Radiation-resistant mutants of Salmonella typhimurium LT2: development and characterization

Comparative assessment of UV-C radiation and non-thermal plasma for inactivation of foodborne fungal spores suspension in vitro

Fungal contamination poses a persistent challenge to industries, particularly in food, healthcare, and clinical sectors, due to the remarkable resilience of fungi in withstanding conventional control methods. In this context, our research delves into the comparative efficacy of UV radiation and non-thermal plasma (NTP) on key foodborne fungal contaminants - Alternaria alternata, Aspergillus niger, Fusarium culmorum, and Fusarium graminearum. The study examined the impact of varying doses of UV radiation on the asexual spores of all mentioned fungal strains. Simultaneously, the study compared the effects of UV radiation and NTP on the metabolic activity of cells after spore germination and their subsequent germination ability. The results revealed that UV-C radiation (254 nm) did not significantly suppress the metabolic activity of cells after spore germination. In contrast, NTP exhibited almost 100% effectiveness on both selected spores and their subsequent germination, except for A. niger. In the case of A. niger, the effectiveness of UV-C and NTP was nearly comparable, showing only a 35% decrease in metabolic activity after 48 hours of germination, while the other strains (A. alternata, F. culmorum, F. graminearum) exhibited a reduction of more than 95%. SEM images illustrate the morphological changes in structure of all tested spores after both treatments. This study addresses a crucial gap in existing literature, offering insights into the adaptation possibilities of treated cells and emphasizing the importance of considering exposure duration and nutrient conditions (introduction of fresh medium). The results highlighted the promising antimicrobial potential of NTP, especially for filamentous fungi, paving the way for enhanced sanitation processes with diverse applications.

Experimental evolution of extremophile resistance to ionizing radiation

A growing number of known species possess a remarkable characteristic - extreme resistance to the effects of ionizing radiation (IR). This review examines our current understanding of how organisms can adapt to and survive exposure to IR, one of the most toxic stressors known. The study of natural extremophiles such as Deinococcus radiodurans has revealed much. However, the evolution of Deinococcus was not driven by IR. Another approach, pioneered by Evelyn Witkin in 1946, is to utilize experimental evolution. Contributions to the IR-resistance phenotype affect multiple aspects of cell physiology, including DNA repair, removal of reactive oxygen species, the structure and packaging of DNA and the cell itself, and repair of iron-sulfur centers. Based on progress to date, we overview the diversity of mechanisms that can contribute to biological IR resistance arising as a result of either natural or experimental evolution.

Impact of the Resistance Responses to Stress Conditions Encountered in Food and Food Processing Environments on the Virulence and Growth Fitness of Non-Typhoidal Salmonellae

The success of Salmonella as a foodborne pathogen can probably be attributed to two major features: its remarkable genetic diversity and its extraordinary ability to adapt. Salmonella cells can survive in harsh environments, successfully compete for nutrients, and cause disease once inside the host. Furthermore, they are capable of rapidly reprogramming their metabolism, evolving in a short time from a stress-resistance mode to a growth or virulent mode, or even to express stress resistance and virulence factors at the same time if needed, thanks to a complex and fine-tuned regulatory network. It is nevertheless generally acknowledged that the development of stress resistance usually has a fitness cost for bacterial cells and that induction of stress resistance responses to certain agents can trigger changes in Salmonella virulence. In this review, we summarize and discuss current knowledge concerning the effects that the development of resistance responses to stress conditions encountered in food and food processing environments (including acid, osmotic and oxidative stress, starvation, modified atmospheres, detergents and disinfectants, chilling, heat, and non-thermal technologies) exerts on different aspects of the physiology of non-typhoidal Salmonellae, with special emphasis on virulence and growth fitness.

Development of Antimicrobial Phototreatment Tolerance: Why the Methodology Matters

Due to rapidly growing antimicrobial resistance, there is an urgent need to develop alternative, non-antibiotic strategies. Recently, numerous light-based approaches, demonstrating killing efficacy regardless of microbial drug resistance, have gained wide attention and are considered some of the most promising antimicrobial modalities. These light-based therapies include five treatments for which high bactericidal activity was demonstrated using numerous in vitro and in vivo studies: antimicrobial blue light (aBL), antimicrobial photodynamic inactivation (aPDI), pulsed light (PL), cold atmospheric plasma (CAP), and ultraviolet (UV) light. Based on their multitarget activity leading to deleterious effects to numerous cell structures-i.e., cell envelopes, proteins, lipids, and genetic material-light-based treatments are considered to have a low risk for the development of tolerance and/or resistance. Nevertheless, the most recent studies indicate that repetitive sublethal phototreatment may provoke tolerance development, but there is no standard methodology for the proper evaluation of this phenomenon. The statement concerning the lack of development of resistance to these modalities seem to be justified; however, the most significant motivation for this review paper was to critically discuss existing dogma concerning the lack of tolerance development, indicating that its assessment is more complex and requires better terminology and methodology.

