1 Centre de Recherches du Service de Santé des Armées Emile Pardé, Département de Biologie des Agents Transmissibles, 24 Avenue des Maquis du Grésivaudan, B.P. 87, F-38702 La Tronche; 2 Service de Biologie Médicale, Hôpital d'Instruction des Armées Begin, 69 Avenue de Paris, F-94160 Saint-Mandé, France
Received 30 April 2004; returned 24 July 2004; revised 9 September 2004; accepted 20 September 2004
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Abstract |
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Methods: MICs were determined by agar dilution in MuellerHinton medium.
Results: Among the antibiotics tested, lower MICs were obtained with imipenem, ceftazidime, piperacillin, piperacillin/tazobactam, doxycycline and minocycline. Fluoroquinolones and aminoglycosides had poor activities. A single clinical isolate of B. pseudomallei was resistant to ceftazidime, co-amoxiclav and doxycycline but remained susceptible to imipenem.
Conclusions: Although B. mallei MICs are often lower, the overall results underline the importance of resistance in both species. The susceptibilities measured are consistent with the current recommendations for the treatment of B. pseudomallei and B. mallei infections.
Keywords: melioidosis , glanders , MICs , resistance , biowarfare
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Introduction |
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B. pseudomallei and B. mallei are considered as potential biological warfare or bioterrorism agents and have been included in the B list of the CDC.3 B. mallei and B. pseudomallei are intrinsically resistant to a wide range of antimicrobial agents including ß-lactam antibiotics, aminoglycosides and macrolides.4,5 However, few antibiotic susceptibility studies of B. mallei have been performed.6,7 As there is a need for effective treatments and post-exposure prophylaxis, the objective of this study was to assess the in vitro susceptibilities of a large panel of strains of B. mallei and B. pseudomallei to a wide variety of antibiotics.
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Materials and methods |
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Results and discussion |
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Whatever the antibiotic family, the present results demonstrated no differences between the strains isolated from humans and those isolated from animal or environmental sources.
Consistent with previous studies, the isolates tested in this protocol were highly resistant to amoxicillin, ticarcillin, cefoxitin, cefoperazone, cefsulodin and aztreonam. One strain, isolated from human infection, appeared resistant to ceftazidime with an MIC of 64 mg/L (MICs of ceftazidime for the other strains were between 14 mg/L). This strain also presented a cross-resistance to ticarcillin/clavulanate, doxycycline and minocycline and was categorized intermediate for co-amoxiclav with an MIC of 16 mg/L. All other strains of B. mallei and B. pseudomallei were susceptible to co-amoxiclav. Cefotaxime activity was low but was partially restored by clavulanate or tazobactam, demonstrating the effectiveness of ß-lactamase inhibitors against the two species. The resistance profiles of both species for ß-lactam antibiotics were similar; however, lower MICs were observed in B. mallei for ß-lactam inhibitor combinations. MICs of piperacillin were low (0.1258 mg/L) and those of the combination of piperacillin and tazobactam were three- or four-fold lower, suggesting that this antibiotic was hydrolysed by the chromosomal ß-lactamases of both species.
All the isolates were susceptible to imipenem. This antibiotic was, with doxycycline and minocycline, one of the most active antibiotics tested. This has been observed previously in a study involving 211 clinical strains,5 and is of interest because this antibiotic is considered as a good alternative to ceftazidime in the treatment of disseminated disease. It has been recommended by the European Agency for the Evaluation of Medicinal Products (EMEA) for the treatment of suspected or confirmed melioidosis.9 The low MICs encountered with minocycline and doxycycline are also of interest because doxycycline has been used alone for the treatment of localized infection and has also been recommended by the EMEA in association with imipenem or meropenem for the treatment of severe cases of melioidosis.9 Despite the lack of recommendations, we assume that oral doxycycline could be useful for post-exposure prophylaxis.10
