M. tuberculosis, the causative agent of Tuberculosis (TB), is an ancient pathogen that has plagued mankind for over 70,000 years. In 2018, TB was responsible globally for 10 million new infections and 1.5 million deaths - more than any other infectious disease. M. tuberculosis bacilli have evolved to thrive in the harsh, nutrient limited environment within the host alveolar macrophage and evade the constant pressure of immune cell mediated killing. This evolution has led to the bacilli developing phenotypic adaptions that concurrently, drastically decrease their susceptibility to many antimicrobials. The basis for phenotypic adaptions is signalling to detect an environmental stimulus and to mediate an appropriate response. To this end, M. tuberculosis and other mycobacteria have evolved a robust cyclic AMP (cAMP) signalling system with multiple cAMP producing and cAMP binding effector proteins. Several of these proteins have already been shown to regulate virulence, carbon metabolism and essential gene expression. However, the link between cAMP signalling and antimicrobial susceptibility in mycobacteria has not previously been investigated.
In this project, I identified a new cAMP degrading phosphodiesterase enzyme (Rv1339) and used it as a tool to significantly decrease intrabacterial levels of cAMP in mycobacteria. The effect of this in M. smegmatis mc2155 was to increase antimicrobial susceptibility. By using a combination of metabolomics, RNA-sequencing, antimicrobial susceptibility assays and bioenergetics analysis, I was able to characterise the potential mechanism behind this increased susceptibility. I was also able to begin preliminary work required to investigate this link in M. tuberculosis H37Rv. This work represents a proof-of-concept that targeting cAMP signalling is a promising new avenue for antimicrobial development, and expands our understanding of cAMP signalling in mycobacteria.