Physiology of Highly Radioresistant Escherichia coli After Experimental Evolution for 100 Cycles of Selection

Ionizing radiation (IR) is lethal to most organisms at high doses, damaging every cellular macromolecule via induction of reactive oxygen species (ROS). Utilizing experimental evolution and continuing previous work, we have generated the most IR-resistant Escherichia coli populations developed to date. After 100 cycles of selection, the dose required to kill 99% the four replicate populations (IR9-100, IR10-100, IR11-100, and IR12-100) has increased from 750 Gy to approximately 3,000 Gy. Fitness trade-offs, specialization, and clonal interference are evident. Long-lived competing sub-populations are present in three of the four lineages. In IR9, one lineage accumulates the heme precursor, porphyrin, leading to generation of yellow-brown colonies. Major genomic alterations are present. IR9 and IR10 exhibit major deletions and/or duplications proximal to the chromosome replication terminus. Contributions to IR resistance have expanded beyond the alterations in DNA repair systems documented previously. Variants of proteins involved in ATP synthesis (AtpA), iron-sulfur cluster biogenesis (SufD) and cadaverine synthesis (CadA) each contribute to IR resistance in IR9-100. Major genomic and physiological changes are emerging. An isolate from IR10 exhibits protein protection from ROS similar to the extremely radiation resistant bacterium Deinococcus radiodurans, without evident changes in cellular metal homeostasis. Selection is continuing with no limit to IR resistance in evidence as our E. coli populations approach levels of IR resistance typical of D. radiodurans.

Experimental Evolution of Extreme Resistance to Ionizing Radiation in Escherichia coli after 50 Cycles of Selection

In previous work (D. R. Harris et al., J Bacteriol 191:5240-5252, 2009, https://doi.org/10.1128/JB.00502-09; B. T. Byrne et al., Elife 3:e01322, 2014, https://doi.org/10.7554/eLife.01322), we demonstrated that Escherichia coli could acquire substantial levels of resistance to ionizing radiation (IR) via directed evolution. Major phenotypic contributions involved adaptation of organic systems for DNA repair. We have now undertaken an extended effort to generate E. coli populations that are as resistant to IR as Deinococcus radiodurans After an initial 50 cycles of selection using high-energy electron beam IR, four replicate populations exhibit major increases in IR resistance but have not yet reached IR resistance equivalent to D. radiodurans Regular deep sequencing reveals complex evolutionary patterns with abundant clonal interference. Prominent IR resistance mechanisms involve novel adaptations to DNA repair systems and alterations in RNA polymerase. Adaptation is highly specialized to resist IR exposure, since isolates from the evolved populations exhibit highly variable patterns of resistance to other forms of DNA damage. Sequenced isolates from the populations possess between 184 and 280 mutations. IR resistance in one isolate, IR9-50-1, is derived largely from four novel mutations affecting DNA and RNA metabolism: RecD A90E, RecN K429Q, and RpoB S72N/RpoC K1172I. Additional mechanisms of IR resistance are evident.IMPORTANCE Some bacterial species exhibit astonishing resistance to ionizing radiation, with Deinococcus radiodurans being the archetype. As natural IR sources rarely exceed mGy levels, the capacity of Deinococcus to survive 5,000 Gy has been attributed to desiccation resistance. To understand the molecular basis of true extreme IR resistance, we are using experimental evolution to generate strains of Escherichia coli with IR resistance levels comparable to Deinococcus Experimental evolution has previously generated moderate radioresistance for multiple bacterial species. However, these efforts could not take advantage of modern genomic sequencing technologies. In this report, we examine four replicate bacterial populations after 50 selection cycles. Genomic sequencing allows us to follow the genesis of mutations in populations throughout selection. Novel mutations affecting genes encoding DNA repair proteins and RNA polymerase enhance radioresistance. However, more contributors are apparent.

