International Journal of Radiation Biology
ISSN: 0955-3002 (Print) 1362-3095 (Online) Journal homepage: http://www.tandfonline.com/loi/irab20
Comment on Castillo et al. (2015) Jonathan I. Katz To cite this article: Jonathan I. Katz (2016) Comment on Castillo et al. (2015), International Journal of Radiation Biology, 92:3, 169-170, DOI: 10.3109/09553002.2016.1135265 To link to this article: http://dx.doi.org/10.3109/09553002.2016.1135265
Published online: 15 Feb 2016.
Submit your article to this journal
Article views: 10
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=irab20 Download by: [University of Saskatchewan Library]
Date: 25 February 2016, At: 23:59
INTERNATIONAL JOURNAL OF RADIATION BIOLOGY, 2016 VOL. 92, NO. 3, 169–170 http://dx.doi.org/10.3109/09553002.2016.1135265
LETTER
Comment on Castillo et al. (2015)
Downloaded by [University of Saskatchewan Library] at 23:59 25 February 2016
Sir In their paper, Castillo et al. (2015) report that reducing levels of ionizing radiation below background inhibits growth in the two species of bacteria, Shewanella oneidensis and Deinococcus radiodurans. This result has the remarkable implication that background levels of radiation, at which there is a very small probability of even one ionizing event in a bacterium in its replication time, are sufficient to facilitate its growth. In this study natural backgrounds were simulated by a gamma-ray irradiation of approximately 100 nGy h 1. For bacteria with a generation time of 3 h, the energy deposition was about 3 10 7 J kg 1 per generation. The mean energy deposition per ionization is about 30 eV (5 10 18 J), so that simulated background corresponds to about 6 1010 ionizations per kg per generation. The mass of a S. oneidensis bacterium is about 6 10 16 kg, while that of a D. Radiodurans bacterium is about 8 10 15 kg. Hence the number of ionizations per S. oneidensis per generation is about 4 10 5 while that per D. radiodurans per generation is about 5 10 4. Only tiny fractions of the bacteria experience even a single ionization during their lifetimes. The actual fractions are even smaller than these numbers indicate because ionizations are clustered along the recoil paths of electrons that Compton scatter the irradiating gamma rays. Hormesis cannot be acting through a response to ionization during the lifetime of a bacterium. Perhaps a single bacterium lucky enough to have an ionization event produces a super-vigorous clone that dominates its colony. This would imply an incubation time for the clone to dominate its colony and increase the measured replication rate; no such incubation time is evident in the data. If the reported result is valid, it must be produced by exposure of the growth medium to background radiation long (thousands of hours) before it is incorporated into the
bacteria. It would be necessary that the hormetic effect be effective in a large fraction of the bacteria growing in that medium; most bacterial cells must contain at least one molecule produced by ionization. Some free radical or other species produced by ionizing radiation must have long enough lifetime to accumulate to concentrations of at least one per bacterium. This hypothesis could be tested by lengthy (thousands of hours) exposure of the growth medium to background radiation prior to incubating bacteria in a low radiation environment, and vice versa; it predicts that long prior exposure would be hormetic, but not exposure during the brief period of bacterial growth. Experiments of this type could be conducted at higher radiation intensities (and with shorter durations) to determine if hormesis is produced by irradiation during bacterial growth or if radiation pre-conditions the growth medium. Mammalian cells are much bigger than bacteria. Further, a long-lived multicellular organism might be affected by events in a small fraction of its cells. These arguments are therefore not applicable to the possibility of hormesis in mammals, either at background radiation levels or at higher levels.
References Castillo H, Schoderbek D, Dulal S, Escobar G, Wood J, Nelson R, Smith G. Stress induction in the bacteria Shewanella oneidensis and Deinococcus radiodurans in response to below-background ionizing radiation. Int J Rad Biol 2015;91:749–756.
