RelA regulates virulence and intracellular survival of Francisella novicida

Abstract

Analysis of the genome ofFrancisella tularensishas revealed few regulatory systems, and how the organism adapts to conditions in different niches is poorly understood. The stringent response is a global stress response mediated by (p)ppGpp. The enzyme RelA has been shown to be involved in generation of this signal molecule in a range of bacterial species. We investigated the effect of inactivation of therelAgene inFrancisellaby generating a mutant inFrancisella novicida. Under amino acid starvation conditions, therelAmutant was defective for (p)ppGpp production. Characterization showed the mutant to grow similarly to the wild-type, except that it entered stationary phase later than wild-type cultures, resulting in higher cell yields. TherelAmutant showed increased biofilm formation, which may be linked to the delay in entering stationary phase, which in turn would result in higher cell numbers present in the biofilm and reduced resistance toin vitrostress. The mutant was attenuated in the J774A macrophage cell line and was shown to be attenuated in the mouse model of tularaemia, but was able to induce a protective immune response. Therefore, (p)ppGpp appears to be an important intracellular signal, integral to the pathogenesis ofF. novicida.