Inactivation of RNA Viruses by Gamma Irradiation: A Study on Mitigating Factors

Effective inactivation of biosafety level 4 (BSL-4) pathogens is vital in order to study these agents safely. Gamma irradiation is a commonly used method for the inactivation of BSL-4 viruses, which among other advantages, facilitates the study of inactivated yet morphologically intact virions. The reported values for susceptibility of viruses to inactivation by gamma irradiation are sometimes inconsistent, likely due to differences in experimental protocols. We analyzed the effects of common sample attributes on the inactivation of a recombinant vesicular stomatitis virus expressing the Zaire ebolavirus glycoprotein and green fluorescent protein. Using this surrogate virus, we found that sample volume and protein content of the sample modulated viral inactivation by gamma irradiation but that air volume within the sample container and the addition of external disinfectant surrounding the sample did not. These data identify several factors which alter viral susceptibility to inactivation and highlight the usefulness of lower biosafety level surrogate viruses for such studies. Our results underscore the need to validate inactivation protocols of BSL-4 pathogens using “worst-case scenario” procedures to ensure complete sample inactivation.

Emergence of pulsed electric fields resistance in Salmonella enterica serovar Typhimurium SL1344

In this investigation we selected and isolated a culture derived from Salmonella enterica serovar Typhimurium SL1344 with stable increased resistance to pulsed electric fields (PEF) after repeated rounds of PEF treatment and outgrowth of survivors. The resulting culture showed a higher resistance to PEF treatments under different treatment conditions. The acquisition of PEF resistance was only observed in stationary phase cells. The cytoplasmic membrane of the resistant variant showed a higher resilience against PEF treatments, since a lower permeabilization degree was observed after PEF treatments, in comparison to the parental strain. Resistance to PEF was also accompanied by a higher tolerance to acidic pH, hydrogen peroxide and ethanol, but not to heat. The occurrence of a PEF resistant variant in S. enterica serovar Typhimurium SL1344 emphasizes the need to further study the mechanisms of inactivation and resistance by PEF for an adequate design of safe treatments.

Effects of gamma irradiation on the viability and phenotypic characteristics of Salmonella Enteritidis inoculated into specific-pathogen-free eggs

The goal of this study was to determine the effects of various levels of gamma irradiation on the phenotypic characteristics of 20 strains of Salmonella Enteritidis inoculated separately into specific-pathogen-free shell eggs. Bacterial strains were inoculated into egg yolks and exposed to (60)Co radiation at doses of 0.49 to 5.0 kGy. The eggs were maintained at 25°C and analyzed for the presence of Salmonella on days 1, 2, 4, and 7, and the recovered Salmonella isolates were characterized biochemically. All strains were resistant to doses of 0.49, 0.54, 0.59, 0.8, and 1 kGy; colony counts were ≥10(5) CFU/ml of egg yolk except for one strain, which was detected at 96 h and at 7 days after irradiation at 1 kGy, with a population reduction of 2 log CFU/ml. For the other evaluated doses, 12 strains (60.0%) were resistant at 1.5 kGy and 7 strains (35.0%) were resistant at 3.0 kGy. Among all analyzed strains, 5.0 kGy was more effective for reducing and/or eliminating the inoculated bacteria; only two (10%) strains were resistant to this level of irradiation. Salmonella colony counts were significantly reduced (P < 0.01) with increasing doses from the day 1 to 7 of observation, when microbial growth peaked. Loss of mobility, lactose fermentation, citrate utilization, and hydrogen sulfide production occurred in some strains after irradiation independent of dose and postirradiation storage time. Increases in antibiotic susceptibility also occurred: seven strains became sensitive to β-lactams, two strains became sensitive to antifolates, and one strain each became sensitive to fluoroquinolone, phenicol, nitrofurans, tetracyclines, and aminoglycosides. The results indicate that up to 5.0 kGy of radiation applied to shell eggs inoculated with Salmonella Enteritidis at 4 log CFU per egg is not sufficient for complete elimination of this pathogen from this food matrix.

Experimental evolution of ultraviolet radiation resistance in Escherichia coli

Ultraviolet (UV) light is a major cause of stress, mutation, and mortality in microorganisms, causing numerous forms of cellular damage. Nevertheless, there is tremendous variation within and among bacterial species in their sensitivity to UV light. We investigated direct and correlated responses to selection during exposure to UV. Replicate lines of Escherichia coli K12 were propagated for 600 generations, half with UV and half as a control without UV. All lines responded to selection, and we found strong positive and negative correlated responses to selection associated with increased UV resistance. Compared to Control populations, UV-selected populations increased in desiccation and starvation resistance approximately twofold but were 10 times more sensitive to hypersalinity. There was little evidence for a persistent large competitive fitness cost to UV resistance. These results suggest that natural variation in UV resistance may be maintained by trade-offs for resistance to other abiotic sources of mortality. We observed an average twofold increase in cell size by the UV-selected populations, consistent with a structural mode of adaptation to UV exposure having preadaptive and maladaptive consequences to other abiotic stresses.

