Susceptibility of the Bacteroides fragilis group in the United States in 1981

Myopia

Myopia, also known as short-sightedness or near-sightedness, is a very common condition that typically starts in childhood. Severe forms of myopia (pathologic myopia) are associated with a risk of other associated ophthalmic problems. This disorder affects all populations and is reaching epidemic proportions in East Asia, although there are differences in prevalence between countries. Myopia is caused by both environmental and genetic risk factors. A range of myopia management and control strategies are available that can treat this condition, but it is clear that understanding the factors involved in delaying myopia onset and slowing its progression will be key to reducing the rapid rise in its global prevalence. To achieve this goal, improved data collection using wearable technology, in combination with collection and assessment of data on demographic, genetic and environmental risk factors and with artificial intelligence are needed. Improved public health strategies focusing on early detection or prevention combined with additional effective therapeutic interventions to limit myopia progression are also needed.

The emergence of antimicrobial resistance in environmental strains of the Bacteroides fragilis group

Anaerobic bacteria of the genus Bacteroides are a large group of commensal microorganisms that colonize the human and animal digestive tract. The genus Bacteroides and the closely related genus Parabacteroides include the Bacteroides fragilis group (BFG) of potentially pathogenic bacteria which are frequently isolated from patients with anaerobic infections. The aim of this study was to assess the antimicrobial resistance of environmental strains of the Bacteroides fragilis group. Strains were isolated from human feces, hospital wastewater, influent (UWW) and effluent (TWW) wastewater from a wastewater treatment plant (WWTP), and from the feces of lab rats as a negative control to monitor the entire route of transmission of BFG strains from humans to the environment. The resistance of 123 environmental BFG strains to six antibiotic groups was analyzed with the use of culture-dependent methods. Additionally, the presence of 25 genes encoding antibiotic resistance was determined by PCR. The analyzed environmental BFG strains were highly resistant to the tested antibiotics. The percentage of resistant strains differed between the analyzed antibiotics and was determined at 97.56% for ciprofloxacin, 49.59% for erythromycin, 44.71% for ampicillin, 35.77% for tetracycline, 32.52% for amoxicillin/clavulanic acid, 26.83% for chloramphenicol, 26.01% for clindamycin, 11.38% for moxifloxacin, and 8.94% for metronidazole. The highest drug-resistance levels were observed in the strains isolated from UWW and TWW samples. The mechanisms of antibiotic-resistance were determined in phenotypically resistant strains of BFG. Research has demonstrated the widespread presence of genes encoding resistance to chloramphenicol (100% of all chloramphenicol-resistant strains), tetracyclines (97.78% of all tetracycline-resistant strains), macrolides, lincosamides and streptogramins (81.97% of all erythromycin-resistant strains). Genes encoding resistance to β-lactams and fluoroquinolones were less prevalent. None of the metronidazole-resistant strains harbored the gene encoding resistance to nitroimidazoles. BFG strains isolated from UWW and TWW samples were characterized by the highest diversity of antibiotic-resistance genes and were most often drug-resistant and multidrug-resistant. The present study examines the potential negative consequences of drug-resistant and multidrug-resistant BFG strains that are evacuated with treated wastewater into the environment. The transmission of these bacteria to surface water bodies can pose potential health threats for humans and animals; therefore, the quality of treated wastewater should be strictly monitored.

Susceptibility trending of blood isolates of the Bacteroides fragilis group over a 12-year period to clindamycin, ampicillin-sulbactam, cefoxitin, imipenem, and metronidazole

Numerous reports have described a steady overall increase in resistance among clinical isolates of the Bacteroides fragilis group to several antimicrobial agents, particularly clindamycin. Determination of resistance rates is significantly influenced by the number of isolates of each species within the B. fragilis group tested. Historically, the B. fragilis species has remained the most susceptible to most antimicrobials when compared to non-B. fragilis species. This study compares the effect of a gradually changing ratio of blood isolates of B. fragilis to non-B. fragilis species tested by broth micro-dilution over a 12-year period on selected antimicrobial agents. In 1987, the ratio of blood isolates of B. fragilis to non-B. fragilis was 68% to 32%; in 1991 it was 59% to 41%; and in 1999 it was 51% to 49%. Both metronidazole and imipenem showed the least changes because of their inherent high activity against all species. For clindamycin, decreases in susceptibility ranged from 84% to 64% for B. fragilis compared to 58% to 67% for non-B. fragilis species. Ampicillin-sulbactam showed a decrease in susceptibility in B. fragilis and non-B. fragilis species, but was highest in 1999 when the ratio of non-B. fragilis species was the highest. Overall resistance rates to cefoxitin varied from 8% to 25% during the testing years and was consistently higher among the non-B. fragilis species. These comparisons indicate that the ratio of B. fragilis group species isolated from the blood has changed over the last 12 years and has appreciably affected the resistance rates to some commonly used anti-anaerobic agents. Whether the noted changes in species isolation rates are a result of selective antibiotic pressure or other factors is yet to be determined.

In vitro susceptibilities of the Bacteroides fragilis group species: change in isolation rates significantly affects overall susceptibility data

A comparison of antimicrobial susceptibility data of species of the Bacteroides fragilis group for 1989-1990 and 1998-1999 studies showed statistically significant increases or decreases in in vitro activity. Overall significant increases in resistance were noted for ampicillin-sulbactam and clindamycin, while significant decreases in resistance were noted for ertapenem and cefoxitin. Susceptibilities to piperacillin-tazobactam, imipenem, meropenem, and trovafloxacin remained virtually the same for the two studies. Importantly, a change in the rates of isolation of the various species showed the B. fragilis species comprised 58% of the isolates in 1989 to 1990 and 45% of the isolates in 1998 to 1999. This change in rates of isolation of B. fragilis versus non-B. fragilis species had an overall effect on susceptibility data.

