In vitro evaluation of the effect of linezolid and levofloxacin on Bacillus anthracis toxin production, spore formation and cell growth.

BACKGROUND

Owing to its ability to form spores and toxins, Bacillus anthracis is considered a bioterror agent. Although current therapeutic strategies can be effective, treatment does not prevent sporulation and toxin production.

OBJECTIVES

To quantify the combined effect of a protein synthesis inhibitor and a bactericidal agent on B. anthracis toxin production, sporulation and cell growth.

METHODS

Susceptibility and synergy titrations were conducted on B. anthracis Sterne and 03-0191 strains using linezolid and levofloxacin. The effect of antibiotic exposure on cell viability was evaluated using a continuous medium replacement model. In vitro static models were used to study the effect of linezolid and levofloxacin on sporulation and toxin production. Spores were quantified using the heat shock method. Toxin was quantified via commercial ELISA.

RESULTS

Synergy titrations indicated that the combination was synergistic or indifferent; however, in all models antagonism was observed. In the spore model, linezolid resulted in the lowest sporulation rates, while combination therapy resulted in the highest. In the toxin model, linezolid prevented toxin production altogether.

CONCLUSIONS

This study advances our understanding of the effects of combination therapy on B. anthracis infection. Used alone, linezolid therapy abolishes toxin production and reduces sporulation. These results suggest that studies using a step-wise approach using linezolid initially to stop sporulation and toxin production followed by levofloxacin to rapidly kill vegetative B. anthracis can be recommended.