Nationwide study of the susceptibility of the Bacteroides fragilis group in the United States

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.

The Surgical Infection Society guidelines on antimicrobial therapy for intra-abdominal infections: evidence for the recommendations

Revised guidelines for the use of antimicrobial therapy in patients with intra-abdominal infections were recently developed by the Therapeutic Agents Committee of the Surgical Infection Society (Mazuski et al., Surg Infect 2002;3:161-173). These were based, insofar as possible, on evidence published over the past decade. The objective of this document is to describe the process by which the Committee identified and reviewed the published literature utilized to develop the recommendations and to summarize the results of those reviews. English-language articles published between 1990 and 2000 related to antimicrobial therapy for intra-abdominal infections were identified by a systematic MEDLINE search and an examination of references included in recent review articles. If current literature with regard to a specific issue was lacking, relevant articles published prior to 1990 were identified. All prospective randomized controlled trials, as well as other articles selected by the Committee, were evaluated individually and collectively. Data with regard to patient numbers, types of infections, and results of interventions were abstracted. Studies were categorized according to their design, and all included trials were graded according to quality. On the basis of this evidence, the Committee formulated recommendations for antimicrobial therapy for intra-abdominal infections and graded those recommendations. After receiving comments from invited reviewers and the general membership of the Society, the guidelines were finalized and submitted to the Council of the Surgical Infection Society for approval. The final recommendations related to the selection of patients needing therapeutic antimicrobials, acceptable antimicrobial regimens, duration of antimicrobial use, and the identification and treatment of higher-risk patients. Although numerous publications pertaining to these topics were identified, but nearly all of the prospective randomized controlled trials represented comparisons of different antimicrobial regimens for the treatment of intra-abdominal infections. A few prospective trials evaluated the need for therapeutic antimicrobial therapy in patients with peritoneal contamination following abdominal trauma. The quality of these prospective trials was highly variable. Many did not limit enrollment to patients with complicated intra-abdominal infections, lacked blinding of treatment assignment, did not provide a complete description of the criteria used to determine therapeutic success or failure, failed to identify the reasons why patients were excluded from analysis, or did not include an intention-to-treat analysis. For many issues, no prospective randomized controlled trials were encountered, and guidelines had to be formulated using evidence from studies with historical controls or uncontrolled data, or on the basis of expert opinion

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.

Ureidopenicillins and beta-lactam/beta-lactamase inhibitor combinations

Although research and development of new penicillins have declined, penicillins continue to be essential antibiotics for the treatment and prophylaxis of infectious diseases. The most recent additions are the ureidopenicillins and beta-lactam/beta-lactamase inhibitor combinations. This article reviews the spectrum of activity, toxicity, pharmacokinetics, and clinical uses of the ureidopenicillins, and the beta-lactam/beta-lactamase inhibitor combination agents.

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.

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

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

Longitudinal study of susceptibilities of species of the Bacteroides fragilis group to five antimicrobial agents in three medical centers

A total of 579 clinical isolates of the Bacteroides fragilis group collected from three Canadian hospitals were tested for susceptibility to five antimicrobial agents by using an agar dilution method. During the 4-year survey, isolates from intra-abdominal infections were collected from the following sites: abdominal abscesses (48%), peritoneal fluid (39%), blood (10%), and bile (3%). B. fragilis was the most prevalent species (35.4%), followed by B. thetaiotaomicron (19.2%), B. ovatus (15.9%), and B. vulgatus (11%). No metronidazole- or imipenem-resistant strains were found during the survey. Resistance profiles varied among the different species tested: 7.8, 2.9, and 7.3% of B. fragilis strains (n = 205) and 68.1, 17.2, and 9.4% of non-B. fragilis strains (n = 373) were resistant to cefotetan, cefoxitin, and clindamycin, respectively. B. fragilis and B. vulgatus demonstrated lower resistance rates than B. thetaiotaomicron, B. ovatus, B. distasonis, and B. caccae. During the study, rates of resistance to cefotetan and clindamycin fluctuated but rates of resistance to cefoxitin increased, particularly at one center. These data indicate a need to determine the susceptibility patterns of the B. fragilis group periodically at each hospital.

