Evolutionary genomics identifies host-directed therapeutics to treat intracellular bacterial infections

Abstract

Obligate intracellular bacteria shed essential biosynthetic pathways during their evolution towards host dependency, providing an opportunity for host-directed therapeutics. Using Rickettsiaceae as a model, we employed a novel computational pipeline called PoMeLo to systematically compare this cytosolic family of bacteria to the related Anaplasmataceae, which reside in a membrane-bound vacuole in the host cell. We identified 20 metabolic pathways that have been lost since the divergence of Anaplasmataceae and Rickettsiaceae, corresponding to the latter’s change to a cytosolic niche. We hypothesized that drug inhibition of these host metabolic pathways would reduce the levels of metabolites available to the bacteria, thereby inhibiting bacterial growth. We tested 22 commercially available inhibitors for 14 of the identified pathways and found that the majority (59%) reduced bacterial growth at concentrations that did not induce host cell cytotoxicity. Of these, 5 inhibitors with an IC50 under 5 μM were tested to determine whether their mode of inhibition was bactericidal or bacteriostatic. Both mycophenolate mofetil, an inhibitor of inosine-5’-monophosphate dehydrogenase in the purine biosynthesis pathway, and roseoflavin, an analog of riboflavin, displayed bactericidal activity. A complementary unbiased mass spectrometry-based metabolomics approach identified 14 pathways impacted by Rickettsia infection based on alterations in metabolite levels. Strikingly, 11 of these (79%) overlapped with those identified by our computational predictions. These in vitro validation studies support the feasibility of a novel evolutionary genomics-guided approach for host-directed antibiotic drug development against obligate pathogens.