Persister Cells - a Plausible Outcome of Neutral Coevolutionary Drift

Abstract

The phenomenon of bacterial persistence - a non-inherited antibiotic tolerance in a minute fraction of the bacterial population, was observed more than 70 years ago. Nowadays, it is suggested that "persister cells" undergo an alternative scenario of the cell cycle; however, pathways involved in its emergence are still not identified. We present a mathematically grounded scenario of such possibility. We have determined that population drift in the space of multiple neutrally coupled mutations, which we called "neutrally coupled co-evolution" (NCCE), leads to increased dynamic complexity of bacterial populations via appearance of cells capable of carrying out a single cell cycle in two or more alternative ways and that universal properties of the coupled transcription-translation system underlie this phenotypic multiplicity. According to our hypothesis, modern persister cells have derived from such cells and regulatory mechanisms that govern the consolidation of this phenomenon represented the trigger. We assume that the described type of neutrally coupled co-evolution could play an important role in the origin of extremophiles, both in bacteria and archaea.

Figure 1

The diagram depicting processes considered in the simple model of the cell cycle. V – ≪cell≫ volume, С – generalized synthesis factor, R – generalized growth factor, S_c – synthesis initiation of the factor C, S_r – synthesis initiation of the factor R, I_c – polysomes that synthesize the factor C, I_r – polysomes that synthesize the factor R, k_c,s – rate constant for the synthesis of the factor C, k_r,s – rate constant for the synthesis of the factor R, Y- general law of ≪cell≫ growth, α_r – the number of growth factor molecules consumed during a single cell cycle. The growth process is schematically shown by the ≪cell≫ size increase from left to right.

Figure 2

Correlation between relative rates of factor c synthesis and dilution and its concentration in the ≪cell≫. (A) Equations (8 and 11), curve 1 – relative rate of synthesis s_c/C (F₁); curve 2 – relative rate of dilution s_r/α_r (F₃); calculations were made for k_e/n = 1.35 min⁻¹, α_r = 500000 unit/cell, K_r = 1000 unit/cell. (B,C) Equation (20), curve 1 – relative rate of synthesis S_c/c (F₂), curve 2 – relative rate of dilution S_r/α_r (F₃), calculations were made for α_r = 250000 unit/cell, K_r = 5000 unit/cell (B) K_r = 1000 unit/cell (C). Values of the remaining parameters are k_c = 3500 unit/cell/min, k_r = 25000 unit/cell/min, K_c = 15000 unit/cell. Abscissa – concentration of the generalized synthesis factor c (unit/cell), ordinate – values of the relative rates of synthesis and dilution (con. unit).

Figure 3

Scenario of the appearance of ≪persister≫ cells. As a result of the neutrally coupled co-evolution and due to nonlinear properties of the coupled transcription-translation system, a population of ≪ancient cells≫ (1) consisting of cells capable of carrying out a single cell cycle in one and only way gave rise to another population of ≪ancient cells≫ (2) consisting of cells of a qualitatively new type, namely capable of carrying out a single cell cycle in several alternative ways (phenotypes). The transition between phenotypes is a random event that occurs due to fluctuations in concentrations of the intracellular components during cell division. The population of ≪modern cells≫ capable of surviving under stress conditions (3) originated from the ≪ancient population≫ of cells (2), in which, in the course of adaptive evolution, regulatory contours of a trigger-type were formed and consolidated at the genetic level, which included both phenotypes in the norm of reaction. Drug-sensitive cells and ≪persister≫ cells of this population represent genetically identical cells that carry out a cell cycle in alternative ways. The transition occurs under stress.