In vitro activities of metronidazole and its hydroxy metabolite against Bacteroides spp

Metronidazole is metabolized to two major oxidative products: an acid metabolite and a hydroxy metabolite. While the activity of the acid metabolite is negligible, the activity of the hydroxy metabolite is approximately 65% of the activity of the parent drug. Pharmacokinetic studies of metronidazole and its hydroxy metabolite have shown that the MICs of both compounds remain above the MICs for most anaerobic organisms over an 8-h dosing interval. By a checkerboard assay, the combined activities of metronidazole and the hydroxy metabolite were examined against 4 quality control strains of Bacteroides species. Macrobroth tube dilutions were set up with Wilkins-Chalgren broth. Serial twofold dilutions of each agent were performed to achieve final concentrations ranging from 0.06 to 4.0 micrograms/ml. The MICs for Bacteroides fragilis and B. distasonis were 1.0 microgram/ml for both parent drug and metabolite. For B. thetaiotamicron and B. ovatus, the MICs of metronidazole and the hydroxy metabolite were 1.0 and 2.0 micrograms/ml, respectively. Synergy was determined by calculating the fractional inhibitory concentration (FIC) index. The interpretative criteria for the FIC index were as follows: synergy, FIC < or = 0.5; partial synergy, 0.51 to 0.75; indifference, FIC 0.76 to 4.0; and antagonism, FIC > 4.0. Partial synergy was observed for the four anaerobes tested, with FIC indices ranging from 0.63 to 0.75. On the basis of this data, in vitro susceptibilities to agents such as metronidazole may ultimately require reevaluation to account for active metabolites.

A five-year multicenter study of the susceptibility of the Bacteroides fragilis group isolates to cephalosporins, cephamins, penicillins, clindamycin, and metronidazole in the United States

Over 2800 clinical strains of the Bacteroides fragilis group were collected during a 5-year period from ten geographically separate sites and tested for their susceptibility to various antimicrobial agents using a broth microdilution method. Among the cephalosporins, ceftizoxime was the most active (13% resistance) and importantly exhibited relatively equal activity against both B. fragilis species and non-B. fragilis species. Cefotaxime exhibited similar activity with an overall resistance rate of 18%. Both ceftriaxone and cefoperazone were appreciably less active cephalosporins especially against non-B. fragilis species. With regard to cephamycins, cefoxitin (MIC90, 32 micrograms/ml) was more active than cefotetan (MIC90, > or = 256 micrograms/ml) and cefmetazole (MIC90, 64 micrograms/ml). Non-B. fragilis species were highly resistant to cefotetan and cefmetazole. Imipenem was highly active against all strains with the exception of four strains of B. fragilis. Ampicillin-sulbactam, amoxicillin-clavulanate, piperacillin-tazobactam, and cefoperazone-sulbactam were all highly active with resistance rates < 2%. No resistance was detected to metronidazole, whereas 14% of isolates were resistant to clindamycin. When compared with other studies, these findings underscore the wide variability in susceptibility patterns reported nationwide and the need to continue monitoring these patterns to aid in choosing the most active compounds for therapy.

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.

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.

Susceptibilities of members of the Bacteroides fragilis group to 11 antimicrobial agents

The susceptibilities of 200 clinical isolates of the Bacteroides fragilis group to 11 antimicrobial agents were determined by the broth microdilution method of the National Committee for Clinical Laboratory Standards. All isolates were susceptible to imipenem and ticarcillin-clavulanic acid. The rates of resistance to cefoxitin and clindamycin were low (4 and 6%, respectively), while those to ceftizoxime and cefotetan were higher (10.5 and 24%, respectively).

