MglA (Macrophage Growth Locus A) is a pleiotropic transcription factor controlling the expression pattern of more than 100 genes in Francisella novicida, including all the ORFs located on the pathogenicity island. To further probe the role of MglA in the pathogenicity of the related strain Francisella tularensis LVS, we generated a mutant LVS strain in which the mglA locus was disrupted by insertion of a non-polar selectable marker cassette.
In vitro and in vivo analysis of the phenotype associated with the mglA null mutation in comparison to the wild-type parental strain and to a complementation strain (ΔmglA:mglA), established that the mutated strain ΔmglA: (i) cannot multiply in vitro in macrophages, (ii) is severely attenuated in a murine model of infection, exhibiting over 10,000 and 10,000,000 fold decrease in virulence by intranasal (IN) administration, and intraperitoneal (IP) route of infection respectively, (iii) unlike wild type LVS, the mutant strain cannot multiply in the lungs, liver and spleen of infected animals following IP administration, (iv) the ΔmglA mutant do not disseminate to target organs (e.g. liver and spleen) following IN administration, (v) Infection by ΔmglA bacteria elicits a significant humoral response, and systemic IP administration of high doses of ΔmglA cells results in full protection of animals against a subsequent IP challenge with high doses of the virulent wild-type strain.
These results indicate that MglA plays a major role in F. tularensis LVS virulence as previously shown for F. novicida. Our studies suggest that inactivation of MglA may serve as a platform for the development of an improved attenuated vaccine of virulent F. tularensis strains.