Previous infection studies in mice have revealed that mutation of the serine protease HtrA in a number of pathogenic bacteria results in attenuated virulence properties (Li et al., 1996; Humphreys et al., 1999; Wilson et al., 2006; Yuan et al., 2008; Lewis et al., 2009; De Stoppelaar et al., 2013). The gene discussed is HTRA1; the disease is infection.