Jonathan I. Katz Department of Physics, McDonnell Center for the Space Sciences, Washington University, St. Louis, MO, USA [email protected]
Response to Dr Katz Sir We thank Professor Katz for his thoughtful comments on our recent IJRB paper and we respond below to his various points: (1) We agree with his statement that only a fraction of the cells experience a single ionization event in our control incubations (and even less so in our shielded treatment!), but, similar to many physics experiments seeking to document subparticle or astronomic events that increase the
size of their detectors to detect rare events, we also have increased the size of our ‘detector’ to represent large populations of cells. The gene expression data presented in Figure 3 for example represents samples from four separate wells representing over a billion cells of Shewanella at 24 h, and the expression data represent this ensemble of detectors. Also note that the data are averaged from three such experiments replicated over time. (2) In interpreting these results, probably more important than considering cell hit rate, is that cells are stressing in response to the reduced hit rate. So the environmental cue
170
(3)
Downloaded by [University of Saskatchewan Library] at 23:59 25 February 2016
(4)
(5)
(6)
J. I. KATZ
is not radiation hits, but rather the downward change of hits, the absence of normal, background levels of radiation. Regarding the limited time for a cell to experience the radiation treatments, in a typical bacterial growth curve, the generation time does not equal the lifetime of a cell, the physicochemical status of that cell is passed on to the daughter cells due to bacterial binary fission. So, there is more time for cell response to these radiation treatments than the cell doubling time. Instead of the generation of a ‘super-vigorous clone that dominates’ we would posit that the amplification of the radiation signals is carried out by cell-to-cell communication similar to the Bystander Effect in mammalian cells (Blyth and Sykes 2011) and stress-response release of outer membrane vesicles (Kulp and Kuehn 2010) or diffusible molecules (Li and Nair 2012) in bacteria. These mechanisms of course remain to be validated in our future work. Professor Katz presents a hypothesis and an approach to test the effect that the culture media may have on the background radiation-treated cells. However, the stress response was observed in the shielded treatment, and it this deprivation of radiation effect that we intend to focus on. We agree that results presented have limitations in application to multicellular organisms. We are in the process of performing similar experiments with the nematode, Caenorhabditis elegans to address this and expand to test if these responses occur across the tissues of a multicellular organism.
Lastly, we are in agreement with Dr Katz that this is an unusual cell response to vanishingly low levels of radiation
that is challenging to explain. Just the observation that the bacteria can sense and respond to the difference in these low-level radiation fields is remarkable and will provide the basis for much interesting research to come.
References Blyth BJ, Sykes PJ. 2011. Radiation-induced bystander effects: What are they, and how relevant are they to human radiation exposures? Radiat Res 176:139–157. Kulp A, Kuhn MJ. 2010. Biological functions and biogenesis of secreted bacterial outer membrane vesicles. Ann Rev Microbiol 64:163–184. Zhi L, Nair SK. 2012. Quorum sensing: How bacteria can coordinate activity and synchronize their response to external signals? Protein Sci 21:1043–1417.
Hugo Castillo1, Donald Schoderbek2, Santosh Dulal3, Gabriela Escobar1, Jeffrey Wood4, Roger Nelson4 & Geoffrey Smith1 1 Department of Biology, New Mexico State University, Las Cruces, NM, USA, 2 Department of Agriculture, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada, 3 School of Medicine, University of North Carolina, Chapel Hill NC, and 4 Department of Energy-Carlsbad Field Office, Carlsbad, NM, USA [email protected]
ISSN: 0955-3002 (Print) 1362-3095 (Online) Journal homepage: http://www.tandfonline.com/loi/irab20
Comment on Castillo et al. (2015) Jonathan I. Katz To cite this article: Jonathan I. Katz (2016) Comment on Castillo et al. (2015), International Journal of Radiation Biology, 92:3, 169-170, DOI: 10.3109/09553002.2016.1135265 To link to this article: http://dx.doi.org/10.3109/09553002.2016.1135265
Published online: 15 Feb 2016.
Submit your article to this journal
Article views: 10
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=irab20 Download by: [University of Saskatchewan Library]
Date: 25 February 2016, At: 23:59
INTERNATIONAL JOURNAL OF RADIATION BIOLOGY, 2016 VOL. 92, NO. 3, 169–170 http://dx.doi.org/10.3109/09553002.2016.1135265
LETTER
Comment on Castillo et al. (2015)
Downloaded by [University of Saskatchewan Library] at 23:59 25 February 2016
Sir In their paper, Castillo et al. (2015) report that reducing levels of ionizing radiation below background inhibits growth in the two species of bacteria, Shewanella oneidensis and Deinococcus radiodurans. This result has the remarkable implication that background levels of radiation, at which there is a very small probability of even one ionizing event in a bacterium in its replication time, are sufficient to facilitate its growth. In this study natural backgrounds were simulated by a gamma-ray irradiation of approximately 100 nGy h 1. For bacteria with a generation time of 3 h, the energy deposition was about 3 10 7 J kg 1 per generation. The mean energy deposition per ionization is about 30 eV (5 10 18 J), so that simulated background corresponds to about 6 1010 ionizations per kg per generation. The mass of a S. oneidensis bacterium is about 6 10 16 kg, while that of a D. Radiodurans bacterium is about 8 10 15 kg. Hence the number of ionizations per S. oneidensis per generation is about 4 10 5 while that per D. radiodurans per generation is about 5 10 4. Only tiny fractions of the bacteria experience even a single ionization during their lifetimes. The actual fractions are even smaller than these numbers indicate because ionizations are clustered along the recoil paths of electrons that Compton scatter the irradiating gamma rays. Hormesis cannot be acting through a response to ionization during the lifetime of a bacterium. Perhaps a single bacterium lucky enough to have an ionization event produces a super-vigorous clone that dominates its colony. This would imply an incubation time for the clone to dominate its colony and increase the measured replication rate; no such incubation time is evident in the data. If the reported result is valid, it must be produced by exposure of the growth medium to background radiation long (thousands of hours) before it is incorporated into the
bacteria. It would be necessary that the hormetic effect be effective in a large fraction of the bacteria growing in that medium; most bacterial cells must contain at least one molecule produced by ionization. Some free radical or other species produced by ionizing radiation must have long enough lifetime to accumulate to concentrations of at least one per bacterium. This hypothesis could be tested by lengthy (thousands of hours) exposure of the growth medium to background radiation prior to incubating bacteria in a low radiation environment, and vice versa; it predicts that long prior exposure would be hormetic, but not exposure during the brief period of bacterial growth. Experiments of this type could be conducted at higher radiation intensities (and with shorter durations) to determine if hormesis is produced by irradiation during bacterial growth or if radiation pre-conditions the growth medium. Mammalian cells are much bigger than bacteria. Further, a long-lived multicellular organism might be affected by events in a small fraction of its cells. These arguments are therefore not applicable to the possibility of hormesis in mammals, either at background radiation levels or at higher levels.
