Exposure of Deinococcus radiodurans to both static magnetic fields and gamma radiation: observation of cell recuperation effects

Abstract

The extremophilic bacterium Deinococcus radiodurans displays an extraordinary ability to withstand lethal radiation effects, due to its complex mechanisms for both proteome radiation protection and DNA repair. Published results obtained recently at this laboratory show that D. radiodurans submitted to ionizing radiation results in its DNA being shattered into small fragments which, when exposed to a “static electric field’ (SEF), greatly decreases cell viability. These findings motivated the performing of D. radiodurans exposed to gamma radiation, yet exposed to a different exogenous physical agent, “static magnetic fields” (SMF). Cells of D. radiodurans [strain D.r. GY 9613 (R1)] in the exponential phase were submitted to ⁶⁰Co gamma radiation from a gamma cell. Samples were exposed to doses in the interval 0.5–12.5 kGy, while the control samples were kept next to the irradiation setup. Exposures to SMF were carried out with intensities of 0.08 T and 0.8 T delivered by two settings: (a) a device built up at this laboratory with niobium magnets, delivering 0.08 T, and (b) an electromagnet (Walker Scientific) generating static magnetic fields with intensities from 0.1 to 0.8 T. All samples were placed in a bacteriological incubator at 30 °C for 48 h, and after incubation, a counting of colony forming units was performed. Two sets of cell surviving data were measured, each in triplicate, obtained in independent experiments. A remarkable similarity between the two data sets is revealed, underscoring reproducibility within the 5% range. Appraisal of raw data shows that exposure of irradiated cells to SMF substantially increases their viability. Data interpretation strongly suggests that the increase of D. radiodurans cell viability is a sole magnetic physical effect, driven by a stochastic process, improving the efficiency of the rejoining of DNA fragments, thus increasing cell viability. A type of cut-off dose is identified at 10 kGy, above which the irradiated cellular system loses recovery and the cell survival mechanism collapses.

Appendices

Appendix A

Assuming a constant Magnetic Force F_m(B) and a viscous force inside the cell proportional to the velocity, f = γv (γ is the viscosity coefficient), both acting on a DNA fragment with mass m, from Dynamics it gives

$$ {F}_m-f=m.a\to {F}_m-\gamma v=m\frac{dv}{dt}, $$

(16)

whose solution is

$$ v(t)=\frac{F_m}{\gamma }.\left\{1-{e}^{\left[-\frac{\gamma }{m}t\right]}\right\}, $$

(17)

Therefore, the velocity of each DNA fragment would increase asymptotically up to $\frac{F_m}{\gamma }$, after exposure to SMF over a long period of time.

Appendix B

Be N(D) the number of surviving cells after irradiation with a dose D. If the dose is changed by a differential increment dD, the number of surviving cells decreases by dN_s, that is,

$$ d{N}_S={N}_S\left(D+ dD\right)-{N}_S(D)<0. $$

(18)

Also, dN_s is directly proportional to both N_s(D) and dD; thus,

$$ d{N}_s\propto {N}_S(D) dD\ \mathrm{or}\ d{N}_s=-\uplambda {N}_S(D) dD\to \frac{dN_s}{N_S}=-\lambda dD, $$

(19)

and integrating Eq. (19) from (0; N₀) to (D; N_s) renders

$$ {N}_S(D)={N}_0.{e}^{\left[-\uplambda .\mathrm{D}\right]}. $$

(20)

Interestingly, Eq. (20) contains an important statistical concept as evidenced by rewriting Eq. 19 as

$$ \frac{1}{N_S}.\left(\frac{d{N}_S}{dD}\right)=-\uplambda . $$

(21)

We note that $\frac{d{N}_S}{dD}$ is the variation of N_s per unit dose interval, which is a probability in the phase-space. Dividing this probability by N_s gives a relative probability, or, a normalized probability,$\frac{1}{N_S}.\left(\frac{d{N}_S}{dD}\right)$, where its absolute value is a constant (λ).