In vitro susceptibility of Bacillus anthracis to various antibacterial agents and their time-kill activity.
OBJECTIVE
To investigate the in vitro acquisition of resistance to antibiotics by Bacillus anthracis.


METHODS
The in vitro activities of 18 antibacterial agents against two strains of B. anthracis, the Sterne strain and the Russian anthrax vaccine strain ST-1, were tested by determining the MICs and by measuring the rates of antibiotic kill at 5x and 10x MIC.


RESULTS
The fluoroquinolones ciprofloxacin, ofloxacin, levofloxacin and moxifloxacin, the beta-lactams penicillin G and amoxicillin, the macrolide clarithromycin, the ketolide telithromycin, as well as clindamycin, rifampicin and quinupristin/dalfopristin had MICs in the range of 0.03-0.25 mg/L. Minocycline had an MIC of 0.03 mg/L, as did penicillin, against the ST-1 strain. Ciprofloxacin had an MIC of 0.03 mg/L against both strains. Erythromycin, vancomycin and the oxazolidinone linezolid were less active (MIC 0.5-2.5 mg/L). Ceftriaxone was the least active, having an MIC of 8.0 mg/L. Chloramphenicol was inactive (MIC > 256 mg/L). Quinupristin/dalfopristin, rifampicin and moxifloxacin showed the most rapid bacterial killing, achieving a complete eradication of detectable organisms (2 log(10) reduction within 0.5-3 h and 4 log(10) reduction within 0.5-4 h for both strains at concentrations of 5x and 10x the MIC). The beta-lactams and vancomycin demonstrated a 2-4 log(10) reduction within 5-15 h. Ceftriaxone had a similar effect to penicillin and amoxicillin against the ST-1 strain, but a slower effect than these two beta-lactams against the Sterne strain. The macrolides, tetracyclines and linezolid demonstrated a lower kill rate, while chloramphenicol did not kill at all.


CONCLUSIONS
These data expand on the spectrum of agents recommended for the treatment of anthrax (ciprofloxacin, penicillin G and tetracyclines) and add new options, such as other fluoroquinolones, amoxicillin, rifampicin and quinupristin/dalfopristin, as potential therapeutic agents.
Athamna
A
A
The Triangle Research And Development Center, Kfar-Qaraa, Israel.
Massalha
M
M
Athamna
M
M
Nura
A
A
Medlej
B
B
Ofek
I
I
Bast
D
D
Rubinstein
E
E
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001408:Bacillus anthracis / Q000187:drug effects* / Q000254:growth & development; D004351:Drug Resistance; D008826:Microbial Sensitivity Tests; D013997:Time Factors
D000900:Anti-Bacterial Agents