Antimicrobial susceptibility testing of Francisella tularensis with a modified Mueller-Hinton broth

Abstract

A modified Mueller-Hinton broth was developed to perform antimicrobial susceptibility tests on F. tularensis. Adequate growth of the organism was obtained within 24 h of inoculation, and minimum inhibitory concentration (MIC) could be read at that time. The 15 selected strains of F. tularensis and 5 reference quality control strains were tested in this medium with 36 antimicrobial agents. The MIC of the aminoglycosides and tetracycline increased 1 to 3 dilutions in this medium compared with those in the usual medium, but the other antimicrobial agents were not consistently affected by the medium. Even though the medium caused an increase in MIC, the aminoglycosides and tetracyclines remained very active in vitro against F. tularensis. Other antimicrobial agents effective in vitro were chloramphenicol, erythromycin, ceftazidime, moxalactam, cefotaxime, ceftriaxone, and Sch 29482 (a cephalosporin).