Aldehyde accumulation in <i>Mycobacterium tuberculosis</i> with defective proteasomal degradation results in copper sensitivity

ABSTRACT Mycobacterium tuberculosis is a major human pathogen and the causative agent of tuberculosis disease. M. tuberculosis is able to persist in the face of host-derived antimicrobial molecules nitric oxide and copper. However, M. tuberculosis with defective proteasome activity is highly sensitive to nitric oxide and copper, making the proteasome an attractive target for drug development. Previous work linked nitric oxide susceptibility with the accumulation of para -hydroxybenzaldehyde in M. tuberculosis mutants with defective proteasomal degradation. In this study, we found that para -hydroxybenzaldehyde accumulation was also responsible for copper sensitivity in these strains. We showed that exogenous addition of para -hydroxybenzaldehyde to wild-type M. tuberculosis cultures sensitized bacteria to copper to a degree similar to that of a proteasomal degradation mutant. We determined that para -hydroxybenzaldehyde reduced the production and function of critical copper resistance proteins of the regulated in copper repressor (RicR) regulon. Further, we extended these Cu-sensitizing effects to an aldehyde that M. tuberculosis may face within the macrophage. Collectively, this study is the first to mechanistically propose how aldehydes can render M. tuberculosis susceptible to an existing host defense and could support a broader role for aldehydes in controlling M. tuberculosis infections. IMPORTANCE M. tuberculosis is a leading cause of death by a single infectious agent, causing 1.5 million deaths annually. An effective vaccine for M. tuberculosis infections is currently lacking, and prior infection does not typically provide robust immunity to subsequent infections. Nonetheless, immunocompetent humans can control M. tuberculosis infections for decades. For these reasons, a clear understanding of how mammalian immunity inhibits mycobacterial growth is warranted. In this study, we show aldehydes can increase M. tuberculosis susceptibility to copper, an established antibacterial metal used by immune cells to control M. tuberculosis and other microbes. Given that activated macrophages produce increased amounts of aldehydes during infection, we propose host-derived aldehydes may help control bacterial infections, making aldehydes a previously unappreciated antimicrobial defense.