Using single-cell technology, we showed previously that, in E. coli, the ubiquitous bacterial stress alarmone (p)ppGpp (Magic Spot) is a central regulator of both spontaneous and environmentally induced persistence1. The (p)ppGpp level varied stochastically in a population of exponentially growing cells and the high (p)ppGpp level in the rare cells induced persistence. Persister cell formation depended on 10 type II toxin – antitoxin (TA) modules encoding RNases that inhibit translation by cleavage of mRNA or rRNA2, 3. A similar mechanism underlies persister formation by Salmonella4.
Recently, Jan Michiels’ group showed that a type I TA module (hokB/sokB) can induce persistence by a mechanism that also depends on (p)ppGpp and, and surprisingly, the highly conserved GTPase Obg5. Type I TAs encode small proteins that depolarize the cell membrane and confer membrane damage and rapid cell killing when overexpressed whereas moderate expression depletes the ATP pool5, 6, 7. Expression of these highly toxic proteins is repressed by cis-acting antisense RNAs. A complex mRNA folding pathway allows the mRNA to escaping irreversible inactivation by the antisense and expression of the toxin in the absence of transcription8.
Together, these results reveal Magic Spot as the central regulator and toxin - antitoxins as the central effectors of persistence in E. coli and other enterics.
1. Maisonneuve, E., Castro-Camargo, M. & Gerdes, K. (p)ppGpp controls bacterial persistence by stochastic induction of toxin-antitoxin activity. Cell 154, 1140-1150 (2013).
2. Germain, E., Roghanian, M., Gerdes, K. & Maisonneuve, E. Stochastic induction of persister cells by HipA through (p)ppGpp-mediated activation of mRNA endonucleases. Proc Natl Acad Sci U S A 112, 5171-6 (2015).
3. Maisonneuve, E., Shakespeare, L.J., Jørgensen, M.G. & Gerdes, K. Bacterial persistence by RNA endonucleases. Proc.Natl.Acad.Sci.U.S.A 108, 13206-13211 (2011).
4. Helaine, S. et al. Internalization of Salmonella by macrophages induces formation of nonreplicating persisters. Science 343, 204-8 (2014).
5. Verstraeten, N. et al. Obg and Membrane Depolarization Are Part of a Microbial Bet-Hedging Strategy that Leads to Antibiotic Tolerance. Mol Cell 59, 9-21 (2015).
6. Gerdes, K. et al. Mechanism of Postsegregational Killing by the Hok Gene-Product of the parB System of Plasmid R1 and Its Homology with the RelF Gene-Product of the Escherichia coli relB Operon. EMBO Journal 5, 2023-2029 (1986).
7. Gerdes, K., Rasmussen, P.B. & Molin, S. Unique Type of Plasmid Maintenance Function - Postsegregational Killing of Plasmid-Free Cells. Proceedings of the National Academy of Sciences of the United States of America 83, 3116-3120 (1986).
8. Moller-Jensen, J., Franch, T. & Gerdes, K. Temporal translational control by a metastable RNA structure. J Biol Chem 276, 35707-13 (2001).