Evolution of anaerobe susceptibility testing in the United States

Susceptibility testing of anaerobes has been described for approximately 45 years, some 15 years fewer than descriptions for aerobic organisms. During that time period, >16 methods, >16 media, and a host of other variables have been described in the United States literature, culminating in the most recent standards published by the National Committee for Clinical Laboratory Standards (NCCLS) in 2001. These new guidelines include a single agar dilution reference method as well as an alternative minimal inhibitory concentration method validated by extensive multilaboratory collaborative trials. In addition, the Food and Drug Administration-approved E-test has proved valuable as a user-friendly alternative to NCCLS methods. Highlights of the "evolution" of various methods, their variations and their failures, factors that affected the development of the current standards, and the rationale for susceptibility testing of anaerobes are discussed herein.

Multilaboratory comparison of anaerobe susceptibility results using 3 different agar media

A 5-laboratory study was performed that used the National Committee for Clinical Laboratory Standards (NCCLS) reference agar dilution method with 3 media formulations to determine whether the use of different media would affect minimum inhibitory concentration (MIC) results. Wilkins-Chalgren, Brucella-based blood agar (BRU), and Wilkins-Chalgren agar plus blood (WCB) and 6 antibiotics (clindamycin, cefoxitin, ceftizoxime, piperacillin, metronidazole, and trovafloxacin) were evaluated with 58 isolates. The MIC values were compared, and a significant correlation of >0.80 was demonstrated for all media and each antibiotic/organism group. The cumulative rate of errors for all antibiotics was 0.1%. These data indicate that a change in the NCCLS reference medium for testing of anaerobic bacteria susceptibility to either BRU or WCB will not affect the MIC results for the antibiotics and organisms evaluated.

Susceptibility of 497 clinical isolates of gram-negative anaerobes to trovafloxacin and eight other antibiotics

Trovafloxacin is a novel investigational trifluoronaphthyridone antibiotic with broad-spectrum activity against Gram-positive and Gram-negative organisms. Its in-vitro activity and those of eight other antimicrobial agents were evaluated against 497 clinical isolates of Gram-negative anaerobic bacteria by the agar dilution method. Trovafloxacin had excellent activity, with a minimum inhibitory concentration (MIC) range of <0.03-4 microg/ml, against all species. Out of the 497 isolates tested, 496 (99.5%) were inhibited by a concentration of < or = 2.0 microg/ml of trovafloxacin; the remaining two strains were inhibited by a concentration of 4.0 microg/ml. The MIC50s and MIC90s were 0.12 microg/ml and 1.0 microg/ml, respectively. Meropenem, imipenem and piperacillin/tazobactam were also very active. Overall, at the MIC90s, trovafloxacin was as active as meropenem, slightly more active than metronidazole and imipenem, twice as active as amoxicillin-clavulanic acid, five times more active than piperacillintazobactam and 68 times more active than clindamycin. About 21% of the isolates were resistant to cefoxitin, 30% to clindamycin and 40% to piperacillin. Five species in the Bacteroides fragilis group of isolates were highly resistant to metronidazole (MIC >128 microg/ml). In general, the relatively more resistant species were the B. vulgatus, B. ovatus, B. thetaiotaomicron, and B. fragilis sensu stricto, in that order. All the isolates of the B. fragilis group and about 50% of the Prevotella spp. were beta-lactamase positive. Trovafloxacin certainly holds promise as an alternative drug for therapy of anaerobic infections.

Multicenter study of in vitro susceptibility of the Bacteroides fragilis group, 1995 to 1996, with comparison of resistance trends from 1990 to 1996

Antimicrobial resistance, including plasmid-mediated resistance, among the species of the Bacteroides fragilis group is well documented. An analysis of the in vitro susceptibility of B. fragilis group species referred between 1995 and 1996 as well as during a 7-year (1990 to 1996), prospective, multicenter survey of over 4,000 clinical isolates of B. fragilis group species was undertaken to review trends in the percent resistance to and geometric mean MICs of the antibiotics tested. There was a trend toward a decrease in the geometric mean MICs of most beta-lactam antibiotics, while the percent resistance to most agents was less affected. Within the species B. fragilis, the geometric mean MICs showed significant (P < 0.05) decreases for piperacillin-tazobactam, ticarcillin-clavulanate, piperacillin, ticarcillin, ceftizoxime, cefotetan, and cefmetazole; a significant increase was observed for clindamycin and cefoxitin. For the non-B. fragilis species, a significant decrease in the geometric mean MICs was observed for meropenem, ampicillin-sulbactam, ticarcillin-clavulanate, piperacillin, ticarcillin, ceftizoxime, and cefmetazole; a significant increase was observed for cefoxitin. Significant increases in percent resistance were observed within the B. fragilis strains for ticarcillin and ceftizoxime and within the non-B. fragilis isolates for cefotetan. Significant increases in percent resistance among all B. fragilis group species were observed for clindamycin, while imipenem showed no significant change in resistance trends. The trend analysis for trovafloxacin was limited to 3 years, since the quinolone was tested only in 1994, 1995, and 1996. During the 7 years analyzed, there was no resistance to metronidazole or chloramphenicol observed. The data demonstrate that resistance among the B. fragilis group species has decreased in the past several years, the major exception being clindamycin. The majority of the resistance decrease has been for the beta-lactams in B. fragilis, compared to other species. The reasons for these changes are not readily apparent.

