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

Resistance trends of the Bacteroides fragilis group over a 10-year period, 1997 to 2006, in Madrid, Spain

The changes in susceptibilities of Bacteroides fragilis group strains isolated in our hospital from 1997 to 2006 were studied. A total of 1,343 clinical strains were included. The study showed differences in the resistance rates in the different species of the group. Increasing resistance to clindamycin and moxifloxacin was observed. Susceptibility to imipenem, piperacillin-tazobactam, and metronidazole remained unchanged.

Antimicrobial susceptibilities of Bacteroides fragilis and Bacteroides thetaiotaomicron strains isolated from clinical specimens and human intestinal microbiota

Species of Bacteroides fragilis group bacteria are the most prevalent pathogens and have the highest resistance rates to antimicrobial agents among anaerobic bacteria. Infections due to these micro-organisms often originate from patient's own intestinal microbiota. The objective of the study was to determine and compare the susceptibility profiles of clinical and intestinal B. fragilis and B. thetaiotaomicron strains against certain antimicrobials. Isolates were identified by conventional methods and API-20 A. Susceptibility tests were performed according to recommendations of NCCLS (M 11-A4) agar dilution methods. Beta-lactamase production was determined with nitrocefin discs. Forty-five clinical isolates (33 B. fragilis and 12 B. thetaiotaomicron) were from following sites: blood (n:8), intra-abdominal abscess (n:7), soft tissue (n:26), and miscellaneous foci of infection (n:4). Fifty B. fragilis and 60 B. thetaiotaomicron isolates from intestinal microbiota of individuals with no history of antimicrobial treatment within last 30 days were also examined. Beta-lactamase production was detected in 93% of clinical and 99% of intestinal isolates. The organisms including intestinal isolates were uniformly susceptible to metronidazole. The MIC90s of other antibiotics and resistance rates of all clinical isolates to those antibiotics were as follows: 256 microg/mL (93%) for ampicillin, 128 microg/mL (13%) for piperacillin, 64 microg/mL (11%) for cefoxitin, 1 microg/mL (2%) for amoxicillin-clavulanate, 0.5 microg/mL (2%) for imipenem, >256 microg/mL (36%) for clindamycin, 8 microg/mL (2%) for chloramphenicol. Intestinal isolates demonstrated similar resistance rates and MIC90s. Metronidazole, imipenem, amoxicillin-clavulanate seem to be effective drugs against these bacteria in Turkey.

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

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

In vitro activity of tigecycline against Bacteroides species

OBJECTIVES

To ascertain the current susceptibility patterns of members of the Bacteroides fragilis group in our hospital and to assess the in vitro activity of tigecycline against these organisms.

METHODS

A total of 400 non-duplicate clinical isolates of the B. fragilis group collected from 2000 to 2002 were studied. Susceptibility testing was performed according to the reference agar dilution method described by the NCCLS. The following antimicrobials were tested: tigecycline, clindamycin, metronidazole, chloramphenicol, cefoxitin, imipenem, amoxicillin-clavulanate and piperacillin-tazobactam.

RESULTS

All strains were susceptible to metronidazole and chloramphenicol. For clindamycin and cefoxitin, the overall susceptibility rates were 59.5% and 83%, respectively. Imipenem and piperacillin-tazobactam were the most active beta-lactam agents tested. Tigecycline inhibited 89.8% of the strains at a concentration of 8 mg/L with an MIC range of <or=0.01 to >16 mg/L. By comparing the MIC50 and MIC90 values of tigecycline among the various species of the group, B. fragilis, Bacteroides thetaiotaomicron and Bacteroides vulgatus were the most susceptible (MIC50/MIC90s of 0.5-1/8 mg/L).

CONCLUSIONS

Tigecycline exhibited activity against most isolates of the B. fragilis group tested. These results indicate that tigecycline may be useful in the treatment and prophylaxis of infections involving these organisms.

A Bacteroides thetaiotamicron porin that could take part in resistance to beta-lactams

The aim of this study was to investigate porin absence or deficiency in two Bacteroides thetaiotaomicron strains resistant to amoxicillin combined with clavulanic acid. Their outer membrane protein (OMP) extracts and those of two susceptible strains were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and compared to detect differences between the strains. A protein band of interest at around 70 kDa electro-eluted for each strain, was tested in a liposome swelling assay. A decrease in initial absorbency was noted for the two susceptible strains but not for the two resistant strains. The liposome swelling of the two susceptible strains was directly visualized by photon microscopy and then photographed. This suggested a B. thetaiotaomicron porin of around 70 kDa could take part in resistance to beta-lactams.

In vitro activities of tigecycline (GAR-936) against recently isolated clinical bacteria in Spain

The antimicrobial activities of tigecycline (GAR-936) were compared with those of other agents against 1,087 strains recently isolated in 12 Spanish medical centers. Tigecycline showed activity against a wide spectrum of aerobic and anaerobic bacteria, including strains such as methicillin-resistant Staphylococcus aureus, coagulase-negative staphylococci, penicillin-resistant Streptococcus pneumoniae, Enterococcus faecium, Acinetobacter baumannii, and Stenotrophomonas maltophilia.

In vitro activities of MK-0826 and 16 other antimicrobials against Bacteroides fragilis group strains

The activity of MK-0826, a new carbapenem, against 309 Bacteroides fragilis group strains was investigated and compared with that of 11 other beta-lactam and 5 non-beta-lactam agents. MK-0826 showed excellent activity (MICs ranged from < or =0.06 to 4 microg/ml). The new carbapenem may be useful in the treatment of mixed anaerobic infections involving B. fragilis group strains.

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.