Inhibitory and bactericidal activity of selected beta-lactam agents alone and in combination with beta-lactamase inhibitors compared with that of cefoxitin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group

The inhibitory activity of five beta-lactam agents, alone and in combination with a beta-lactamase inhibitor, was compared with that of cefoxitin and metronidazole against 300 beta-lactamase producing Bacteroides fragilis group isolates. Each of the beta-lactamase inhibitors significantly potentiated the activity of the respective beta-lactam. In the presence of clavulanate, the MIC90 (minimum inhibitory concentration) values of amoxicillin and ticarcillin were reduced 64-fold and 32-fold, respectively. Similarly, sulbactam enhanced the activity of ampicillin and cefoperazone 16-fold and 8-fold, respectively, whereas tazobactam potentiated the activity of piperacillin 16-fold. Few strains were resistant to the beta-lactam-beta-lactamase inhibitor combinations and were comprised of strains of B. fragilis, B. thetaiotamicron, and B. distasonis. Of the strains, 7% were resistant to cefoxitin, and none to metronidazole. Using time-kill kinetic studies, the bactericidal activity of the various beta-lactam agents, with and without beta-lactamase inhibitors, was determined and compared with that of cefoxitin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the B. fragilis group. Overall, metronidazole was the most bactericidal agent with all isolates being killed with less than or equal to 4 micrograms/ml at 24 hr. Ampicillin-sulbactam was the next most bactericidal agent with all isolates being killed with less than or equal to 16/8 micrograms/ml of ampicillin-sulbactam at 24 hr. Amoxicillin-clavulanate and cefoperazone-sulbactam had bactericidal activity similar to that of ampicillin-sulbactam. Piperacillin-tazobactam and ticarcillin-clavulanate were bactericidal at higher concentrations with all isolates killed with 64 micrograms/ml of piperacillin and 128 micrograms/ml of ticarcillin combined with their respective beta-lactamase inhibitors. None of the beta-lactam agents alone was able to kill more than 19 of the 26 isolates. We conclude that beta-lactam agents combined with beta-lactamase inhibitors have both inhibitory and bactericidal activity against cefoxitin-resistant members of the B. fragilis group provided that the concentrations achieved for these combinations are at the upper limits for maximum recommended dosing. Although isolates of the B. fragilis group have been reported to produce unusual beta-lactamases that are refractory to beta-lactamase inhibitors, none of the cefoxitin-resistant isolates tested in this study were resistant to the beta-lactam-beta-lactamase inhibitor combinations.

Pharmacokinetics and tissue penetration of single-dose cefotetan used for antimicrobial prophylaxis in patients undergoing colorectal surgery

The pharmacokinetics and tissue penetration of cefotetan were studied after a single injection of 2 g given intravenously for antimicrobial prophylaxis to 16 consecutive patients undergoing colorectal surgery. Concentrations in tissue greater than or equal to the MIC for 90% of the main pathogens tested were considered adequate. The elimination half-life at beta phase was 4.6 +/- 1.4 h, the total body clearance was 0.75 +/- 0.19 ml/kg/min, and the volume of distribution was 260 +/- 71 ml/kg. At the time of incision (33 +/- 16 min after the injection), cefotetan concentrations were 14.2 +/- 7 micrograms/g in abdominal-wall fat, 16.4 +/- 1 micrograms/g in epiploic fat, and 163 +/- 62 mg/liter in serum. At the time of surgical anastomosis (151 +/- 54 min), cefotetan concentrations were 33.3 +/- 6 micrograms/g in the colonic wall and 73 +/- 34 mg/liter in serum. Upon closure of the abdomen (216 +/- 76 min), cefotetan concentrations were 6.3 +/- 3 micrograms/g in abdominal-wall fat, 6.1 +/- 4 micrograms/g in epiploic fat, and 64 +/- 38 mg/liter in serum. Cefotetan tissue penetration was 10% into abdominal and epiploic fat and 46% into the colonic wall. Levels in tissue were compared with the MIC for 90% of the most frequently encountered pathogenic germs (Staphylococcus aureus, Bacteroides fragilis, and Escherichia coli). Adequate concentrations in tissue were obtained up to anastomosis but not upon closure. The authors therefore recommend the injection of an additional dose of 1 g before closure in order to ensure optimal efficacy throughout the surgical procedure.

