JAC Advance Access published online on June 22, 2007
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkm213
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Susceptibility of Bacillus anthracis, Bacillus cereus, Bacillus mycoides, Bacillus pseudomycoides and Bacillus thuringiensis to 24 antimicrobials using Sensititre® automated microbroth dilution and Etest® agar gradient diffusion methods
1 Center for Biological Defense, College of Public Health, University of South Florida, 3602 Spectrum Boulevard, Tampa, FL 33612, USA 2 Florida Department of Health, Bureau of Laboratories, 3602 Spectrum Boulevard, Tampa, FL 33612, USA
Received 2 March 2007; returned 26 April 2007; revised 17 May 2007; accepted 18 May 2007
* Corresponding author. Tel: +1-813-974-3873; Fax: +1-813-974-1479; E-mail: vluna{at}health.usf.edu
Objectives: To examine susceptibilities of Bacillus anthracis and related species to 24 antimicrobials using and concurrently comparing two methods.
Methods: Twenty-four antimicrobials were tested against 95 isolates of the Bacillus cereus group including 18 B. anthracis, 42 B. cereus, 5 Bacillus mycoides, 5 Bacillus mycoides/pseudomycoides, 6 Bacillus pseudomycoides and 19 Bacillus thuringiensis to determine their MICs, MIC ranges, MIC50s and MIC90s with Etest® and Sensititre® at 30 and 35°C for 18, 24 and 48 h.
Results: Both methods yielded near-identical results at both temperatures for all antimicrobials except trimethoprim/sulfamethoxazole. Resistance to trimethoprim/sulfamethoxazole in 97% (92/95) was not always evident until tests were incubated for 48 h at 30°C. All B. anthracis isolates were susceptible to 22 antimicrobials and resistant to trimethoprim/sulfamethoxazole while three isolates were erythromycin-intermediate. Whereas the B. thuringiensis were resistant to the ß-lactams, two B. cereus, one B. mycoides, five B. pseudomycoides and two B. mycoides/pseudomycoides were susceptible. Three B. cereus were solely clindamycin-resistant. Of the seven erythromycin-intermediate or -resistant B. cereus, three were resistant to clindamycin and one was resistant to clarithromycin and clindamycin. One B. mycoides was intermediately resistant to quinupristin/dalfopristin and meropenem and one was clindamycin-resistant. All B. pseudomycoides were clindamycin-resistant with one quinupristin/dalfopristin-resistant. Two B. mycoides/pseudomycoides were intermediately resistant to quinupristin/dalfopristin and clindamycin and a third was intermediately resistant to clindamycin alone. All isolates were susceptible to chloramphenicol, ciprofloxacin, gatifloxacin, gentamicin, levofloxacin, linezolid, moxifloxacin, rifampicin, streptomycin, tetracycline, tigecycline and vancomycin.
Conclusions: This paper expands the list of therapeutic or prophylactic antimicrobials potentially effective against B. cereus group isolates using two testing methods that produced comparable results.
Key Words: method comparison , minimum inhibition concentration , trimethoprim , ß-lactams
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