Antimicrobial susceptibility tests on anaerobic oral mixed cultures in periodontal diseases

The ecosystem of the dental plaque in periodontal diseases is very complex: the study of such micro-organisms, which are mostly strict anaerobes, requires the use of specific techniques under conditions of strict anaerobiosis. The aim of the present study was to design a rapid method to evaluate the activity of antimicrobials on mixed bacterial plaque of subjects with periodontal diseases. The study was carried out using a computerised instrument generally used for simultaneous diagnostic tests with aerobic bacteria. Operative and methodological modifications were made to obtain conditions of strict anaerobiosis and the balanced growth of all the microbial forms present in the mixed cultures of the plaque. Penicillins and cephalosporins were active on all the samples, whereas colistin, gentamicin, kanamycin and nalidixic acid showed no activity. Clindamycin, tetracycline, erythromycin and penicillin G were effective only against some samples. The activity of the antimicrobials towards isolated strains was analogous to that towards the corresponding mixed culture.

Cephalosporin Antibiotics

The last three decades have seen the emergence of a cornucopia of antimicrobials in the cephalosporin class. This becomes even more apparent when one realizes that over 60 pages of the 1993 AHFS drug information text is dedicated to the cephalosporin class.1 Due to similarities in nomenclature and the proliferation of available agents, the clinician is constantly challenged by this confusing array of antimicrobials. The intent of this manuscript will be to distill the volumes of information available on cephalosporins into a more manageable format and close with a summary of the most clinically important cephalosporin antimicrobials.

Genetic and biochemical analysis of a novel Ambler class A beta-lactamase responsible for cefoxitin resistance in Bacteroides species

A clinical isolate of Bacteroides vulgatus was resistant to tetracycline, clindamycin, ampicillin, cephaloridine, cefoxitin, and other beta-lactam antibiotics except imipenem. beta-Lactam resistance was mediated by a membrane-associated, clavulanate-sensitive cephalosporinase capable of degrading cephalosporins and penicillins. Cefoxitin also was degraded but at a slow rate. The cefoxitin resistance (Fxr) determinant was cloned from B. vulgatus genomic libraries that were prepared in Escherichia coli and then mated with Bacteroides fragilis for the identification of Fxr strains. Analysis of B. fragilis strains with the cloned Fxr determinant revealed the presence of a new beta-lactamase protein with the physical and enzymatic properties of the beta-lactamase found in the original B. vulgatus isolate. The beta-lactamase gene (cfxA) was subcloned on a 2.2-kb DraI-HindIII fragment, and the nucleotide sequence was determined. These results showed that cfxA encoded a protein of 321 amino acids and 35,375 molecular weight. Mutant strains in which the cfxA structural gene was disrupted by insertional inactivation lost both Fxr and beta-lactamase activity. Comparison of CfxA with other beta-lactamases showed a relationship with the active-site serine beta-lactamases in the Ambler molecular class A, although CfxA had apparently diverged significantly. This was exemplified by the substitution in CfxA at 13 of 25 amino acid residues previously identified as being invariant in class A beta-lactamases. These results suggest that CfxA may represent a new class A homology group which diverged very early.

Identification of a circular intermediate in the transfer and transposition of Tn4555, a mobilizable transposon from Bacteroides spp

Transmissible cefoxitin (FX) resistance in Bacteroides vulgatus CLA341 was associated with the 12.5-kb, mobilizable transposon, Tn4555, which encoded the beta-lactamase gene cfxA. Transfer occurred by a conjugation-like mechanism, was stimulated by growth of donor cells with tetracycline (TC), and required the presence of a Bacteroides chromosomal Tcr element. Transconjugants resistant to either FX, TC, or both drugs were obtained, but only Fxr Tcr isolates could act as donors of Fxr in subsequent matings. Transfer of Fxr could be restored in Fxr Tcs strains by the introduction of a conjugal Tcr element from Bacteroides fragilis V479-1. A covalently closed circular DNA form of Tn4555 was observed in donor cells by Southern hybridization, and the levels of this circular transposon increased significantly in cells grown with TC. Both the cfxA gene and the Tn4555 mobilization region hybridized to the circular DNA, suggesting that this was a structurally intact transposon unit. Circular transposon DNA purified by CsCl-ethidium bromide density gradient centrifugation was used to transform Tcs B. fragilis 638, and Fxr transformants were obtained. Both the circular form and the integrated Tn4555 were observed in transformants, but the circular form was present at less than one copy per chromosomal equivalent. Examination of genomic DNA from Fxr transformants and transconjugants revealed that Tn4555 could insert at a wide variety of chromosomal sites. Multiple transposon insertions were present in many of the transconjugants, indicating that there was no specific barrier to the introduction of a second transposon copy.

In vitro susceptibility of clinical isolates of Bacteroides fragilis and Bacteroides thetaiotaomicron in Japan

A nationwide survey of the susceptibility of 433 isolates of Bacteroides fragilis and 149 isolates of Bacteroides thetaiotaomicron was conducted from December 1986 through November 1989 in Japan. These strains were collected from 16 university hospitals and one metropolitan hospital. Metronidazole was the most active drug against both species, with no resistant isolates found. The activity of imipenem and sulbactam-cefoperazone was good, with very low resistance rates determined in Bacteroides fragilis (1.4% and 1.6%, respectively) and in Bacteroides thetaiotaomicron (3.4% for both drugs), and was comparable to that of metronidazole. Cefoxitin, cefmetazole, cefotetan, cefbuperazone, latamoxef and ceftizoxime were found to be more active against Bacteroides fragilis, for which resistance rates were 3.2 to 9.5%, than against Bacteroides thetaiotaomicron, for which resistance rates were 18.1 to 21.8%. Rates of piperacillin resistance in the two species were 12.9% and 26.8%, respectively. Clindamycin was very active at a low concentration (MIC50 of 0.39 to 1.56 mg/l), but 24% and 27.5% of Bacteroides fragilis and Bacteroides thetaiotaomicron isolates, respectively, were resistant to this agent.

