Coxiella burnetii and Burkholderia pseudomallei are Gram-negative intracellular pathogens found in the environment and are the causative agents of Q-fever and melioidosis, respectively. Both diseases are endemic in Australia, are a challenge to diagnose and their treatment is often very difficult. Complicating this further is that currently there is no vaccine available against B. pseudomallei and the only available vaccine for Q-fever, Q-Vax, requires pre-vaccination testing due to the risk of severe adverse reactions, limiting its use as a countermeasure to contain outbreaks of this highly infectious organism. Therefore the identification of new drug targets against these organisms is essential.
Macrophage infectivity potentiators (Mips) are proteins that catalyze the folding of proline-containing proteins through their peptidyl prolyl cis-trans isomerase (PPIase) activity and have been shown to play an important role in the virulence of several pathogenic bacteria. To this end, the Mip proteins of C. burnetii and B. pseudomallei exhibit virulence-associated PPIase activity. Our data shows that inhibition of Mip represents a potentially novel target for antimicrobial therapies against intracellular pathogens. Using a protease coupled PPIase assay, we have demonstrated that a selection of pipecolic acid-derived compounds have inhibitory properties against recombinant Mip proteins from C. burnetii and B. pseudomallei. Importantly, these compounds also inhibit both C. burnetii growth in axenic media and intracellular survival in vitro, validating these molecules as novel therapeutics. Furthermore these compounds also reduce the cytotoxic effects of B. pseudomallei on macrophages demonstrating that these Mip inhibitors have broad-spectrum activity.