Activities of new antimicrobial agents (trovafloxacin, moxifloxacin, sanfetrinem, and quinupristin-dalfopristin) against Bacteroides fragilis group: comparison with the activities of 14 other agents

The antimicrobial activities of trovafloxacin, moxifloxacin, sanfetrinem, quinupristin-dalfopristin, and 14 other antimicrobial agents against 218 Bacteroides fragilis group strains were determined. A group of 10 imipenem-resistant strains were also tested. Imipenem, meropenem, and sanfetrinem had the lowest MICs of all of the beta-lactams. Quinupristin-dalfopristin inhibited all of the strains at 2 microg/ml. Overall, the MICs of trovafloxacin and moxifloxacin for 90% of the strains tested were 1 and 2 microg/ml, respectively.

In-vitro susceptibilities of species of the Bacteroides fragilis group to newer beta-lactam agents

The in-vitro activities of imipenem and four beta-lactam-beta-lactamase inhibitor combinations were tested against 816 strains of the Bacteroides fragilis group, and compared with other anti-anaerobic agents. None of the strains was resistant to metronidazole, and only one was resistant to chloramphenicol. Mezlocillin and piperacillin were moderately active, while clindamycin was the least active. Rates of resistance varied between various species. The new beta-lactam agents tested showed excellent activity; piperacillin-tazobactam and imipenem were the most active. The emergence of strains that are resistant to these agents, observed in this study, suggests there is a need to perform periodic antimicrobial susceptibility tests.

Broth disk elution method for anaerobic bacteria: a collaborative study to assess its reliability for clinical purposes

A collaborative study involving seven laboratories was undertaken to evaluate the reproducibility and the reliability of the broth disk elution test against anaerobic bacteria by comparing with the reference agar dilution method. A two breakpoint broth test was also evaluated. Assays were performed using the same testing conditions (i.e. medium, temperature, atmosphere and incubation time). One hundred Gram-negative and Gram-positive clinical isolates were initially studied. Overall agreement of 98.5% and 97.5%, were found for disk elution and the two breakpoint tests, respectively. In order to assess the reliability of the disk elution test, two different lots (LOT1 and LOT2) of disks of piperacillin and clindamycin were selected, to obtain two final concentrations after dilution (10 and 60 mg/mL; 1 and 4 mg/mL, respectively). Two hundred and eighty assays were performed against one strain of both Bacteroides fragilis(piperacillin MIC, 8.0 mg/mL; clindamycin MIC, <0.5 mg/mL) and Bacteroides thetaiotaomicron(piperacillin MIC, 16.0 mg/mL; clindamycin MIC, <0.5 mg/mL). With LOT 1, considering both species and both antibiotics, the agreement among six laboratories ranged from 85% to 100% (P > 0.05) with the higher concentration. Overall agreement among all laboratories was 91%. No optimal agreement (>90%) for clindamycin-Bacteroides thetaiotaomicron using the LOT1 (77%) was found. Since this finding was not observed with LOT2 (100% agreement), discrepancies were attributed to variation between lots. Overall agreement with LOT2 was 100% for all centres. The present study indicates that the broth disk elution method proved to be a reliable and suitable alternative for routine susceptibility testing for anaerobic bacteria, as a resistance screening method for clinical purposes.

Imipenem/cilastatin: an update of its antibacterial activity, pharmacokinetics and therapeutic efficacy in the treatment of serious infections

The prototype carbapenem antibacterial agent imipenem has a very broad spectrum of antibacterial activity, encompassing most Gram-negative and Gram-positive aerobes and anaerobes, including most beta-lactamase-producing species. It is coadministered with a renal dehydropeptidase inhibitor, cilastatin, in order to prevent its renal metabolism in clinical use. Extensive clinical experience gained with imipenem/cilastatin has shown it to provide effective monotherapy for septicaemia, neutropenic fever, and intra-abdominal, lower respiratory tract, genitourinary, gynaecological, skin and soft tissues, and bone and joint infections. In these indications, imipenem/cilastatin generally exhibits similar efficacy to broad-spectrum cephalosporins and other carbapenems and is at least equivalent to standard aminoglycoside-based and other combination regimens. Imipenem/cilastatin is generally well tolerated by adults and children, with local injection site events, gastrointestinal disturbances and dermatological reactions being the most common adverse events. Seizures have also been reported, occurring mostly in patients with impaired renal function or CNS pathology, or with excessive dosage. Although it is no longer a unique compound, as newer carbapenems such as meropenem are becoming available, imipenem/cilastatin nevertheless remains an important agent with established efficacy as monotherapy for moderate to severe bacterial infections. Its particular niche is in treating infections known or suspected to be caused by multiresistant pathogens.

Cross-resistance to beta-lactam-beta-lactamase inhibitor combinations and clindamycin among cefoxitin-resistant and cefoxitin-susceptible strains of the Bacteroides fragilis group

In the present study the cross-resistance rates of cefoxitin-resistant and cefoxitin-susceptible strains of the Bacteroides fragilis group were compared with regard to beta-lactam-beta-lactamase inhibitor combinations and clindamycin. Piperacillin-tazobactam was the most active agent tested with an overall resistance rate of 0.2% and no resistance among cefoxitin-resistant strains. Ticarcillin-clavulanate was the least active combination with an overall resistance rate of 1.6% but a resistance rate of 13.1% among cefoxitin-resistant strains. For ampicillin-sulbactam, amoxicillin-clavulanate, and clindamycin resistance, rates of cefoxitin-resistant versus cefoxitin-susceptible strains were 9.1% and 0.4%, 8.3% and 0.9%, and 28% and 12.9%, respectively.