A novel brain heart infusion broth supports the study of common Francisella tularensis serotypes

Francisella tularensis Schu S4, LVS and U112 have become model organisms for the study of Francisella pathogenesis, and represent a cross section of the different F. tularensis subspecies. Both Schu S4 and LVS are fastidious organisms, requiring medium fortified with supplements and nutrients for enhanced growth. Chamberlains defined medium, Tryptone Soy Broth supplemented with cysteine (TSBc), and cation-adjusted Mueller-Hinton broth (CAMHB) supplemented with 2% IsoVitaleX are typically used in the cultivation of these bacteria. In this report, we describe a simple brain heart infusion broth formulation that can be used to obtain superior growth characteristics in all of these model organisms, and can support bacterial growth from low inoculum. Surprisingly, CAMHB, which is favored in the literature for culturing Schu S4 and LVS, induced the worst growth characteristics of the four formulations studied. To expand on these observations, an additional seven strains of F. tularensis, representing types A.I, A.II, and B were selected from the Department of Defense United Culture Collection (UCC) and a comparative analysis of their growth characteristics performed in the four broth formulations. Results demonstrate differences in the growth characteristics of Francisella species that are significantly influenced by both strain type and the choice of growth medium. Though four of the five additional Type A strains displayed superior growth characteristics in Chamberlain's defined medium, growth characteristics of all three model organisms, as well the Type B strains, were enhanced by the new BHI-based broth formulation. We conclude that this medium represents the optimal choice for cultivation of the three model organisms used for Francisella research.