Analysis of rRNA restriction fragment length polymorphisms from Bacteroides spp. and Bacteroides fragilis isolates associated with diarrhea in humans and animals

The Escherichia coli rRNA operon rrnB was used as a 32P-labeled hybridization probe in Southern blots of genomic DNAs from representative strains of the saccharolytic, gram-negative, obligate anaerobes of the genus Bacteroides. Control experiments with the B. fragilis type strain ATCC 25285 established that nearly identical rRNA fragment patterns were produced when either the E. coli rrnB gene probe or homologous rRNA isolated from B. fragilis was used as the probe. In addition, it was shown that a specific 16S or 23S rrnB gene probe also could be used to produce fragment patterns suitable for analysis. Thirty-one strains from 8 of the 10 recognized Bacteroides species were then examined. The resulting autoradiographs revealed specific fragment patterns for all but one (B. ovatus) of the species tested. Restriction fragment length polymorphisms were observed for many of the strains tested, but these differences did not hinder species classification. The five B. ovatus strains examined did not form a distinct group, and their rRNA fragment patterns displayed a marked heterogeneity. The same approach was applied to a unique set of enterotoxin-producing B. fragilis strains isolated from animals and humans with diarrhea. The results demonstrated that these strains were in fact B. fragilis and that they produce rRNA fragment patterns closely related to those of the type strain ATCC 25285. This set of strains did not appear to form a separate subgroup or genotype within the B. fragilis species, and there were no distinguishable restriction fragment length polymorphisms that could be used to specifically separate enterotoxin-producing strains from nonenterotoxigenic strains.

Conjugal transfer of antibiotic resistance factors in Bacteroides fragilis: the btgA and btgB genes of plasmid pBFTM10 are required for its transfer from Bacteroides fragilis and for its mobilization by IncP beta plasmid R751 in Escherichia coli

Transferable plasmids play an important role in the dissemination of clindamycin-erythromycin resistance in Bacteroides fragilis. We previously described the isolation and properties of pBFTM10, a 14.9-kb ClnR transfer factor from B. fragilis TMP10. We also reported the isolation of a transfer-deficient deletion derivative of pBFTM10 contained in the B. fragilis-Escherichia coli shuttle vector pGAT400. In the present study we used pGAT400 and a similar shuttle vector, pGAT550, to characterize and sequence a region of pBFTM10 required for its transfer from B. fragilis to B. fragilis or E. coli recipients and for its mobilization by the broad-host-range plasmid R751 from E. coli donors to E. coli recipients. Deletion of certain BglII restriction fragments from pBFTM10 resulted in partial or complete loss of transfer ability. Tn1000 insertions into this same region also resulted in altered transfer properties. We used the sites of Tn1000 insertions to determine the DNA sequence of the transfer region. Two potential open reading frames encoding proteins of 23.2 and 33.8 kDa, corresponding to two genes, btgA or btgB, were identified in the sequence. Tn1000 insertions within btgA or btgB or deletion of all or portions of btgA or btgB resulted in either a transfer deficiency or greatly reduced transfer from B. fragilis donors and alterations in mobilization by R751 in E. coli. A potential oriT sequence showing similarity in organization to the oriT regions of the IncP plasmids was also detected. Thus, pBFTM10 encodes and requires at least two proteins necessary for efficient transfer from B. fragilis. These same functions are expressed in E. coli and are required for mobilization by R751.

Susceptibility testing of anaerobic bacteria: myth, magic, or method?

The demand for susceptibility testing of anaerobes has increased, yet consensus as to procedure and interpretation in this area has not been achieved. While routine testing of anaerobic isolates is not needed, certain isolates in specific clinical settings should be tested. Also, laboratories may monitor their local antibiograms by doing periodic surveillance batch testing. The National Committee for Clinical Laboratory Standards has published a protocol of methods approved for susceptibility testing of anaerobic bacteria. Both agar and broth microdilution are included; however, the broth disk elution method is no longer approved by the National Committee for Clinical Laboratory Standards because of method-related interpretive errors. A number of newer methods are undergoing evaluation and seem promising. Clinicians and microbiologists reviewing susceptibility reports should be aware of sources of variability in the test results. Variables in susceptibility testing of anaerobes include the media and methods used, organisms chosen for testing, breakpoints chosen for interpretation, antibiotic, and determination of endpoint. Clustering of MICs around the breakpoint may lead to significant variability in test results. Adherence of testing laboratories to approved methods and careful descriptions of the method and the breakpoints used for interpretation would facilitate interlaboratory comparisons and allow problems of emerging resistance to be noted. A variety of resistance mechanisms occurs in anaerobic bacteria, including the production of beta-lactamase and other drug-inactivating enzymes, alteration of target proteins, and inability of the drug to penetrate the bacterial wall. Antimicrobial resistance patterns in the United States and abroad are described.