A Bacteroides tetracycline resistance gene represents a new class of ribosome protection tetracycline resistance

The ribosome protection type of tetracycline resistance (Tcr) has been found in a variety of bacterial species, but the only two classes described previously, Tet(M) and Tet(O), shared a high degree of amino acid sequence identity (greater than 75%). Thus, it appeared that this type of resistance emerged recently in evolution and spread among different species of bacteria by horizontal transmission. We obtained the DNA sequence of a Tcr gene from Bacteroides, a genus of gram-negative, obligately anaerobic bacteria that is phylogenetically distant from the diverse species in which tet(M) and tet(O) have been found. The Bacteroides Tcr gene defines a new class of ribosome protection resistance genes, Tet(Q), and has a deduced amino acid sequence that was only 40% identical to Tet(M) or Tet(O). Like tet(M) and tet(O), tet(Q) appears to have spread by horizontal transmission, but only within the Bacteroides group.

Microbiological study of foot-rot in lambs: isolation, elastolytic activity and antimicrobial susceptibility

A microbiological study was made of an outbreak of foot-rot in lambs. Elastolytic activity was detected in all 10 Bacteroides nodosus strains isolated from the hooves of 9 affected animals. Among 14 other isolated strictly anaerobic microorganisms belonging to different species or genera only 4 strains showed a low degree of elastolysis. The 14 antimicrobial agents studied effectively inhibited "in vitro" growth of B. nodosus.

Survey of Bacteroides fragilis group susceptibility patterns in Canada

The in vitro activities of penicillin, clindamycin, chloramphenicol, metronidazole, piperacillin, piperacillin-tazobactam, ticarcillin, ticarcillin-clavulanate, ampicillin-sulbactam, cefoxitin, ceftizoxime, cefotetan, moxalactam, and imipenem against 348 Bacteroides fragilis group isolates collected from six Canadian cities during 1990 were determined by the National Committee for Clinical Laboratory Standards (NCCLS) agar dilution technique. All isolates were susceptible to chloramphenicol, metronidazole, piperacillin-tazobactam, and imipenem. For the other antibiotics tested, the following resistance rates were observed: penicillin, 97%; clindamycin, 9%; piperacillin, 19%; ticarcillin, 31%; ticarcillin-clavulanate, 0.28%; ampicillin-sulbactam, 0.85%; cefoxitin, 26%; ceftizoxime, 15%; cefotetan, 53%; and moxalactam, 17%. Susceptibility profiles to beta-lactam antibiotics varied among the different species tested: B. fragilis and Bacteroides vulgatus demonstrated lower resistance rates than Bacteroides distasonis and indole-positive Bacteroides thetaiotaomicron and Bacteroides ovatus. Ceftizoxime results should be interpreted cautiously, because the MICs obtained with the recommended NCCLS control strain were lower than expected.

Pharmacokinetics and tissue penetration of a single 1,000-milligram, intravenous dose of metronidazole for antibiotic prophylaxis of colorectal surgery

The levels of metronidazole in serum and tissue penetration of metronidazole were studied after prophylactic administration in 11 patients undergoing elective colorectal surgery. A single dose of 1,000 mg given intravenously was administered before surgery. Adequate drug levels in serum (greater than or equal to MIC for 90% of strains tested [MIC90] for Bacteroides fragilis) were found in all patients throughout the procedure. Mean peak (15-min) and last-determined (24-h) metronidazole levels in serum were 28.8 +/- 8 and 4.2 +/- 1.7 mg/liter, respectively. The beta-phase elimination half-life was 9.5 +/- 2.3 h, and the clearance and apparent volume of distribution were 57 +/- 13 ml/min and 0.7 +/- 0.1 liter/kg, respectively. In the colonic wall at surgical anastomosis, tissue metronidazole levels greater than or equal to MIC90 for B. fragilis were found in 91% of patients. In the abdominal wall fat and epiploic fat, tissue metronidazole levels greater than or equal to MIC90 for B. fragilis were found in 40 to 60% of patients at surgical incision and closure. No anaerobic infection occurred during the study.

