1 National Institute for Communicable Diseases, National Health Laboratory Service, Johannesburg; 2 MRC/University of the Witwatersrand/NHLS Respiratory and Meningeal Pathogens Research Unit, Johannesburg, South Africa; 3 Department of International Health, Emory University, Atlanta, GA; 4 Abbott Laboratories, Abbott Park, Chicago, IL, USA
Received 3 April 2003; returned 28 April 2003; revised 4 June 2003; accepted 10 June 2003
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Abstract |
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Materials and methods: The MICs of 26 isolates of B. anthracis from South Africa and Zimbabwe, as well as the Sterne vaccine strain and a type culture strain, were determined by agar dilution.
Results: The most active antimicrobial agents were the novel ketolide ABT 773, new and conventional fluoroquinolones, and doxycycline; macrolides were intermediately active. The lack of activity of extended-spectrum cephalosporins against B. anthracis was confirmed.
Conclusions: Susceptibility to conventional antibiotics was in keeping with previous studies. Two new fluoroquinolones and a ketolide showed promising in vitro activity that would support their further evaluation in animal models of anthrax.
Keywords: anthrax, antibiotics, fluoroquinolones, macrolides, ketolides
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Introduction |
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Materials and methods |
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Two B. anthracis isolates were obtained from human infections acquired in Zimbabwe; the Sterne vaccine strain, 34F2, was a gift of Dr M. Henton, Onderstepoort Veterinary Research Institute (OVRI), Pretoria; NCTC 2606 was a culture collection isolate from the UK; all the other isolates were from domestic or wild animals in southern Africa, donated by Dr M. Henton (OVRI). Isolates had been stored in semi-solid agar; purity was checked by plating onto horse blood agar. The control organisms used were Staphylococcus aureus ATCC 29213 and Bacillus cereus ATCC 11778.
Antibiotics tested
Clarithromycin, erythromycin, cefdinir, cefditoren, ciprofloxacin, temafloxacin, tosufloxacin and the experimental agents cethromycin (ABT 773, a ketolide), olamufloxacin (HSR 903) and ABT 492 (fluoroquinolones), were all obtained from Abbott Laboratories, Abbott Park, IL, USA. Doxycycline was supplied by Sigma (S. Africa), Midrand, South Africa.
Determination of antimicrobial susceptibility
We used the agar dilution method to determine MICs, according to the method of the National Committee for Clinical Laboratory Standards (NCCLS).7 All work with bacteria was done in a class 2 biosafety cabinet in the biosafety level 3 laboratory of the National Health Laboratory Service, Johannesburg, South Africa. Inocula of B. anthracis were prepared by subculturing several colonies of an overnight growth on blood agar into trypticase soy broth, incubating at 37°C for 46 h, and adjusting the turbidity to the equivalent of a 0.5 McFarland standard. A multipoint replicator with 3 mm diameter pins was used to inoculate MuellerHinton agar with an estimated final inoculum size of 104 cfu/spot.7 For each antimicrobial tested, growth control plates without incorporated antimicrobials were similarly inoculated. Incubation was at 37°C for 1820 h. The MICs at which 50% and 90% of isolates were inhibited (MIC50 and MIC90, respectively) were determined.
ß-Lactamase detection
We used the chromogenic cephalosporin method (Nitrocefin: Oxoid, Basingstoke, UK) to test for constitutive ß-lactamase production. Nitrocefin solution (four drops) was added to overnight 1 mL nutrient broth cultures of the B. anthracis isolates; the presence of a red colour on inspection after 30 min of incubation at 37°C indicated a positive reaction. A ß-lactamase-producing Branhamella catarrhalis strain was used as a positive control.
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Results and discussion |
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The data in this report would therefore support the further evaluation of both cethromycin (ABT 773) and ABT 492 against anthrax in animal models. The priority may be justifiably given to ABT 773 on the basis that it represents a new class of agent with activity against this pathogen.
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Acknowledgements |
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Footnotes |
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References |
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