Servicio de Microbiología, Hospital de Móstoles, 28935 Móstoles, Madrid, Spain
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Abstract |
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Introduction |
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Materials and methods |
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MICs were determined by a dilution method on WilkinsChalgren agar (Difco Laboratories, Detroit, MI, USA) for the following antibiotics: meropenem (Zeneca, Pontevedra, Spain), clindamycin (Upjohn, Crawley, UK), metronidazole (Sigma, St Louis, MO, USA), cefoxitin (Sigma), trovafloxacin (Pfizer, Groton, CT, USA) and co-amoxiclav (SmithKline Beecham, Toledo, Spain).
Final concentrations were 0.5256 mg/L of clindamycin, 0.128 mg/L of meropenem, 464 mg/L of cefoxitin, 0.258 mg/L of metronidazole, 1/0.516/8 mg/L of co-amoxiclav and 0.068 mg/L of trovafloxacin. Microorganisms were incubated on blood agar for 48 h at 35°C in an anaerobic chamber to ensure purity and viability. MIC plates were also incubated at 35°C for 48 h in an anaerobic atmosphere. Quality control strains used throughout the study were B. fragilis ATCC 25285 and Bacteroides thetaiotaomicron ATCC 29741. All results were interpreted according to NCCLS criteria.3
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Results and discussion |
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B. fragilis group infections require early treatment, usually before susceptibility results are available, particulary bearing in mind the fact that the susceptibility tests used for these microorganisms are more complicated and take longer than the techniques routinely used for aerobic bacteria. In addition, in recent years increasing resistance to antimicrobial agents as well as regional and institutional differences in the rates of resistance have been reported for the B. fragilis group.2,4 Consequently, the results of periodic susceptibility studies are useful and necessary as a guide in the choice of agents for prophylaxis and empirical treatment.
The results obtained in this study confirm the excellent in vitro activity of metronidazole with 100% of the strains susceptible and with very low MICs (0.5 mg/L) in all cases. These data concur with those of previous studies4,5 although exceptional strains with resistance to metronidazole have been described.6 Meropenem also displayed high efficacy against the B. fragilis group and no resistant strain was detected. Resistance to the carbapenems has been occasionally and infrequently recorded.4 Worthy of attention among our strains are one with an MIC of 4 mg/L and four with MICs of 2 mg/L. These high MICs, one or two dilutions below the breakpoint, should draw attention to the progressive reduction in the overall susceptibility to carbapenems previously described by other authors.4 A similar situation was found with co-amoxiclav. Although the resistance percentage in our study (3%) agrees with that described in previous work,7 we must draw attention to the frequency of strains with intermediate susceptibility (MIC 8/4 mg/L) reaching 13%, and the increase in the MIC90, which coincides precisely with the breakpoint. No strain with resistance to trovafloxacin was found in our isolates. There are few previous studies evaluating the susceptibility of the B. fragilis group to trovafloxacin. Although some describe low percentages of resistance, variable according to the pH,8 the majority concur with our results.9
The MICs obtained for cefoxitin should be interpreted with care. The MIC distribution displays a mode coinciding with the MIC considered by the NCCLS as intermediate in susceptibility. In addition, 18 of the sensitive strains had an MIC of 16 mg/L, only one dilution below the breakpoint, and 16 of the resistant strains had an MIC of 64 mg/L, only one dilution above the breakpoint. Overall, 70 of the 100 isolates had MICs grouped in three dilutions, the breakpoint ± one dilution. This means that the resistant and susceptible percentages (18 and 46%) may change to 18 and 82% or to 54 and 46%, according to whether the strains with intermediate susceptibility are considered susceptible or resistant. Therefore, although the clinical implication of this grouping of MICs obtained in vitro is not known, the figures involved make the use of this agent inadvisable in infections caused by the B. fragilis group.