Comparative in vitro susceptibility of Bacteroides and Fusobacterium isolated from footrot in sheep to 28 antimicrobial agents

The agar dilution method was used to determine the bacteriostatic activity of 28 antimicrobial agents against 141 strains to the genus Bacteroides and 29 strains from the genus Fusobacterium. All organisms were isolated from clinical cases of ovine footrot. The strains were isolated from 125 Merino sheep, over a period of 2 years, from January 1987 to December 1988. The three ureidopenicillins studied (azlocillin, mezlocillin and piperacillin) proved to be the most effective antimicrobial agents. Chloramphenicol, metronidazole and tinidazole effectively inhibited the growth of Bacteroides spp., while phosphomycin was active against Fusobacterium spp.

Susceptibilities of Bacteroides and Fusobacterium spp. from foot rot in goats to 10 beta-lactam antibiotics

The agar dilution method was used to determine the bacteriostatic activities of 10 beta-lactam antibiotics against 132 strains belonging to the genus Bacteroides and 25 strains belonging to the genus Fusobacterium, all isolated from clinical cases of caprine foot rot. The three ureidopenicillins studied proved to be the most effective antimicrobial agents.

Comparative activities of newer beta-lactam agents against members of the Bacteroides fragilis group

A nationwide susceptibility survey of 557 isolates of the Bacteroides fragilis group was continued in 1986. The most active beta-lactam drugs were imipenem and ticarcillin-clavulanic acid, which had 0.2 and 1.7% resistance, respectively. The rank order of activity of beta-lactam drugs was imipenem, ticarcillin-clavulanic acid, cefoxitin, piperacillin, moxalactam, ceftizoxime, cefotetan, cefotaxime, cefoperazone, and ceftazadime.

Imipenem resistance in Bacteroides distasonis mediated by a novel beta-lactamase

Imipenem is a highly active drug against the Bacteroides fragilis group of organisms. On the basis of a nationwide survey of over 500 isolates, it was found that the frequency of imipenem resistance was less than 0.1%. We have a highly resistant Bacteroides distasonis isolate, TAL7860, for which the following MICs (micrograms per milliliter) were determined by agar dilution: cefoxitin, greater than 128; moxalactam, greater than 128; piperacillin, greater than 128; imipenem, 16; and SCH34343, 16. Resistance was shown to involve both a beta-lactamase and an outer membrane permeability barrier. beta-Lactamase kinetics studies with several beta-lactams, including imipenem, revealed similar hydrolytic efficiency in comparison with those found for the B. fragilis strains. An imipenem outer membrane permeability barrier was detected for TAL7860, which was approximately sixfold more effective for B. fragilis TAL3636 and TAL2480. Significant inhibition of nitrocefin destruction was also shown with sulbactam and clavulanic acid at 10 mumol and dithiothreitol at 10 mM. No inhibition was seen with 10 mM EDTA. Differences in physicochemical properties and inhibition studies suggest that this beta-lactamase is different from the imipenem-inactivating metallo-beta-lactamase previously described in B. fragilis. We demonstrated a significant permeability barrier to clavulanic acid and sulbactam, which resulted in loss of synergism between these clinically employed beta-lactamase inhibitors and beta-lactam drugs. The novel beta-lactamase activity in conjunction with a limited permeability in TAL7860 resulted in resistance to all commonly employed beta-lactams, including the newest and most potent beta-lactam drugs.

In vitro comparison of cefoperazone/sulbactam with selected antimicrobials against 300 bacteroides isolates. Inhibitory activity and time-kill kinetic studies

The susceptibilities of 258 Bacteroides fragilis group isolates and 42 other Bacteroides species isolates against cefoperazone, cefoperazone/sulbactam (2:1 ratio) and selected other antimicrobials were determined by broth microdilution method. All isolates were susceptible to cefoperazone/sulbactam, ampicillin/sulbactam, ticarcillin/clavulanate, metronidazole, and imipenem. Other antibiotics showed variables levels of resistance (5-30%). Killing curves with the cef/sulb against selected B. fragilis group isolates were performed and showed excellent bactericidal activity at two to four times the minimum inhibitory concentration (MIC) after 12 hr incubation, even against the isolates with high cefoperazone MICs (greater than or equal to 64 micrograms ml). There was no regrowth at 24 hr. Cefoperazone/sulbactam is a compound with excellent inhibitory and bactericidal activity against B. fragilis group isolates.

Evolution in antibiotic susceptibility of Bacteroides fragilis group strains in France based on periodic surveys

Anaerobic antibiotic susceptibility testing with standardized methods enabled the authors to speculate on the evolution of antibiotic resistance within the Bacteroides fragilis group strains. Cefoxitin resistance was stable (0-5%) until 1985 and gradually increased later. Clindamycin resistance emerged in 1980 with a stable 10% resistance rate until 1986. An increase in clindamycin resistance developed in three institutions in 1987. No change was detected for imipenem and metronidazole, while piperacillin resistance increased since 1986. A comparison of antibiotic resistance rates is discussed for anaerobes in different countries.

Comparative in vitro antimicrobial susceptibilities of a special panel of 74 Bacteroides fragilis group isolates in Wilkins-Chalgren agar with and without sheep blood

Seventy-four cefoxitin-resistant Bacteroides fragilis group isolates were tested by the serial twofold agar dilution method for susceptibility to imipenem and other agents in medium with and without 5% sheep blood. Imipenem (MIC for 90% of strains tested, 1 microgram/ml) and metronidazole (MIC for 90% of strains tested, 2 micrograms/ml) were the two most active agents. The addition of 5% sheep blood to the medium had little or no effect on the activity of the antibiotics tested.

