In vitro susceptibility of Bacillus anthracis to various antibacterial agents and their time-kill activity

Objectives: 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 5× and 10× MIC. Results: The fluoroquinolones ciprofloxacin, ofloxacin, levofloxacin and moxifloxacin, the β-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 log₁₀ reduction within 0.5-3 h and 4 log₁₀ reduction within 0.5-4 h for both strains at concentrations of 5× and 10× the MIC). The β-lactams and vancomycin demonstrated a 2-4 log₁₀ 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 β-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.