin-tazobactam (128 mg/L) and metronidazole (32 mg/L). Ninety-six percent of the isolates were beta-lactamase producers.

CONCLUSIONS

Antimicrobial resistance among the B. fragilis group is increasing.

A MATE family multidrug efflux transporter pumps out fluoroquinolones in Bacteroides thetaiotaomicron

We cloned a gene, bexA, that codes for a multidrug efflux transporter from the chromosomal DNA of Bacteroides thetaiotaomicron ATCC 29741 by using an Escherichia coli DeltaacrAB DeltaacrEF mutant as a host. Although the initial recombinant construct contained other open reading frames, the presence of bexA alone was sufficient to confer to the E. coli host elevated levels of resistance to norfloxacin, ciprofloxacin, and ethidium bromide. Disruption of bexA in B. thetaiotaomicron made the strain more susceptible to norfloxacin, ciprofloxacin, and ethidium bromide, showing that this gene is expressed in this organism and functions as a multidrug efflux pump. The deduced BexA protein sequence was homologous to the protein sequence of Vibrio parahaemolyticus NorM, a multidrug efflux transporter, and thus, BexA belongs to the multidrug and toxic compound extrusion (MATE) family.