Belgian Collaborative Study of the in-vitro susceptibility of the Bacteroides fragilis group. A Belgian Collaborative Study Group

The antimicrobial susceptibility of clinical isolates of the Bacteroides fragilis group obtained from nine large Belgian university and community hospitals was studied. Of the beta-lactam antibiotics tested, none were active against 100% of the isolates. Piperacillin was active against 93% of the strains at a breakpoint of 64 micrograms/ml. The percentage of isolates inhibited at 16 micrograms/ml (and 32 micrograms/ml) for the 7-alpha-methoxy antibiotics was: cefoxitin 84; latamoxef 87; and cefotetan 64. B. fragilis tended to be more susceptible to all beta-lactams than the other members of the group. Among the non beta-lactam antibiotics, the resistance rate for minocycline, doxycycline, clindamycin and erythromycin (at a breakpoint of 4 micrograms/ml) were 1%, 4%, 14% and 34%, respectively. Like the beta-lactams, clindamycin was less active against non B. fragilis species of the group. Chloramphenicol, metronidazole and tinidazole were still active against 100% of the isolates at their breakpoint values (8 micrograms/ml).

Susceptibility of 114 isolates of the Bacteroides fragilis group to imipenem and eight other antimicrobial agents

The minimal inhibitory concentrations (MICs) of the newer beta-lactam antibiotic, imipenem, were compared with those of cefoxitin, cefotetan, penicillin, amoxycillin, ticarcillin and metronidazole against 114 clinical isolates of the Bacteroides fragilis group of anaerobic organisms. The ability of clavulanic acid, a beta-lactamase inhibitor, to potentiate the in vitro activity of amoxycillin and ticarcillin was also studied. Using an agar dilution technique we found imipenem to be the most active beta-lactam antibiotic tested having an MIC50 of 0.25 microgram/ml and inhibiting all isolates at a concentration of 4 micrograms/ml. Metronidazole had comparable activity with a MIC50 of 0.5 microgram/ml and all isolates inhibited by 1 microgram/ml. Cefoxitin and cefotetan showed similar activity both with a MIC50 of 8 micrograms/ml against the B. fragilis group, while penicillin, amoxycillin and ticarcillin all had a MIC50 of 16 micrograms/ml. Clavulanic acid significantly reduced the MIC50 of amoxycillin and ticarcillin to 0.5 micrograms/ml and 0.25 micrograms/ml, respectively.

Recovery of anaerobic bacteria from clinical specimens in 12 years at two military hospitals

Examination of 15,844 clinical specimens submitted over 12 years (1973 to 1985) to the anaerobic microbiology laboratories in two military hospitals demonstrated the recovery of anaerobic bacteria in 4,458 (28.1%) specimens. The specimens yielded 6,557 anaerobic isolates (1.47 isolates per specimen). Bacteroides spp. accounted for 43% of all isolates; anaerobic gram-positive cocci, 26%; Clostridium spp., 7%; and Fusobacterium spp., 4%. Bacteroides spp. predominated in abscesses, obstetrical and gynecological (OBG) infections, abdominal infections, cysts, wounds, and tumors. Members of the Bacteroides fragilis group accounted for 44% of all Bacteroides spp., and of them, B. fragilis was mostly isolated in abscesses, wounds, abdomen, and blood. Pigmented Bacteroides spp. accounted for 21% of all Bacteroides sp. isolates and were mostly isolated in sinus, eye, chest, bone, and ear infections. Bacteroides melaninogenicus accounted for 42% of this group's isolates. Bacteroides bivius accounted for 9% of Bacteroides spp., and most isolates were found in OBG infections. Anaerobic gram-positive cocci were mostly isolated in OBG infections, abscesses, and wounds. The predominant anaerobic gram-positive cocci were Peptostreptococcus magnus (18%), Peptostreptococcus asaccharolyticus (17%), Peptostreptococcus anaerobius (16%), and Peptostreptococcus prevotii (13%). Clostridium spp. were mostly isolated from wounds, abscesses, abdominal infections, and blood. The predominant strain was Clostridium perfringens (48%). Fusobacterium spp. were recovered in abscesses and abdominal and OBG infections. The predominant isolate was Fusobacterium nucleatum (47%). These data illustrate the relative frequency of the different anaerobic bacteria in a variety of infections and demonstrate the predominance of certain isolates at different sites.

Management of infection following intra-abdominal trauma

Intra-abdominal infections following abdominal trauma often involve the gastrointestinal aerobic and anaerobic bacterial flora. These organisms possess various virulence factors and exhibit potential synergy between them. The intra-abdominal infection is biphasic, with the Enterobacteriaceae as the major pathogens in the peritonitis stage, and the Bacteroides fragilis group predominant in the abscess stage. Experiments with animals and experience in human beings support the need to use single or combined antimicrobial agent therapy that is effective against both Enterobacteriaceae and the B fragilis group.

Susceptibility of the Bacteroides fragilis group in the United States: analysis by site of isolation

An ongoing survey of the susceptibility of the Bacteroides fragilis group of bacteria was continued at New England Medical Center in 1984 and 1985. A total of 1,229 strains were obtained from eight centers in the United States. These results were compared with those for 1,847 isolates tested in 1981 through 1983. The most active beta-lactam drugs were imipenem and ticarcillin-clavulanic acid (Timentin), which had a less than 1% resistance rate. No metronidazole- or chloramphenicol-resistant isolates were found during the 5 years of the study. Isolates obtained from blood, perinatal, and bone sites of infection were more resistant to a variety of antimicrobial agents. Susceptibility patterns of the members of the B. fragilis group varied at the eight hospitals and among species. These data indicate the need for determining the susceptibility patterns for the B. fragilis group of organisms at each hospital.