Comparison of the bactericidal activity of clindamycin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group

Time-kill kinetic methodology was used to evaluate the bactericidal activity of cefoxitin, cefotetan, clindamycin, and metronidizole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group. Overall, metronidazole was the most bactericidal agent, with all isolates being killed with less than or equal to 4 micrograms/ml at 24 hr. Clindamycin was the next most bactericidal agent, with 20 of 26 isolates being killed with less than 16 micrograms/ml. Six isolates with clindamycin MICs greater than or equal to 64 micrograms/ml were not killed at 24 hr, with concentrations as high as 256 micrograms/ml. Cefoxitin and cefotetan were the least bactericidal agents tested. Seven isolates with MICs of greater than or equal to 64 micrograms/ml to each agent demonstrated a lack of killing at 24 hr, with concentrations of the respective agent as high as 256 micrograms/ml. At concentrations with either agent of 32 micrograms/ml, the remaining 19 isolates were killed at 24 hr. Of the six B. fragilis isolates resistant to clindamycin, four were also resistant to both cefoxitin and cefotetan. We conclude that in hospitals with cefoxitin-resistant B. fragilis group isolates, metronidazole would provide appropriate therapy.

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.

In vitro antibacterial activity of trospectomycin (U-63,366F) against anaerobic bacteria and aerobic gram-positive cocci in Chile

The in vitro activity of trospectomycin sulfate was compared with those of several antimicrobials, against 301 anaerobic bacteria and 613 aerobic Gram-positive cocci. Trospectomycin was about 4- to 32-fold more active than was spectinomycin. Trospectomycin exhibited consistently good activity against all Bacteroides fragilis group isolates, except Bacteroides vulgatus, and against all other anaerobes comparable or higher to that of clindamycin. The trospectomycin's activity was most similar to that of vancomycin, even against methicillin-resistant Staphylococcus aureus and methicillin-resistant Staphylococcus epidermidis.

Pharmacokinetics and tissue penetration of a single dose of ornidazole (1,000 milligrams intravenously) for antibiotic prophylaxis in colorectal surgery

Levels in serum and tissue penetration of ornidazole were studied after a single intravenous injection of 1,000 mg given to 14 patients for prophylaxis of surgical infection. They were scheduled for elective colorectal surgery. Adequate levels in blood (greater than or equal to MIC for 90% of Bacteroides fragilis strains tested) were found in all patients throughout the procedure and up to hour 24. Mean-maximal (15 min) and last-determined (24 h) ornidazole levels in serum were 24 +/- 5.2 and 6.3 +/- 1.4 mg/liter, respectively. beta-Phase elimination half-life was 14.1 +/- 2.7 h, and clearance and apparent volume of distribution were 47 +/- 12 ml/min and 0.9 +/- 0.13 liters/kg, respectively. In all patient, adequate levels in tissue were found in the abdominal wall and the epiploic fat at time of incision and in the colonic wall at time of anastomosis. At time of closure, all but one patient had adequate levels in tissue in the abdominal wall and the epiploic fat. No anaerobic nor aerobic infection occurred in the study patients.

In-vitro antimicrobial susceptibility of Bacteroides and Fusobacterium isolated from footrot in goats

The agar dilution method was used to determine the antimicrobial activity of 17 antimicrobial agents against 132 strains belonging to the genus Bacteroides and 25 strains from the genus Fusobacterium, all isolated from 120 clinical cases of caprine footrot between October 1987 and November 1988. Josamycin, chloramphenicol and rifampin proved to be the most effective antibiotics in vitro. Significant resistance was found to the other antimicrobial agents studied.

