A Proteomic Signature of Dormancy in the Actinobacterium: Micrococcus luteus

Dormancy is a protective state in which diverse pathogenic and non-pathogenic bacteria curtail metabolic activity to survive external stresses, including antibiotics. Evidence suggests dormancy consists of a continuum of interrelated states including viable-but-non-culturable (VBNC) and persistence states that contribute to the antibiotic tolerance. Reactivation from latent infection are observed in many serious pathogens including Mycobacterium turberculosis, Staphylococcus, Streptococcus, and Borrelia bacteria. Despite the obvious threat presented by dormant bacteria, the protein mechanisms regulating these dormancy states are not well understood. We have studied VBNC dormancy in Micrococcus luteus NCTC 2665 by tandem mass spectrometry-based quantitative proteomics to uncover some of these mechanisms. M. luteus is a nonpathogenic actinobacterium exhibiting a uniquely well-defined and reproducible VBNC state induced by nutrient deprivation. Dormant M. luteus demonstrated a global loss of protein diversity accompanied by increased levels of eighteen proteins that are conserved across actinobacteria including M. tuberculosis. Four of these proteins have been previously associated with latent tuberculosis, but the other 14 proteins are novel protein targets for dormancy studies. We have developed rapid methods to quantitate dormancy-related proteins across growth phases by targeted proteomics. The proteins upregulated during dormancy implicate important roles for anaplerotic metabolism, redox and amino acid metabolism, ribosomal regulatory processes, and nucleoid associated proteins in dormancy. Our data show that M. luteus is a viable model system for dissecting the protein mechanisms underlying dormancy and we identified new protein targets for future studies on therapeutics active against dormant bacterial infections, which is a severe limitation of current antibiotics.