Radioresistance development of DNA repair deficient Escherichia coli DH5α in ground beef subjected to electron beam at sub-lethal doses

PURPOSE

Electron beam (e-beam) efficiently and non-thermally inactivates microorganisms in food by lethal DNA changes (direct effects) and free radicals from water radiolysis (in-direct effects). Non-pathogenic Escherichia coli DH5α (α substrain of DH5 described by Hanahan 1985 , 'DH' stands for Douglas Hanahan) is a microorganism that lacks DNA repair capability, resulting in high radiosensitivity. Studying microbial inactivation of E. coli DH5α repeatedly subjected to sub-lethal e-beam in ground beef may enhance understanding of microbial radioresistance. The objective of this study was to determine if repetitive processing with e-beam at sub-lethal doses increases D-value (e-beam dose required to inactivate one log of microbial population) of E. coli DH5α in ground beef.

MATERIALS AND METHODS

Survivors from the highest e-beam dose were isolated and incubated in ground beef for the next cycle of e-beam processing. Five cycles were conducted. To acclimatise E. coli DH5α, first two cycles used low doses. D-values were determined following the third cycle.

RESULTS

D-values increased (p < 0.05) significantly with each cycle. Thus, E. coli DH5α has a capability to develop greater radioresistance under these experimental conditions. Following the third cycle D-values were 0.32 ± 0.006 and 0.32 ± 0.002 kGy for survivors enumerated on non-selective and selective media, respectively; the fourth cycle 0.39 ± 0.007 and 0.40 ± 0.019 kGy; and the fifth cycle 0.46 ± 0.006 and 0.46 ± 0.020 kGy. D-values on non-selective and selective media were similar (p > 0.05) indicating absence of cell recovery in E. coli DH5α.

CONCLUSIONS

E. coli DH5α increases radioresistance to e-beam as a result of repetitive exposure to sub-lethal doses despite its DNA repair deficiency.

Oxidative stress resistance in Deinococcus radiodurans

Deinococcus radiodurans is a robust bacterium best known for its capacity to repair massive DNA damage efficiently and accurately. It is extremely resistant to many DNA-damaging agents, including ionizing radiation and UV radiation (100 to 295 nm), desiccation, and mitomycin C, which induce oxidative damage not only to DNA but also to all cellular macromolecules via the production of reactive oxygen species. The extreme resilience of D. radiodurans to oxidative stress is imparted synergistically by an efficient protection of proteins against oxidative stress and an efficient DNA repair mechanism, enhanced by functional redundancies in both systems. D. radiodurans assets for the prevention of and recovery from oxidative stress are extensively reviewed here. Radiation- and desiccation-resistant bacteria such as D. radiodurans have substantially lower protein oxidation levels than do sensitive bacteria but have similar yields of DNA double-strand breaks. These findings challenge the concept of DNA as the primary target of radiation toxicity while advancing protein damage, and the protection of proteins against oxidative damage, as a new paradigm of radiation toxicity and survival. The protection of DNA repair and other proteins against oxidative damage is imparted by enzymatic and nonenzymatic antioxidant defense systems dominated by divalent manganese complexes. Given that oxidative stress caused by the accumulation of reactive oxygen species is associated with aging and cancer, a comprehensive outlook on D. radiodurans strategies of combating oxidative stress may open new avenues for antiaging and anticancer treatments. The study of the antioxidation protection in D. radiodurans is therefore of considerable potential interest for medicine and public health.

Ionising radiation triggers fat accumulation in white adipose tissue

PURPOSE

To investigate changes in gonadal white adipose tissue and lipogenesis-related gene expression induced by radiation exposure.