Differences in the in vitro inhibitory and bactericidal activity of ceftizoxime, cefoxitin, cefotetan, and penicillin G against Bacteroides fragilis group isolates. Comparison of time-kill kinetic studies with MIC values

Nineteen strains of the Bacteroides fragilis group were used to determine the bactericidal activity of ceftizoxime, cefoxitin, cefotetan, and penicillin G with time-kill kinetics studies. Each antimicrobial agent was tested at subinhibitory (1/2 X MIC), inhibitory (1 X MIC), and suprainhibitory (4 X MIC) concentrations. Penicillin G exhibited virtually no sustained bactericidal activity at any of the antimicrobial concentrations tested. At subinhibitory concentrations, ceftizoxime was considerably more bactericidal than cefoxitin or cefotetan: At 12 hr, ceftizoxime killed 89% of the inoculum, whereas cefoxitin and cefotetan killed 35% and 33% of the inoculum, respectively. At inhibitory concentrations, ceftizoxime was again more bactericidal than cefoxitin and cefotetan: At 12 hr, ceftizoxime killed 90% of the inoculum, whereas cefoxitin and cefotetan killed 78% and 73%, respectively. At suprainhibitory concentrations, all three antimicrobial agents showed comparable bactericidal activity at 12 and 24 hr. Ceftizoxime and cefoxitin had somewhat lower killing rates overall against test strains with high MICs (greater than or equal to 32) versus low MICs (less than or equal to 16). However, at subinhibitory concentrations, ceftizoxime killed the B. fragilis group strains with high or low MIC values more effectively than cefotetan killed strains with low MICs. At the highest antibiotic concentrations tested (4 X MIC), only slight differences were seen in the bactericidal activity of the three compounds, regardless of MICs.

Sequencing the gene for an imipenem-cefoxitin-hydrolyzing enzyme (CfiA) from Bacteroides fragilis TAL2480 reveals strong similarity between CfiA and Bacillus cereus beta-lactamase II

Using a newly constructed Bacteroides fragilis-Escherichia coli cloning shuttle vector, pJST61, we have cloned the cefoxitin (FOX)-imipenem (IMP) resistance determinant from B. fragilis TAL2480. FOX-IMP resistance in this strain results from the production of a periplasmic, Zn2(+)-containing beta-lactamase which hydrolyzes carbapenems and cephamycins and whose activity is resistant to clavulanic acid but sensitive to Zn2(+)-binding reagents, including EDTA. The pJST61 vector permits efficient library construction in E. coli and allows for the transfer of the library to B. fragilis recipients for the screening or selection of specific phenotypes. The library clone containing the FOX-IMP resistance gene was detected after transfer to B. fragilis TM4000 (Fox-Imps) selecting for Foxr. One of the isolates carrying plasmid pJST241 is resistant to FOX and IMP and synthesizes a periplasmic protein with substrate and inhibitor properties identical to those of strain TAL2480. On the basis of deletion analysis, Tn1000 insertion mutations, and DNA sequencing, we have defined the 747-base cfiA (FOX-IMP resistance) gene within the 3.6-kilobase cloned insert in pJST241. The cfiA gene contains an open reading frame that could code for a precursor protein of 249 amino acids and with a molecular mass of 27,260 daltons. A potential signal sequence has been identified at the N terminus of this protein; cleavage within this sequence would result in a protein of 231 amino acids with a molecular mass of 25,249 daltons. The CfiA protein shows remarkable similarities to the exported, Zn2(+)-requiring, type II beta-lactamase Blm proteins from Bacillus cereus 569/H and 5/B/6. Although overall amino acid identity is only 32%, the Zn ligand-binding His and Cys residues are precisely conserved and the amino acids in the vicinity of these sites show strong similarities (greater than 80%) when the CfiA and Blm proteins are compared.

Susceptibilities of species of the Bacteroides fragilis group to 10 antimicrobial agents

A total of 94 clinical isolates of the Bacteroides fragilis group was tested for susceptibility to metronidazole, chloramphenicol, clindamycin, cefoxitin, cefotetan, cefmetazole, moxalactam, mezlocillin, amoxicillin-clavulanic acid, and imipenem. All the strains tested were susceptible to imipenem, metronidazole, amoxicillin-clavulanic acid, and chloramphenicol. The rate of resistance to clindamycin was 21%. The results of this study demonstrate a difference in resistance rates from one species of the B. fragilis group to another.

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.