The most dramatic increase in resistance rates compared with previous reports was found with clindamycin. The prevalence of resistance to this antimicrobial agent was 49%, with an MIC90 of 256 mg/L. Unlike with cefoxitin, the distribution of the MICs obtained was bimodal, so that the great majority of the isolates were found to be either clearly susceptible (0.5 mg/L) or clearly resistant (
128 mg/L). This increase in resistance to clindamycin has been described previously in several reports, but only in one, which was carried out in Spain in 1991,7,10 were similar resistance rates (45%) found. Other contemporary or later studies in Spain7 report resistance percentages below 25%. The geographical differences are obvious. A European multicentre study reported resistance ranging from 0% in Austria and Sweden to 19% in Belgium.2 The high percentage of clindamycin resistance in our study has clear clinical consequences. The endogenous origin of the infections caused by this microorganism suggests that the resistance pattern obtained in this healthy population study does not differ greatly from the one that might be found in those clinical situations in which the B. fragilis group is involved. The use of clindamycin in many institutions as part of empirical treatment of abdominal or genital infections, or its use in surgical prophylaxis protocols in these same areas, should be modified on the basis of resistance increases as in our study. Bearing in mind the great geographical and even institutional variability described in previous studies, our results should serve to draw attention to the need for periodic updating of therapeutic protocols in accordance with susceptibility results.
In our experience meropenem, metronidazole and trovafloxacin display high efficacy, and thus, would be drugs of choice in infection by B. fragilis group organisms. Co-amoxiclav may be a good alternative in spite of an overall tendency to reduced susceptibility. Cefoxitin and clindamycin show a notable increase in resistance rates and their use in empirical treatment in infections by these anaerobes is inadvisable. They should only be used once the results of susceptibility tests are available. Therefore, in view of the continously changing antimicrobial resistance patterns of the B. fragilis group, updated information regarding resistance patterns and, consequently, adequate antimicrobial therapy, would contribute to the avoidance of therapeutic failure in the treatment of infections caused by this pathogen.
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Notes |
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References |
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2 . Phillips, I., King, A., Nord, C. E. & Hoffstedt, B. (1992). Antibiotic sensitivity of the Bacteroides fragilis group in Europe. European Study Group. European Journal of Clinical Microbiology and Infectious Diseases 11, 292304.[ISI][Medline]
3 . National Committee for Clinical Laboratory Standards. (1998). Performance Standards for Antimicrobial Susceptibility TestingEighth Informational Supplement: M100-S8. NCCLS, Wayne, PA.
4 . Snydman, D. R., McDermott, L., Cuchural, G. J., Hecht, D. W., Iannini, P. B., Harrell, L. J. et al. (1996). Analysis of trends in antimicrobial resistance patterns among clinical isolates of Bacteroides fragilis group species from 1990 to 1994. Clinical Infectious Diseases 23, Suppl. 1, S5465.[ISI][Medline]
5 . Turgeon, P., Turgeon, V., Gourdeau, M., Dubois, J. & Lamothe, F. (1994). Longitudinal study of susceptibilities of species of the Bacteroides fragilis group to five antimicrobial agents in three medical centers. Antimicrobial Agents and Chemotherapy 38, 22769.[Abstract]
6 . Rotimi, V. O., Khoursheed, M., Brazier, J. S., Jamal, W. Y. & Khodakhast F. B. (1999). Bacteroides species highly resistant to metronidazole: an emerging clinical problem? Clinical Microbiology and Infection 5, 1669.[Medline]
7 . Baquero, F. & Reig, M. (1992). Resistance of anaerobic bacteria to antimicrobial agents in Spain. European Journal of Clinical Microbiology and Infectious Diseases 11, 101620.[ISI][Medline]
8 . Falagas, M. E., McDermott, L. & Snydman, D. R. (1997). Effect of pH on in vitro antimicrobial susceptibility of the Bacteroides fragilis group. Antimicrobial Agents and Chemotherapy 41, 20479.[Abstract]
9 . Wexler, H. M., Molitoris, E., Molitoris, D. & Finegold, S. M. (1996). In vitro activities of trovafloxacin against 57 strains of anaerobic bacteria. Antimicrobial Agents and Chemotherapy 40, 22325.[Abstract]
10 . Pelaez, M. T., Cercenado, E., Rodriguez-Creixems, M. & Bouza, E. (1991). Resistance of anaerobic bacteria to antimicrobial agents. Reviews Infectious Diseases 13, 183.
Received 10 May 1999; returned 15 September 1999; revised 1 October 1999; accepted 4 January 2000