MATERIALS AND METHODS

Groups of two-month-old C57BL/6 mice were exposed whole-body to ¹³⁷Cs γ-rays at a single dose (5 gray [Gy]) or fractionated doses (1 Gy x 5 times, 0.5 Gy x 10 times, or 0.2 Gy x 25 times). Six months after irradiation, gonadal white adipose tissue was isolated from mice. Two and 25-month-old mice were used as young and old study references. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was used to measure messenger RNA (mRNA) expression of genes related to: (i) Primary lipid metabolism (ATP-citrate lyase [ACL], malic enzyme1 [ME1] and glucose-6-phosphate dehydrogenase 2 [G6PD2]), (ii) glucose uptake (glucose transporter 4 [GLUT4]), (iii) fatty acid synthesis (sterol regulatory element binding transcription factor 1 [SREBP-1c], fatty acid synthetase [FAS] and acetyl-coenzyme A carboxylase beta [ACC]), (iv) triglyceride synthesis (diacylglycerol O-acyltransferase 1 [DGAT1] and diacylglycerol O-acyltransferase 2 [DGAT2]), and (v) adipose-derived hormones (leptin [LEP]).

RESULTS

The weight of gonadal white adipose tissue in the irradiated groups tended to increase compared to the non-irradiated group though the radiation-induced increase in white adipose tissue was only significant for the 5 x 1 Gy group. The mRNA levels of SREBP-1c, ACC, FAS, ACL, GLUT4, ME1 and G6PD2 were relatively lower in γ-irradiated groups than in non-irradiated groups. The mRNA levels of leptin and DGAT were relatively higher than non-irradiated groups. The changes in expression of these lipogenesis-related genes caused by γ-irradiation showed a very similar pattern to changes caused by ageing.

CONCLUSIONS

A physical agent such as γ-rays can trigger biological responses resulting in fat accumulation of gonadal white adipose tissue in mice.

Small-molecule antioxidant proteome-shields in Deinococcus radiodurans

For Deinococcus radiodurans and other bacteria which are extremely resistant to ionizing radiation, ultraviolet radiation, and desiccation, a mechanistic link exists between resistance, manganese accumulation, and protein protection. We show that ultrafiltered, protein-free preparations of D. radiodurans cell extracts prevent protein oxidation at massive doses of ionizing radiation. In contrast, ultrafiltrates from ionizing radiation-sensitive bacteria were not protective. The D. radiodurans ultrafiltrate was enriched in Mn, phosphate, nucleosides and bases, and peptides. When reconstituted in vitro at concentrations approximating those in the D. radiodurans cytosol, peptides interacted synergistically with Mn²⁺ and orthophosphate, and preserved the activity of large, multimeric enzymes exposed to 50,000 Gy, conditions which obliterated DNA. When applied ex vivo, the D. radiodurans ultrafiltrate protected Escherichia coli cells and human Jurkat T cells from extreme cellular insults caused by ionizing radiation. By establishing that Mn²⁺-metabolite complexes of D. radiodurans specifically protect proteins against indirect damage caused by gamma-rays delivered in vast doses, our findings provide the basis for a new approach to radioprotection and insight into how surplus Mn budgets in cells combat reactive oxygen species.

Directed evolution of ionizing radiation resistance in Escherichia coli

We have generated extreme ionizing radiation resistance in a relatively sensitive bacterial species, Escherichia coli, by directed evolution. Four populations of Escherichia coli K-12 were derived independently from strain MG1655, with each specifically adapted to survive exposure to high doses of ionizing radiation. D(37) values for strains isolated from two of the populations approached that exhibited by Deinococcus radiodurans. Complete genomic sequencing was carried out on nine purified strains derived from these populations. Clear mutational patterns were observed that both pointed to key underlying mechanisms and guided further characterization of the strains. In these evolved populations, passive genomic protection is not in evidence. Instead, enhanced recombinational DNA repair makes a prominent but probably not exclusive contribution to genome reconstitution. Multiple genes, multiple alleles of some genes, multiple mechanisms, and multiple evolutionary pathways all play a role in the evolutionary acquisition of extreme radiation resistance. Several mutations in the recA gene and a deletion of the e14 prophage both demonstrably contribute to and partially explain the new phenotype. Mutations in additional components of the bacterial recombinational repair system and the replication restart primosome are also prominent, as are mutations in genes involved in cell division, protein turnover, and glutamate transport. At least some evolutionary pathways to extreme radiation resistance are constrained by the temporally ordered appearance of specific alleles.

