Antimicrobial Susceptibility Testing of Bacillus anthracis : Comparison of Results Obtained by Using the National Committee for Clinical Laboratory Standards Broth Microdilution Reference and Etest Agar Gradient Diffusion Methods

Abstract

We determined the patterns of antimicrobial susceptibility of 65 isolates of Bacillus anthracis (50 historical and 15 recent U.S. clinical isolates) to nine antimicrobial agents using the National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution reference method. The results for the 50 historical B. anthracis isolates obtained by the broth microdilution method were compared to those generated by the Etest agar gradient diffusion method. One isolate of B. anthracis was ␤-lactamase positive and resistant to penicillin (MIC, 128 g/ml); a second isolate, which was ␤-lactamase negative, was borderline penicillin resistant, with the penicillin MICs for the isolate varying from 0.12 to 0.25 g/ml; and the remainder of the isolates were ␤-lactamase negative and penicillin susceptible (MICs, <0.12 g/ml). Approximately 78% of the isolates showed reduced susceptibility to ceftriaxone (MICs, >16 g/ml). All B. anthracis isolates were susceptible to chloramphenicol (MICs, <8 g/ml), ciprofloxacin (MICs, < 1 g/ml), clindamycin (MICs, <0.5 g/ml), rifampin (MICs, <0.5 g/ml), tetracycline (MICs, <0.06 g/ml), and vancomycin (MICs, <2 g/ml) by use of NCCLS breakpoints for staphylococci. All 15 recent B. anthracis isolates from the United States were susceptible to penicillin, doxycycline, and ciprofloxacin. By use of the susceptibility breakpoint for staphylococci of <0.5 g/ml, 97% of the B. anthracis isolates tested would have been categorized as intermediate to erythromycin. No statistically significant difference was found between the results of broth microdilution testing and the results of the Etest method for any of the antimicrobial agents tested; however, the results for penicillin obtained by the Etest were 1 to 9 dilutions lower than those obtained by the broth microdilution method. The differences in the penicillin MICs by the Etest method and the difficulties of reading the Etest results through the glass of a biological safety cabinet may limit the utility of this alternate susceptibility testing method for B. anthracis isolates.

The antimicrobial susceptibility and virulence factors of Bacillus anthracis strains isolated in Croatia

“…The importance of treatment of the disease in humans has been underscored by the bioterrorism events of 2001 in the United States. The Center for Disease Control recommends ciprofloxacin, penicillin and doxycycline for the treatment of human cases and for prophylactic use after exposure and ciprofloxacin has been the drug of choice for prophylactic treatment after exposure to the spores (Mohammed et al, 2002). Penicillin, traditionally the drug of choice for treatment is still recommended in other parts of the world despite reports of resistance against penicillin (Coker et al, 2002).…”

“…Penicillin, traditionally the drug of choice for treatment is still recommended in other parts of the world despite reports of resistance against penicillin (Coker et al, 2002). Streptomycin, gentamicin and chloramphenicol have also been recommended until recently (Mohammed et al, 2002).…”

Antimicrobial susceptibility of Bacillus anthracis strains from Hungary

“…Amoxicillin can be an alternative to penicillin (WHO, 2008). Penicillin and amoxicillin resistance was confirmed in several studies, within a range of 1-12% of the strains examined (Lightfoot et al, 1990;Cavallo et al, 2002;Coker et al, 2002;Mohammed et al, 2002;Turnbull et al, 2004). All Hungarian strains, however, proved to be highly susceptible to penicillin and amoxicillin.…”

“…The extremely resistant spore is able to survive in the soil for decades (Fasanella et al, 2010). Bacillus anthracis is susceptible to most antibiotics used in therapy (Lightfoot et al, 1990;Doğanay and Aydin, 1991;Odendaal et al, 1991;Cavallo et al, 2002;Coker et al, 2002;Mohammed et al, 2002;Turnbull et al, 2004;Luna et al, 2007;Caplan et al, 2009;Habrun et al, 2011;Ortatatli et al, 2012;Quinn et al, 2011). The main recommended antibiotics to treat human anthrax cases are ciprofloxacin and doxycycline, apart from penicillin (WHO, 2008).…”

Evaluation of new antimicrobial agents on Bacillus spp. strains: docking affinity and in vitro inhibition of glutamate-racemase

“…Contrarily, the test compounds probably inhibit cell wall synthesis, which would make them selective for bacterial versus human cells. The MIC of the test compounds is close to that of other antibiotics, such as ceftriaxone (MIC 90 32 µg/mL 24 ), which also inhibit cell wall synthesis.…”