Activity of trospectomycin against Bacteroides fragilis and other Bacteroides species

The in vitro activity of trospectomycin (U-63366; 6'-n-propyl spectinomycin pentahydrate sulfate) was evaluated against 189 clinical isolates of the Bacteroides fragilis group and 65 Bacteroides species isolates. At less than or equal to 8 micrograms/ml, the activity of trospectomycin compared favorably with those of clindamycin and cefoxitin against B. fragilis, Bacteroides distasonis, and Bacteroides vulgatus, and there was no cross resistance to these three drugs among the strains of the B. fragilis group. All the Bacteroides species were susceptible to trospectomycin. The results of this in vitro study indicate that trospectomycin possesses excellent activity against Bacteroides species.

Anaerobic bacteria in jaw cysts

The bacterial flora of the fluid content of 54 cysts of the jaws with a history suggestive of infection were studied. Forty-seven of these cysts were subsequently considered to be infected and from 78.6% of these, positive bacterial cultures were developed. Of the bacterial strains 89.2% were anaerobes and only 10.8% were pure aerobes or facultative anaerobes. Microorganisms isolated from each specimen ranged from 1 to 4 bacterial species. Gram positive anaerobic cocci revealed to be the most frequent bacterial group (36.9%), followed by Gram negative anaerobic rods (29.8%), Gram positive aerobic cocci being the third most common group of the isolates (19.0%). Antibiotic sensitivity tests of the isolated anaerobic cocci to a group of nine antibiotics revealed chloramphenicol and minocycline as the most effective. All anaerobic rods tested, were sensitive to metronidazole.

Susceptibility of anaerobic bacteria to antimicrobial agents

The antimicrobial susceptibility of 1,117 clinical isolates of anaerobic bacteria was determined by the agar dilution technique. Metronidazole was the most active agent; only Propionibacterium acnes and Actinomyces sp. isolates were resistant. Clindamycin and chloramphenical were the next most effective agents. Beta-lactam antibiotics, with the exception of penicillin, were active against most anaerobes other than the Bacteroides fragilis group. At a breakpoint of 8 mg/l, 25% of Fusobacterium spp. and 30% of the non-fragilis Bacteroides spp. were resistant to penicillin. The highest resistance to beta-lactams was seen in the B. fragilis group. Within the indole-positive members of the group, resistance rates of 71% were seen for cefoxitin, 49% for moxalactam, 79% for cefotaxime, 22% for piperacillin and 89% for penicillin. We conclude that metronidazole has the most predictable in vitro activity against common clinical anaerobic isolates and that resistance to beta-lactams was frequent and of potential clinical importance as these latter agents are frequently used in the prophylaxis and therapy of mixed anaerobic infections.

Activity of cefmetazole against anaerobic bacteria

The in vitro activity of cefmetazole versus that of other antimicrobial drugs was assessed against 374 clinical isolates of Bacteroides spp., Clostridium spp., and anaerobic gram-positive cocci. Compared with cefoxitin, cefmetazole showed good activity against Bacteroides fragilis, other Bacteroides species, and anaerobic cocci. It was somewhat less active than cefoxitin against Bacteroides thetaiotaomicron, B. ovatus, B. distasonis, and B. vulgatus and somewhat more active against Clostridium spp.

Clinical importance of cefoxitin-resistant Bacteroides fragilis isolates

Resistance to cefoxitin among species of the Bacteroides fragilis group of organisms has remained low (8%-10%) in a multicenter nationwide survey. However, a statistically significant increase in the percentage of B. fragilis group organisms resistant to cefoxitin was found at Tufts-New England Medical Center from 1981 to 1982. Non fragilis species accounted for most of the resistance. The presence of cefoxitin resistance in B. fragilis isolates correlated with resistance to other antibiotics. The presence of cefoxitin-resistant B. fragilis group organisms also correlated with the presence of other cefoxitin-resistant bacteria. No difference could be detected in therapeutic outcome of patients with cefoxitin-sensitive or cefoxitin-resistant B. fragilis group organisms, regardless of treatment with cefoxitin or other antibiotics.

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.

Susceptibility testing of 7 antibiotics against anaerobic bacteria: comparison of 2 different media and carbon dioxide concentrations

Anaerobic agar (AA) and Danish Blood agar (DBA) were evaluated by a standardized agar diffusion and agar dilution test in 5% CO2. The activity of seven antibiotics (tetracycline, clindamycin, metronidazole, rifamycin, chloramphenicol, penicillin, erythromycin) was tested against 40 anaerobic bacteria, including 3 control strains (Cl.perfringens ATCC 13124, B.fragilis ATCC 25285, B.thetaiotaomicron ATCC 29741). 70% of the strains were resistant to erythromycin in 10% CO2, only 30% in 5% CO2. No evident CO2-effect could be seen with the other antibiotics. Mean MIC for tetracycline was twice as great on AA than DBA. In spite of that, tablet sensitivity testing with tetracycline on AA proved to be more accurate and completely separated the resistant and susceptible strains. For penicillin, the mean MIC was one dilution step higher on AA. No major differences could be seen with the other antibiotics. AA was superior to DBA in providing growth of anaerobes. Measurement on AA was easier, and it was more precise. Except for tetracycline. MIC on control strains fell well within range set by The National Committee for Clinical Laboratory Standards (NCCLS) on AA. Acceptable correlation coefficients were recorded between agar diffusion and agar dilution. Prediction of susceptibility based on zone diameter measurements was very good on AA. Only one discrepancy that could cause change of susceptibility status occurred on AA, while there were 12 on DBA. On DBA, there was poor correlation between MIC, compared with earlier results on the same agar. The 50% inhibitory concentration (IC50) was also measured, but offered no advantage over MIC.

