CtpB Facilitates Mycobacterium tuberculosis Growth in Copper-Limited Niches Original paper

What was studied?

This original study tested how CtpB Facilitates Mycobacterium tuberculosis Growth in Copper-Limited Niches and mapped when this P-type ATPase helps or harms the bacillus during host metal stress. The authors asked whether CtpB supports growth when copper is scarce, how its expression responds to copper chelation, and whether CtpB activity changes fitness when copper surges in host cells. They used controlled overexpression, targeted deletion, genetic complementation, and infection models to link CtpB function to copper import or directed copper delivery to cuproenzymes. The work places CtpB within nutritional immunity, where macrophages first starve bacteria of copper and later flood phagosomes with copper, and it tests if CtpB tunes survival across these swings.

Who was studied?

Researchers worked with Mycobacterium tuberculosis Erdman and M. bovis BCG strains and created ∆ctpB mutants and complemented strains. They expressed M. tuberculosis ctpB or ctpV in M. smegmatis under an inducible promoter to test gain-of-function in copper-rich media. Strains were passed through copper-free Sauton medium to deplete internal copper and then assayed growth with or without copper supplementation. They infected 3T3-L1 adipocytes, a copper-poor niche, and measured replication with or without added copper. They also tested virulence by intravenous infection of DBA/2 mice and monitored survival. Transcription studies probed ctpB induction under copper chelation and the role of sigma factor SigC.

Most important findings

CtpB responds to copper starvation and supports growth in copper-poor settings, yet it can sensitize cells when copper is high. Copper chelation induced ctpB transcription about eight-fold, while forced ctpB expression in M. smegmatis reduced growth only when copper was abundant, a pattern that fits copper import or high-affinity delivery to copper enzymes. Deleting ctpB produced no early defect but, after serial passage in copper-free medium, caused clear growth loss that copper addition partly rescued; the same pattern occurred in Erdman and BCG and reversed with complementation. In adipocytes, ∆ctpB showed a delayed replication defect that copper supplementation removed, and Erdman raised ctpB expression in low copper but not with added copper or zinc.

In DBA/2 mice, ∆ctpB infection shortened median survival by about 42% versus wild type despite similar organ burdens at death, which suggests faster disease progression and a “just-enough copper” advantage when CtpB is absent during copper spikes in vivo. CtpB is highly conserved in the M. tuberculosis complex and other intracellular mycobacteria, consistent with a niche-specific role in host metal pressure. These signals define a microbiome-relevant signature in which CtpB links copper-poor tissue niches (such as adipose) to mycobacterial fitness and ties copper surges in phagosomes to potential toxicity when import runs ahead of efflux and storage.

Key implications

Clinicians can read CtpB as a fitness factor in copper-limited niches and a liability during copper surges. In a microbiome signatures database, pairing ctpB with copper-handling loci (such as ctpV and mymT) and with host contexts that alter copper (adipose stores, activated macrophages) can refine risk for persistence or rapid disease. The adipocyte data support adipose as a copper-poor reservoir where CtpB helps growth, while the mouse survival data suggest that blocking CtpB could slow growth in copper-poor tissue yet might reduce copper-intoxication during phagosome transitions; any intervention should balance these effects. Gene-level reporting that flags ctpB together with sigC-linked import systems and copper efflux capacity can help predict where M. tuberculosis will thrive, when it will face copper injury, and how tissue site and inflammation state shape those outcomes.