A novel broth medium for enhanced growth of Francisella tularensis

Francisella tularensis is a fastidious organism that requires a lengthy incubation time in liquid growth media for detection. The objective of this study was to develop a medium formulation using readily available supplements that enhanced early growth of F. tularensis. Francisella tularensis live vaccine strain was used to evaluate the growth responses for each of the media formulations tested. Growth in brain heart infusion broth supplemented with 2% Vitox, 10% Fildes and 1% histidine (BVFH) resulted in a significant increase in growth after 8 h incubation compared to other media formulations tested (P < 0·001). Virulent strains of F. tularensis grown in BVFH medium demonstrated similar enhanced early growth. Cell densities of 3·9-5·2 log₁₀ CFU per ml were obtained after 24 h of growth in BVFH from a 1-2 cell ml^-1 starting inoculum of the virulent Type A Schu4 strain, indicating the suitability of this medium in rapidly amplifying low starting titres of F. tularensis. Collectively, these results indicate that the novel formulation of the BVFH medium was capable of producing enhanced growth response for F. tularensis.

The need for rapid cultivation of Francisella tularensis is essential for detection and monitoring during natural outbreak events or intentional bioterrorism attacks. The addition of selected supplements into the base medium BHI (BVFH) developed in this study enhanced growth of F. tularensis Type A1, A2 and B strains compared to BHI alone. Growth of these organisms in BVFH will allow for improved response time should a natural or intentional contamination event occur.