Extremely Radiation-Resistant Mutants of a Halophilic Archaeon with Increased Single-Stranded DNA-Binding Protein (RPA) Gene Expression

Extremely halophilic archaea are highly resistant to multiple stressors, including radiation, desiccation and salinity. To study the basis of stress resistance and determine the maximum tolerance to ionizing radiation, we exposed cultures of the model halophile Halobacterium sp. NRC-1 to four cycles of irradiation with high doses of 18-20 MeV electrons. Two independently obtained mutants displayed an LD(50) > 11 kGy, which is higher than the LD(50) of the extremely radiation-resistant bacterium Deinococcus radiodurans. Whole-genome transcriptome analysis comparing the mutants to the parental wild-type strain revealed up-regulation of an operon containing two single-stranded DNA-binding protein (RPA) genes, VNG2160 (rfa3) and VNG2162, and a third gene of unknown function, VNG2163. The putative transcription start site for the rfa3 operon was mapped approximately 40 bp upstream of the ATG start codon, and a classical TATA-box motif was found centered about 25 bp further upstream. We propose that RPA facilitates DNA repair machinery and/or protects repair intermediates to maximize the ionizing radiation resistance of this archaeon.

Was Earth ever infected by martian biota? Clues from radioresistant bacteria

Here we propose that the radioresistance (tolerance to ionizing radiation) observed in several terrestrial bacteria has a martian origin. Multiple inconsistencies with the current view of radioresistance as an accidental side effect of tolerance to desiccation are discussed. Experiments carried out 25 years ago were reproduced to demonstrate that "ordinary" bacteria can develop high radioresistance ability after multiple cycles of exposure to high radiation dosages followed by cycles of recovery of the bacterial population. We argue that "natural" cycles of this kind could have taken place only on the martian surface, and we hypothesize that Mars microorganisms could have developed radioresistance in just several million years' time and, subsequently, have undergone transfer to Earth by way of martian meteorites. Our mechanism implies multiple and frequent exchanges of biota between Mars and Earth.

Technical report: irradiation of food. Committee on Environmental Health

Recent well-publicized outbreaks of foodborne illness have heightened general interest in food safety. Food irradiation is a technology that has been approved for use in selected foods in the United States since 1963. Widespread use of irradiation remains controversial, however, because of public concern regarding the safety of the technology and the wholesomeness of irradiated foods. In this report, we describe the technology, review safety and wholesomeness issues, and give a historical perspective of the public controversy regarding food irradiation.

Escherichia coli mutants resistant to inactivation by high hydrostatic pressure

Alternating cycles of exposure to high pressure and outgrowth of surviving populations were used to select for highly pressure-resistant mutants of Escherichia coli MG1655. Three barotolerant mutants (LMM1010, LMM1020, and LMM1030) were isolated independently by using outgrowth temperatures of 30, 37, and 42 degrees C, respectively. Survival of these mutants after pressure treatment for 15 min at ambient temperature was 40 to 85% at 220 MPa and 0.5 to 1.5% at 800 MPa, while survival of the parent strain, MG1655, decreased from 15% at 220 MPa to 2 x 10(-8)% at 700 MPa. Heat resistance of mutants LMM1020 and LMM1030 was also altered, as evident by higher D values at 58 and 60 degrees C and reduced z values compared to those for the parent strain. D and z values for mutant LMM1010 were not significantly different from those for the parent strain. Pressure sensitivity of the mutants increased from 10 to 50 degrees C, as opposed to the parent strain, which showed a minimum around 40 degrees C. The ability of the mutants to grow at moderately elevated pressure (50 MPa) was reduced at temperatures above 37 degrees C, indicating that resistance to pressure inactivation is unrelated to barotolerant growth. The development of high levels of barotolerance as demonstrated in this work should cause concern about the safety of high-pressure food processing.

Inactivation of Escherichia coli O157:H7, salmonellae, and Campylobacter jejuni in raw ground beef by gamma irradiation

Raw ground beef patties inoculated with stationary-phase cells of Escherichia coli O157:H7, salmonellae, or Campylobacter jejuni were subjected to gamma irradiation (60Co) treatment, with doses ranging from 0 to 2.52 kGy. The influence of two levels of fat (8 to 14% [low fat] and 27 to 28% [high fat]) and temperature (frozen [-17 to -15 degrees C] and refrigerated [3 to 5 degrees C]) on the inactivation of each pathogen by irradiation was investigated. In ascending order of irradiation resistance, the D10 values ranged from 0.175 to 0.235 kGy (C. jejuni), from 0.241 to 0.307 kGy (E. coli O157:H7), and from 0.618 to 0.800 kGy (salmonellae). Statistical analysis revealed that E. coli O157:H7 had a significantly (P < 0.05) higher D10 value when irradiated at -17 to -15 degrees C than when irradiated at 3 to 5 degrees C. Regardless of the temperature during irradiation, the level of fat did not have a significant effect on the D10 value. Salmonellae behaved like E. coli O157:H7 in low-fat beef, but temperature did not have a significant effect when the pathogen was irradiated in high-fat ground beef. Significantly higher D10 values were calculated for C. jejuni irradiated in frozen than in refrigerated low-fat beef. C. jejuni was more resistant to irradiation in low-fat beef than in high-fat beef when treatment was at -17 to -15 degrees C. Regardless of the fat level and temperature during inactivation, these pathogens were highly sensitive to gamma irradiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Microbiological safety of irradiated foods