Beta-lactamase-mediated imipenem resistance in Bacteroides fragilis

Imipenem has excellent antimicrobial activity owing in part to beta-lactamase stability. We found that only 2 of over 350 Bacteroides fragilis group clinical isolates were resistant to imipenem, with an MIC of more than 16 micrograms/ml. These two isolates from the Tufts Anaerobe Laboratory (TAL) were resistant to all other beta-lactam agents tested. The organisms were able to inactivate imipenem in broth cultures and contained similar beta-lactamases that were able to hydrolyze carbapenems, cephamycins, cephalosporins, and penicillins. The molecular sizes of the beta-lactamases in TAL2480 and TAL3636 were estimated to be 44,000 daltons. The novel beta-lactamase contained Zn2+ as a cofactor. An additional factor contributing to resistance was determined. The outer membranes of these two organisms were found to limit free diffusion of the drugs into the periplasm. This novel beta-lactamase, associated with a barrier to drug permeation, resulted in high-grade beta-lactam drug resistance.

Bacteroides fragilis: current susceptibilities, mechanisms of drug resistance, and principles of antimicrobial therapy

The Bacteroides fragilis group of organisms includes the most clinically important anaerobic bacteria. Optimal therapy of infections in which these organisms are involved includes adequate and timely surgical drainage of all collections, debridement of necrotic tissue, optimal nutritional support, and administration of appropriate empiric antibiotics to cover both the aerobic and anaerobic bacterial components of these mixed infections. Special attention must be paid to the B. fragilis group because of its high rate of resistance to many of the commonly used antibiotics. Of the currently available beta-lactam antibiotics, piperacillin has the lowest rate of resistance. Successful antimicrobial agents include clindamycin, chloramphenicol, and metronidazole plus an aminoglycoside. Piperacillin, cefoxitin, and moxalactam can be used with an aminoglycoside or alone if no resistant organisms are revealed on culture and susceptibility testing. Beta-lactam-based regimens are potentially less toxic and may be less costly than those that contain one or more non-beta-lactam antibiotics.

Transfer of beta-lactamase-associated cefoxitin resistance in Bacteroides fragilis

A cefoxitin-resistant Bacteroides fragilis isolate, TAL 4170, which inactivates cefoxitin, was able to transfer beta-lactamase-mediated cefoxitin resistance to a susceptible B. fragilis recipient. Cefoxitin-resistant transconjugants acquired a new beta-lactamase with a pI of 8.1 and were able to inactivate cefoxitin and retransfer cefoxitin resistance. No plasmids were detected in the donor or transconjugants.

Evaluation of broth disk elution methods for susceptibility testing of anaerobic bacteria with the newer beta-lactam antibiotics

Broth disk elution procedures represent one of the most practical means for clinical laboratories to perform routine antibiotic susceptibility tests on anaerobic bacteria. The accuracy of five disk elution test methods and media (including the one to be proposed by the National Committee for Clinical Laboratory Standards) was evaluated for the testing of newer beta-lactam antibiotics, including cefoperazone, cefotaxime, cefoxitin, ceftazidime, ceftizoxime, moxalactam, and piperacillin. Various numbers of antibiotic disks were used to achieve disk elution test concentrations which approximated the highest MIC termed susceptible by the Food and Drug Administration. A group of 88 anaerobes representing many different species was tested in parallel by the five disk elution methods and the National Committee for Clinical Laboratory Standards reference agar dilution procedure. Overall, full agreement between the reference agar dilution MICs and the disk elution category results was 88.3% for PRAS BHI, 84.5% for Schaedler, 85.7% for thioglycolate, and 87.4% for Wilkins-Chalgren broth. Essential agreement (+/- 1 twofold MIC increment from the disk elution concentration) was achieved with 94.6% of PRAS BHI tests, 94.3% of Schaedler tests, 93.6% of thioglycolate tests, and 95.7% of Wilkins-Chalgren tests. Due to growth failures with a number of isolates and difficulties in interpreting results, the use of Wilkins-West broth was discontinued after approximately one-half of the isolates had been tested. The majority of errors with all of the disk elution methods occurred with isolates (most notably members of the Bacteroides fragilis group) having MICs near the single test concentrations used in the disk methods. With the notable exception of tests for the B. fragilis group, the disk elution methods offered acceptable accuracy with the newer beta-lactam antibiotics tested in this study.

Six-year retrospective survey of the resistance of Bacteroides fragilis group species to clindamycin and cefoxitin

Two hundred and forty-six strains of the Bacteroides fragilis group, all clinical isolates, collected at the Wadsworth Veterans Administration Medical Center from 1977 to 1982, were tested for susceptibility to clindamycin and cefoxitin. There was no significant change in resistance to either clindamycin or cefoxitin over the time period tested for any individual species, nor for the B. fragilis group in toto. Striking differences in susceptibility to the two drugs were seen among species of the B. fragilis group. B. fragilis displayed resistance to cefoxitin (32 micrograms/ml) and clindamycin (8 micrograms/ml) of 0.0% and 0.8%, respectively, whereas B. thetaiotaomicron showed resistances of 12.7% to cefoxitin (32 micrograms/ml) and 9% to clindamycin (8 micrograms/ml). B. thetaiotaomicron, B. distasonis, and B. ovatus are distinctly more resistant to cefoxitin than B. fragilis and B. vulgatus. Similarly B. thetaiotaomicron and B. distasonis are much more resistant to clindamycin than are the other B. fragilis group species. It is apparent that determination of species within the B. fragilis group is important in evaluating a potential therapeutic regimen.