Kinetics of Antimicrobial Activity of Amoxicillin/Clavulanic Acid and Metronidazole against β-Lactamase-Producing Bacteroides fragilis Group

Bacteroides fragilis group are the most common anaerobic bacteria isolated in clinical specimens. The use of a beta-lactam with a β-lactamase inhibitor should result in a marked increase in the group's sensitivity to the β-lactams. Since the activity (MIC) shown by the amoxicillin + clavulanic acid combination against Bacteroides fragilis group is good, other parameters of in vitro activity have been studied. This study was also done with metronidazole. The minimum inhibitory concentration (MIC) was determined in 26 strains of Bacteroides fragilis group (14 B. fragilis; 5 B. thetaiotaomicron; 4 B. vulgatus; 3 B. distasonis). Likewise, the minimum bactericidal concentration (MBC), the killing curve, the sub-MIC and post-antibiotic effect were determined. The MIC ranged between 0.5 and 32 mg/l. The MBC was two- to four-fold the MIC for amoxicillin/clavulanic acid, and one- to two-fold the MIC for metronidazole for most strains. The killing curve showed a continuous decrease, sloping most sharply between 0-2 hours and 6-8 hours. Amoxicillin + clavulanic acid showed a post-antibiotic effect between 2 and 4 hours. The inhibitory minimum antibiotic concentration was one-half the MIC for most strains.

Comparison of in vitro antibiograms of Bacteroides fragilis group isolates: differences in resistance rates in two institutions because of differences in susceptibility testing methodology

With 120 clinical isolates of the Bacteroides fragilis group, a comparison of rates of resistance to selected antimicrobial agents by using two susceptibility tests was performed in two medical institutions. The broth microdilution method produced MICs significantly lower than those determined by the agar dilution method. With ceftizoxime and cefoxitin, 88 and 18%, respectively, of the MICs were greater than or equal to 2 twofold dilutions apart. These differences in MIC results produced major interpretive discrepancies for ceftizoxime and cefoxitin, whereas no significant differences in resistance rates were noted for clindamycin and metronidazole.

Antibiotics in acute pancreatitis. Current status and future directions

Infectious complications currently account for 80 percent of deaths from acute pancreatitis. The adjunctive role of antibiotics in the prevention and treatment of secondary pancreatic infections has received insufficient attention. Randomized clinical studies of effective antibiotics for prophylaxis or empiric therapy of pancreatic infections do not currently exist. In their absence, it is not known whether prophylactic antibiotics are useful in patients with acute pancreatitis. Until such studies are available, if antibiotics are to be used, their choice must be based upon indirect criteria: the ability of the antibiotic to effectively penetrate pancreatic tissue and juice, knowledge of the most common pancreatic pathogens, and the ability of the antibiotic to exceed the in vitro concentration (MIC-90) in pancreatic juice for the common pathogens. Recognition of the limited state of knowledge regarding antibiotics in acute pancreatitis may stimulate future investigations.

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.

Patterns of resistance to anaerobic organisms in Australia

This study investigated patterns of resistance in anaerobic organisms isolated at Westmead Hospital in Sydney, Australia, during the years 1987 to 1988. Minimal inhibitory concentrations (MICs) to ampicillin, sulbactam/ampicillin, metronidazole, clindamycin, and cefoxitin were determined by agar dilution for 200 anaerobes from clinically significant infections. Antibiotics active against nearly all of these anaerobes included metronidazole and sulbactam/ampicillin, which demonstrated good activity against beta-lactamase producing Bacteroides spp. with the exception of Bacteroides distasonis. Resistance in non-beta-lactamase producing anaerobes was similar to that seen with ampicillin. As expected, ampicillin resistance was common in the Bacteroides fragilis group where beta-lactamase production was frequent. In addition, beta-lactamase was detected in 33% of other Bacteroides spp. Ampicillin resistance was also seen in 5 to 15% of additional anaerobes that did not produce beta-lactamase. Clindamycin resistance occurred in 4 to 18% of the B. fragilis group. Clindamycin resistance was also seen in 7 to 8% of Clostridium spp. and anaerobic Gram-positive cocci. Resistance to cefoxitin was variable in the B. fragilis group with the highest levels of resistance occurring in the indole-negative subgroup. Resistance in other anaerobes was not commonly seen.