This paper attempts to summarize relevant information on microbiological safety of irradiated foods in the light of previous reports of expert committees and current literature references. After a brief survey of the relative radiation resistance of food-borne microorganisms, the importance of microbial load for dose requirement, and the role of post-irradiation conditions, it addresses the following questions: Could selective changes in the microflora, caused by non-sterilizing radiation doses, make known pathogens more likely to occur, or bring into prominence unfamiliar pathogens? Is it probable that 'mutational' (including adaptive) changes might make pathogens more virulent, more harmful, or more difficult to recognize, and could new pathogens arise in this way? Is it possible that development of radiation-resistant strains might render the antimicrobial irradiation processes ineffective? The present survey of relevant scientific evidence related to these questions reaffirms the basic conclusion of earlier reviews, that microbiological safety of irradiated food is fully comparable with that of foods preserved by other acceptable preservation methods. Similar to other preservation processes, gains in microbiological or keeping quality attained by food irradiation can be and must be safeguarded by proper control in the food irradiation facilities and by proper care of the product before and after processing.

Genetic characterization of hyperresistance to formaldehyde and 4-nitroquinoline-N-oxide in the yeast Saccharomyces cerevisiae

The hyperresistance to 4-nitroquinoline-N-oxide (4-NQO) and formaldehyde (FA) of yeast strains transformed with the multi-copy plasmids pAR172 and pAR184, respectively, is due to the two genes, SNQ and SFA, which are present on these plasmids. Restriction analysis revealed the maximal size of SFA as 2.7 kb and of SNQ as 2.2 kb, including transcription control elements. The presence of the smallest 2.7 kb subclone carrying SFA increased hyperresistance to formaldehyde fivefold over that of the original pAR184 isolate. No such increase in hyperresistance to 4-NQO was seen with the smaller subclones of the pAR172 isolate. Disruption of the SFA gene led to a threefold increase in sensitivity to FA as compared with the wild type. Expression of gene SNQ introduced on a multi-copy vector into haploid yeast mutants rad2, rad3, and snm1 did not complement these mutations that block excision repair.

Genetic mapping of mutations in a highly radiation-resistant mutant of Salmonella typhimurium LT2

The genes involved in the high radiation resistance of mutant R68 of Salmonella typhimurium LT2 were mapped by conjugation. It was observed that the high radiation resistance involved genes localized in two regions of the chromosome, which have been designated as garA and garB for high gamma resistance. The garA gene mapped near gal and uvrB at about 18 map units, and the garB gene mapped near purC at about 49 map units. The resistance of R68 was reduced to the wild-type level by the acquisition of the two wild-type alleles, garA+ and garB+. Recombinants carrying the garA or garB gene repaired single-strand breaks in their DNA faster than did the wild-type strain. However, only those with the garA mutation showed a marked increase in UV irradiation resistance above the wild-type level, whereas those with garB mutation exhibited an increased rate of spontaneous degradation of DNA beyond the level observed in recA cells.

Isolation of radiation-resistant bacteria without exposure to irradiation

Resistance to desiccation was utilized in the selection of highly radiation-resistant asporogenous bacteria from non-irradiated sources. A bacterial suspension in phosphate buffer was dried in a thin film at 25 degrees C and 33% relative humidity. Storage under these conditions for 15 days or more reduced the number of radiation-sensitive bacteria. Further selection for radiation-resistant bacteria was obtained by irradiation of bacteria on velveteen in the replication process, thereby avoiding the toxic effect of irradiated media. The similarity of radiation resistance and identifying characteristics in irradiated and non-irradiated isolates should allay some concerns that highly radiation-resistant bacteria have been permanently altered by radiation selection.

