Stress induction in the bacteriaShewanella oneidensisandDeinococcus radioduransin response to below-background ionizing radiation

Below-Background Ionizing Radiation as an Environmental Cue for Bacteria

“…During mid-exponential phase (8 h in S. oneidensis), six genes related to oxidative stress response (katB, oxyR), DNA repair (recA, lexA), protein folding (dnaK), and a putative efflux pump (SOA0154) were again significantly upregulated as we reported previously (Castillo et al, 2015). Oxidative stress and efflux pump activity was initiated and maintained during lateexponential (13 h) but dissipated by early stationary phase (17 h).…”

“…In our previous report (Castillo et al, 2015), we showed growth inhibition and/or the upregulation of stress-related genes in experiments with limited time points. In the experiment reported here with more site access, S. oneidensis cultures did not show a significant difference in growth in response to the reduced radiation dose (Figure 2A), whereas D. radiodurans growth became inhibited at the beginning of its exponential growth phase and remained significantly different throughout the duration of the experiment ( Figure 2B).…”

“…Shewanella oneidensis and D. radiodurans cultures were grown as previously described (Castillo et al, 2015). In summary, both bacteria were grown in a shielded treatment incubator and a radiation-supplemented control incubator for 24 h as a conditioning period, after which they were transferred into fresh broth to initiate the experiment.…”

“…However, using growth as the only endpoint to measure the effect of a particular environmental variable might be misleading since cells employ multiple physiological mechanisms in response to changing environments to maintain homeostasis. In order to circumvent this limitation, similar low-radiation dose experiments have used different approaches for a more indepth analysis of the cellular response, such as micronucleus assays (Carbone et al, 2009), enzymatic activity (Satta et al, 2002;Carbone et al, 2009;Fratini et al, 2015), mutation assays (Satta et al, 1995(Satta et al, , 2002Fratini et al, 2015), and differential gene expression (Castillo et al, 2015;Fratini et al, 2015). In the present study, we measured the expression of genes related to some of these types of stress, on cells collected at various time points in order to compare their differential expression.…”

“…The first evidence of a deleterious biological response to radiation doses lower than background was reported by Planel et al (1987), who observed a longer generation time in Paramecium tetraurelia and a lower cell number in Synechococcus lividus cultures when these organisms were grown shielded from radiation at dose rates 17 and 6 times lower than background, respectively. Since then, experiments with other organisms have shown similar deleterious effects when exposed to below background levels of radiation, such as lower protection to mutational damage in Saccharomyces cerevisiae (Satta et al, 1995); higher sensitivity to apoptosis, higher intracellular concentration of oxidative stressrelated enzymes, and higher mutation rate induced by gamma rays (after conditioning under below background conditions) in Cricetulus griseus (Satta et al, 2002); growth rate retardation in Mus musculus L5178Y cells (Kawanishi et al, 2012), changes in the concentration of oxidative stress-related enzymes (Fratini et al, 2015), and lower cell number and/or up regulation of DNA repair and oxidative stress genes in Shewanella oneidensis and Deinococcus radiodurans (Smith et al, 2011;Castillo et al, 2015).…”