Susceptibility to antibiotics of Bacillus anthracis strains isolated in Romania

A number of 21 B. anthracis strains isolated from 16 pustules, 2 blood cultures and 3 cerebrospinal fluids during 2000-2004 were studied for their susceptibility to antibiotics. The antibiosusceptibility testing was performed by disk diffusion method, on Mueller-Hinton agar medium. Two of the studied strains exhibited resistance to penicillins, considered until not long ago as antimicrobial agents of choice for the treatment of anthrax infection. The penicillin resistance explained the difficulties encountered during the treatment of these two cases as well as the fatal evolution in one of them. Both penicillin-resistant strains were subsequently tested, by using "in agar" antibiotic dilution method, in order to determine the minimum inhibitory concentrations (MIC) of the respective strains to penicillin G by the help of a serial antibiotic dilution from 16 microg/ml to 0.0075 microg/ml. The MIC values were 0.5 microg/ml and 4 microg/ml respectively, whereas in case of the standard B. anthracis 34F2 Sterne strain was < 0.015 microg/ml. All the 21 B. anthracis tested strains exhibited resistance to the IIIrd generation cephalosporins, as well as to TMP/STX, but were sensitive to tetracyclines and fluoroquinolones, these sensitivity aspects coming into agreement with the literature data. The strains proved to be also susceptible as follows: 13 strains to macrolides, 15 to rifampicin, 16 to chloramphenicol and all 21 to gentamycin; the last antibiotic can be used in association with fluoroquinolones in the treatment of B. anthracis infections. Fluoroquinolones (i.e. ciprofloxacin) become drugs of choice for the treatment of B. anthracis infections if early administered (within the first 24 hrs), in advance of the germ invasion into the lymph system and septicemia, preventing in this way the bacterial multiplication and production of edemathogenic and lethal toxins.