References Castillo H, Schoderbek D, Dulal S, Escobar G, Wood J, Nelson R, Smith G. Stress induction in the bacteria Shewanella oneidensis and Deinococcus radiodurans in response to below-background ionizing radiation. Int J Rad Biol 2015;91:749–756.
Jonathan I. Katz Department of Physics, McDonnell Center for the Space Sciences, Washington University, St. Louis, MO, USA [email protected]
Response to Dr Katz Sir We thank Professor Katz for his thoughtful comments on our recent IJRB paper and we respond below to his various points: (1) We agree with his statement that only a fraction of the cells experience a single ionization event in our control incubations (and even less so in our shielded treatment!), but, similar to many physics experiments seeking to document subparticle or astronomic events that increase the
size of their detectors to detect rare events, we also have increased the size of our ‘detector’ to represent large populations of cells. The gene expression data presented in Figure 3 for example represents samples from four separate wells representing over a billion cells of Shewanella at 24 h, and the expression data represent this ensemble of detectors. Also note that the data are averaged from three such experiments replicated over time. (2) In interpreting these results, probably more important than considering cell hit rate, is that cells are stressing in response to the reduced hit rate. So the environmental cue
170
(3)
Downloaded by [University of Saskatchewan Library] at 23:59 25 February 2016
(4)
(5)
(6)
J. I. KATZ
is not radiation hits, but rather the downward change of hits, the absence of normal, background levels of radiation. Regarding the limited time for a cell to experience the radiation treatments, in a typical bacterial growth curve, the generation time does not equal the lifetime of a cell, the physicochemical status of that cell is passed on to the daughter cells due to bacterial binary fission. So, there is more time for cell response to these radiation treatments than the cell doubling time. Instead of the generation of a ‘super-vigorous clone that dominates’ we would posit that the amplification of the radiation signals is carried out by cell-to-cell communication similar to the Bystander Effect in mammalian cells (Blyth and Sykes 2011) and stress-response release of outer membrane vesicles (Kulp and Kuehn 2010) or diffusible molecules (Li and Nair 2012) in bacteria. These mechanisms of course remain to be validated in our future work. Professor Katz presents a hypothesis and an approach to test the effect that the culture media may have on the background radiation-treated cells. However, the stress response was observed in the shielded treatment, and it this deprivation of radiation effect that we intend to focus on. We agree that results presented have limitations in application to multicellular organisms. We are in the process of performing similar experiments with the nematode, Caenorhabditis elegans to address this and expand to test if these responses occur across the tissues of a multicellular organism.
Lastly, we are in agreement with Dr Katz that this is an unusual cell response to vanishingly low levels of radiation
that is challenging to explain. Just the observation that the bacteria can sense and respond to the difference in these low-level radiation fields is remarkable and will provide the basis for much interesting research to come.
References Blyth BJ, Sykes PJ. 2011. Radiation-induced bystander effects: What are they, and how relevant are they to human radiation exposures? Radiat Res 176:139–157. Kulp A, Kuhn MJ. 2010. Biological functions and biogenesis of secreted bacterial outer membrane vesicles. Ann Rev Microbiol 64:163–184. Zhi L, Nair SK. 2012. Quorum sensing: How bacteria can coordinate activity and synchronize their response to external signals? Protein Sci 21:1043–1417.
Hugo Castillo1, Donald Schoderbek2, Santosh Dulal3, Gabriela Escobar1, Jeffrey Wood4, Roger Nelson4 & Geoffrey Smith1 1 Department of Biology, New Mexico State University, Las Cruces, NM, USA, 2 Department of Agriculture, Food, and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada, 3 School of Medicine, University of North Carolina, Chapel Hill NC, and 4 Department of Energy-Carlsbad Field Office, Carlsbad, NM, USA [email protected]