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 activity of cefoxitin, cefotaxime alone, and in combination with desacetylcefotaxime against the Bacteroides species

The agar dilution method was used to determine the inhibitory activity of cefotaxime (CTX) alone, desacetylcefotaxime (dCTX) alone, CTX plus dCTX, and cefoxitin against 74 clinical isolates of the Bacteroides species recovered from diabetic patients with foot ulcers. The study concluded that the addition of dCTX to CTX increased the inhibitory activity from 45% to 73% for all strains tested and from 50% to 81% among the 32 strains of Bacteroides fragilis. This synergistic interaction against B. fragilis resulted in a four- to nine-fold reduction in the MIC of seven strains (64-128 micrograms/ml, resistant category MICs). While the lowest CTX MIC for B. fragilis was 2 micrograms/ml (four strains), the addition of dCTX also produced a remarkable reduction in susceptible range CTX MICs to 0.05-2 micrograms/ml in 16 strains (50%). The overall susceptibility to cefoxitin and CTX plus dCTX was as follows: 100% and 100% for Bacteroides vulgatus, 50% and 66% for Bacteroides thetaiotaomicron, 100% and 33% for Bacteroides ovatus, and 83% and 82% for Bacteroides species other than the B. fragilis group.

Annual incidence, epidemiology, and comparative in vitro susceptibilities to cefoxitin, cefotetan, cefmetazole, and ceftizoxime of recent community-acquired isolates of the Bacteroides fragilis group

The six species of the Bacteroides fragilis group are potent pathogens and commonly have different susceptibility patterns. We determined the relative annual isolation rate of anaerobic bacteria and the susceptibility of B. fragilis group species isolated during 1987 at two community hospitals. The relative frequencies of isolation of 261 strains were as follows: B. fragilis, 61%; B. thetaiotaomicron, 17%; B. distasonis, 7%; B. vulgatus, 6%; B. ovatus, 5%; and B. uniformis, 4%. A total of 234 recent clinical isolates were tested against cefmetazole, cefotetan, cefoxitin, ceftizoxime, clindamycin, imipenem, and piperacillin by a brucella agar dilution method. Imipenem was the most active agent tested with all but three isolates (two B. vulgatus and one B. distasonis) susceptible to less than 2 micrograms/ml. Of the cephalosporins tested, cefoxitin, cefotetan, and cefmetazole were relatively equal against B. fragilis, with 93 to 98% of strains susceptible to 32 micrograms/ml. Ceftizoxime was less active, with an MIC for 90% of strains tested of 128 micrograms/ml and only 75% of isolates susceptible to 32 micrograms/ml. Against B. ovatus, B. vulgatus, B. thetaiotaomicron, and B. uniformis, cefoxitin showed a two- to threefold-superior activity compared with that of cefotetan and cefmetazole. In general, ceftizoxime was much less active, except against B. distasonis, for which 78% of isolates were susceptible to 32 micrograms/ml compared with 68% for cefoxitin, 19% for cefmetazole, and 16% for cefotetan. Clindamycin and piperacillin showed activity similar to that of cefoxitin, except piperacillin was less active versus B. vulgatus and B. distasonis. We therefore suggest that clinical laboratories determine the species of B. fragilis group isolates as well as perform susceptibility studies on the isolates. Clinicians should be aware that while B. fragilis is the most frequent isolate, 38% of isolates are from other, more resistant B. fragilis group species.

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).

A multicenter study of the in vitro antianaerobic activity of cefotetan compared with other antimicrobial agents

The in vitro antianaerobic activity of cefotetan was compared with that of chloramphenicol, clindamycin, cefoxitin, and penicillin in a multicenter study. Both agar dilution and broth microdilution testing procedures, as described by the National Committee for Clinical Laboratory Standards (NCCLS), were employed; a total of 1,377 strains were examined. Results were interpreted using the U.S. Food and Drug Administration- and NCCLS-recommended criteria. This study indicates that Bacteroides fragilis, Clostridium difficile, and most other clinically significant anaerobic bacteria are susceptible to cefotetan.

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.