Evaluation of in vitro antimicrobial susceptibility of Bacillus anthracis strains isolated during anthrax outbreaks in Italy from 1984 to 2017.

Anthrax, caused by Bacillus anthracis , is a non-contagious infectious disease that affects a wide range of animal species (primarily ruminants), as well as humans. Due to the often fatal outcome in human cases, the quick administration of definitely effective antimicrobials is crucial either as prophylaxis or for the therapy of clinical cases. In this study, a total of 110 B. anthracis strains, temporally, geographically and genetically different, isolated during anthrax outbreaks in Italy from 1984 to 2017, were screened for their susceptibility towards 16 clinically relevant antimicrobial agents using the broth microdilution method. The strains were isolated from various matrices (human, animal and environmental samples) and were representative of thirty distinct genotypes previously identified by 15-loci MLVA. The following antimicrobials have been tested: gentamicin, ceftriaxone, streptomycin, penicillin G, clindamycin, chloramphenicol, vancomycin, linezolid, cefotaxime, tetracycline, erythromycin, rifampin, amoxicillin, ciprofloxacin, doxycycline and trimethoprim. All the isolates were susceptible towards most of the tested antimicrobials with the exception of trimethoprim for which all of them showed high MIC-values. As far as ceftriaxone and cefotaxime are concerned, an intermediate level of susceptibility was recorded. Although the Centers for Disease Control and Prevention recommend the use of doxycycline, ciprofloxacin, penicillin G and amoxicillin for the treatment of human cases and for post exposure prophylaxis to anthrax spores, this study shows a high degree of in vitro susceptibility of B. anthracis to many other antimicrobials, thus suggesting the possibility of an alternative choice for prophylaxis and therapy of infections.