Modulation of the M. tuberculosis cell envelope between replicating and non-replicating persistent bacteria

Abstract

The success of Mycobacterium tuberculosis as a human pathogen depends on the bacterium's ability to persist in a quiescent form in oxygen and nutrient-poor host environments. In vitro studies have demonstrated that these restricting environments induce a shift from bacterial replication to non-replicating persistence (NRP). Entry into NRP involves changes in bacterial metabolism and remodeling of the cell envelope. Findings consistent with the phenotypes observed in vitro have been observed in patient and animal model samples. This review focuses on the cell envelope differences seen between replicating and NRP M. tuberculosis and summarizes the ways in which serine/threonine protein kinases (STPKs) may mediate this process.

Keywords: Dormancy; Lipids; Mycobacterium tuberculosis; Mycolic acids.

Figure 1.. M. tuberculosis cell envelope composition in replicating vs. NRP cultures.

Replicating M. tuberculosis produced peptidoglycan, arabinogalactan, and PDIM, and readily converted exported TMM into TDM. In NRP M. tuberculosis, levels of DATs/PATs and SLs were unaltered while PDIM levels declined. TMM and TDM levels also decreased as they were converted into free mycolic acids. NRP M. tuberculosis accumulated TAGs and WEs. The ratio of LAM to LM was increased in NRP. MmpL3, MmpL7 and MmpL11 are phosphorylated. MmpL3 and MmpL7 export TMM and PDIM, respectively, which are present in replicating and NRP M. tuberculosis. MmpL11 accumulates in late stationary conditions and contributes to NRP through transport of TAGs and WEs. Colored circles next to proteins indicate known STPKs. *, Differentially phosphorylated in stationary phase, (Verma et al., 2017). **, Differentially phosphorylated in hypoxia, (Prisic et al., 2010).

Figure 2.. Mycolic acid biosynthesis is controlled by STPK phosphorylation.

Phosphoproteomic data from PknA and PknB studies indicates that several mycolic acid synthesis proteins are substrates of PknA and/or PknB. In vitro and biochemical data suggests that all enzymes represented above are down regulated by phosphorylation and may be substrates of multiple protein kinases. Closed circles indicate if PknA or PknB phosphorylate each enzyme based on phosphoproteomic data of whole cell lysates from WT and PknA or PknB depletion mutants (Carette et al., 2018; Zeng et al., 2020), and open circles indicate which kinases phosphorylate each enzyme in vitro. * indicate that an enzyme has not been identified as a phosphoprotein in cells.

Figure 3.. Membrane topology of MmpL3, MmpL11, and MmpL7.