With 50% of isolates intermediate or resistant to ciprofloxacin (MIC breakpoint of 2 mg/L), this antibiotic cannot be recommended for treatment and/or prophylaxis. Clinical experience in maintenance therapy has demonstrated a poor efficacy for preventing relapses.11 Both species demonstrated resistance to pefloxacin, ofloxacin and norfloxacin. MICs of gatifloxacin and levofloxacin were equivalent to those of ciprofloxacin for B. pseudomallei. All the strains of this species were resistant to erythromycin and clindamycin, and nearly all were resistant to all the aminoglycosides tested (gentamicin, tobramycin, netilmicin, amikacin). This resistance is due to the presence of a unique multidrug efflux system (AmrAB-OprA) in B. pseudomallei, which is specific for both aminoglycosides and macrolide antibiotics.12 In contrast, MICs of aminoglycosides for B. mallei were lower and all strains appeared susceptible to netilmicin with MICs in the range 0.1250.25 mg/L. All the MICs of clindamycin were high in both species, but only B. pseudomallei exhibited high MICs of erythromycin. This observation is consistent with those obtained with azithromycin.7 B. pseudomallei was categorized as moderately susceptible to chloramphenicol. The MIC50 and MIC90 of co-trimoxazole were 8 and 16 mg/L, respectively, and the majority of the strains were categorized as intermediate or resistant (breakpoints 2/38, >8/152) to this antibiotic. This relative in vitro resistance is not correlated with clinical experience as co-trimoxazole has been traditionally used for the therapy of melioidosis. Such discrepancies between results obtained with co-trimoxazole by different susceptibility testing methods and clinical data have already been documented.1
In conclusion, this study confirms the high level of antibiotic resistance in B. mallei and B. pseudomallei, including towards agents not tested in previous studies. Nevertheless, the overall resistance is lower in B. mallei. The resistance profiles appear to be independent of the origin of isolates. Imipenem, ceftazidime, co-amoxiclav, piperacillin, piperacillin/tazobactam and doxycycline appear as the more effective drugs tested on this panel of isolates. These results remain consistent with the current recommendations for the treatment of melioidosis and glanders. However, the emergence of ceftazidime-resistant clinical isolates and the wide distribution of B. pseudomallei in Southeast Asia increase the risk of malicious use of those resistant strains. Piperacillin/tazobactam could be a useful alternative for treatment of both glanders and melioidosis. Nevertheless, the disparity between in vitro results and clinical response underlines the necessity to validate this association by timekill studies, animal models and clinical experience.
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Acknowledgements |
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Footnotes |
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References |
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2 . Neubauer, H., Meyer, H. & Finke, E. J. (1997). Human glanders. International Review of Armed Forces Medical Services 70, 25865.
3 . Centers for Disease Control and Prevention. (2003). Bioterrorism Agents/Diseases. [Online.] http://www.bt.cdc.gov/agent/agentlist-category.asp (6 September 2004, date last accessed).
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Heine, H. S., England, M. J., Waag, D. M. et al. (2001). In vitro antibiotic susceptibilities of Burkholderia mallei (causative agent of glanders) determined by broth microdilution and E-test. Antimicrobial Agents and Chemotherapy 45, 211921.
5 . Dance, D., Wuthiekanun, V., Chaowagul, W. et al. (1989). The antimicrobial susceptibility of Pseudomonas pseudomallei. Emergence of resistance in vitro and during treatment. Journal of Antimicrobial Chemotherapy 24, 295309.[Abstract]
6 . Batmanov, V. P. (1994). Sensitivity of Pseudomonas mallei to tetracyclines and their effectiveness in experimental glanders. Antibiotiki I Khimioterapiia 39, 337.
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.
Kenny, D. J., Russell, P., Rogers, D. et al. (1999). In vitro susceptibilities of Burkholderia mallei in comparison to those of other pathogenic Burkholderia spp. Antimicrobial Agents and Chemotherapy 43, 27735.
8 . Ashdown, L. R. (1979). Identification of Pseudomonas pseudomallei in the clinical laboratory. Journal of Clinical Pathology 32, 5004.[Abstract]
9 . European Agency for the Evaluation of Medicinal Products. (2002). Guidance document on use of medicinal products for treatment and prophylaxis of biological agents that might be used as weapons of bioterrorism. [Online.] http://www.emea.eu.int/pdfs/human/bioterror/404801.pdf (9 September 2004, date last accessed).
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Russell, P., Eley, S. M., Ellis, J. et al. (2000). Comparison of efficacy of ciprofloxacin and doxycycline against experimental melioidosis and glanders. Journal of Antimicrobial Chemotherapy 45, 8138.
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Chetchotisakd, P., Chaowagul, W., Mootsikapun, P. et al. (2001). Maintenance therapy of melioidosis with ciprofloxacin plus azithromycin compared with cotrimoxazole plus doxycycline. American Journal of Tropical Medicine and Hygiene 64, 247.
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Moore, R. A., DeShazer, D., Reckseidler, S. et al. (1999). Efflux-mediated aminoglycoside and macrolide resistance in Burkholderia pseudomallei. Antimicrobial Agents and Chemotherapy 43, 46570.