Harris Baker posted an update 3 months, 1 week ago
This finding did not seem to be a consequence of initial in vivo survival defects of the aceA mutant, however, as bacterial burdens of all strains tested were equivalent at 2 hours post-infection. The,50-fold decrease in virulence of an ICL-deficient strain of P. aeruginosa is consistent with data previously shown in other infection models. Contrary to the aceA mutant, however, the glcB mutant did not show any attenuation in this model, with similar bacterial recovery seen between this strain and the wild-type PAO1. To our knowledge, this is the first report of an MS-deficient strain of P. aeruginosa being tested in a pulmonary model of infection, although such mutants have been shown to be involved in virulence in an alfalfa seedling model. We have speculated that the ICL-mediated production of succinate from isocitrate is sufficient to promote in vivo survival, despite the accumulation of glyoxylate. Perhaps most interestingly, we found the double aceA glcB mutant to be severely attenuated in this model, much more so than the single aceA mutant, with no recoverable colonies at 48 hours despite equal lung burdens at 2 hours. These data suggest that no bypass mechanisms can circumvent the loss of both enzymes in the glyoxylate shunt. Taken together, these in vivo results demonstrate that glyoxylate shunt inhibitors that have the greatest potential to serve as therapeutic agents against P. aeruginosa need to inhibit both enzymes in the pathway. One potential caveat to this approach, however, is the likelihood that glyoxylate shunt inhibitors would only be efficacious in the pulmonary environment, as evidenced by the lack of virulence differentiation between wild-type PAO1 and its isogenic glyoxylate shunt mutants when mice were challenged in a septicemia model of infection. Additionally, the impact on pulmonary efficacy when non-lipidbased FG-4592 carbon sources are introduced needs to be explored and assessed when considering this target for antimicrobial therapy. Given the clear differences in growth in vitro between wild-type PAO1 and its isogenic glyoxylate shunt mutants, we were intrigued by the possibility that this metabolic pathway could represent a new antibacterial target. Accordingly, we utilized M9 Acetate to screen for compounds that affected growth of wild-type PAO1. We set our cutoff at 40% growth inhibition relative to the untreated controls in each plate, and the screening of approximately 150,000 compounds yielded 498 primary hits, resulting in a 0.3% hit rate. 219 of the primary hits were subjected to a secondary screen that consisted of repeat growth inhibition in M9 Acetate, as well as an assessment of growth inhibition, if any, in M9 Glucose. The latter was included to triage compounds that had intrinsic whole cell activity that was not associated with the functionality of the glyoxylate shunt. Additionally, this assay also served to eliminate any non-specific, detergent-like compounds that had whole cell activity due to membrane disruption rather than a defined mechanism of action. From this secondary screening we identified 21 compounds that showed an acetate-specific pattern of growth inhibition. These hits were further characterized for specific ICLand MS-inhibition, and also for antibacterial spectrum of activity against other clinically-relevant Gram-negative pathogens. In an attempt to confirm that the growth deficiencies seen in the primary and secondary assays were attributable to the specific inhibition of the glyoxylate shunt, the 21 compounds identified above were used to measure IC50 values against purified ICL and MS enzymes. The enzymatic assays used for each protein have been described previously, and involve the detection of specific reaction products for each enzyme in the pathway. While all of these compounds had measurable IC50 values against at least one of the two enzymes, we found that 8 of them demonstrated inhibitory activities against both enzymes in the nanomolar to single-digit micromolar ranges. Given that our in vivo results that suggested that dual-enzyme targeting would likely be required to eradicate an infection through glyoxylate shunt inhibition, together with the fact that resistance development is less likely to occur as rapidly when compounds have more than one target, we decided to further pursue only those compounds that showed inhibition against both ICL and MS. These 8 lead compounds were subjected to traditional MIC testing using both standard Mueller Hinton Broth as well as the M9 Acetate medium that was used for primary screening. Interestingly, all 8 compounds demonstrated significant growth inhibition of 4 different Gram-negative pathogens when tested in M9 Acetate, yet showed little inhibition when these same strains were subjected to MIC testing in nutrient-replete MHB.