Ionizing radiation damage to the folded chromosome of Escherichia coli K-12: sedimentation properties of irradiated nucleoids and chromosomal deoxyribonucleic acid

The structures of the membrane-free nucleoid of Escherichia coli K-12 and of unfolded chromosomal deoxyribonucleic acid (DNA) were investigated by low-speed sedimentation on neutral sucrose gradients after irradiation with 60Co gamma rays. Irradiation both in vivo and in vitro was used as a molecular probe of the constraints on DNA packaging in the bacterial chromosome. The number of domains of supercoiling was estimated to be approximately 180 per genome equivalent of DNA, based on measurements of relaxation caused by single-strand break formation in folded chromosomes gamma irradiated in vivo and in vitro. Similar estimates based on the target size of ribonucleic acid molecules responsible for maintaining the compact packaging of the nucleoid predicted negligible unfolding due to the formation of ribonucleic acid single-strand breaks at doses of up to 10 krad; this was born out by experimental measurements. Unfolding of the nucleoid in vitro by limit digestion with ribonuclease or by heating at 70 degrees C resulted in DNA complexes with sedimentation coefficients of 1,030 +/- 59S and 625 +/- 15S, respectively. The difference in these rates was apparently due to more complete deproteinization and thus less mass in the heated material. These structures are believed to represent intact, replicating genomes in the form of complex-theta structures containing two to three genome equivalents of DNA. The rate of formation of double-strand breaks was determined from molecular weight measurements of thermally unfolded chromosomal DNA gamma irradiated in vitro. Break formation was linear with doses up to 10 krad and occurred at a rate of 0.27 double-strand break per krad per genome equivalent of DNA (1,080 eV/double-strand break). The influence of possible nonlinear DNA conformations on these values is discussed.

Ionizing radiation damage to the folded chromosome of Escherichia coli K-12: repair of double-strand breaks in deoxyribonucleic acid

The extremely gentle lysis and unfolding procedures that have been developed for the isolation of nucleoid deoxyribonucleic acid (DNA; K. M. Ulmer et al., J. Bacteriol. 138:475-485, 1979) yield undamaged, replicating genomes, thus permitting direct measurement of the formation and repair of DNA double-strand breaks at biologically significant doses of ionizing radiation. Repair of ionizing radiation damage to folded chromosomes of Escherichia coli K-12 strain AB2497 was observed within 2 to 3 h of post-irradiation incubation in growth medium. Such behavior was not observed after post-irradiation incubation in growth medium of a recA13 strain (strain AB2487). A model based on recombinational repair is proposed to explain the formation of 2,200 to 2,300S material during early stages of incubation and to explain subsequent changes in the gradient profiles. Association of unrepaired DNA with the plasma membrane is proposed to explain the formation of a peak of rapidly sedimenting material (greater than 3,100S) during the later stages of repair. Direct evidence of repair of double-strand breaks during post-irradiation incubation in growth medium was obtained from gradient profiles of DNA from ribonuclease-digested chromosomes. The sedimentation coefficient of broken molecules was restored to the value of unirradiated DNA after 2 to 3 h of incubation, and the fraction of the DNA repaired in this fashion was equal to the fraction of cells that survived at the same dose. An average of 2.7 double-strand breaks per genome per lethal event was observed, suggesting that one to two double-strand breaks per genome are repairable in E. coli K-12 strain AB2497.

Increased radiation resistance of vegetative Bacillus pumilus

A 4.5-fold increase in vegetative cell radiation resistance of Bacillus pumilus E601, the internationally recognized biological standard for irradiation sterilization, was obtained by the repeated passage of resistant survivors through successive sublethal doses of (60)Co irradiation. This increase in resistance was accompanied by a corresponding increase in spore resistance through the seventh irradiation passage. By the fifteenth passage, the ability for spore formation was lost. Other effects noted by the successive irradiation dosages included loss of motility and pellicle formation, and changes in the Gram reaction, cell morphology, and colonial morphology. Increased resistance was also accompanied by an increased nutritional requirement for specific amino acids. Radiation resistance was not transferred from vegetative cells to spores.

Deoxyribonucleic acid repair in a highly radiation-resistant strain of Salmonella typhimurium

Deoxyribonucleic acid repair was studied in gamma-irradiated wild-type Salmonella typhimurium and in a radiation-resistant derivative 20 times more resistant than wild type. After exposure to 20 or 50 krad, the wild-type strain (DB21) degraded 30 to 50% of its prelabeled DNA into acid-soluble fragments, whereas the radioresistant strain degraded less than 15% after 4 h of incubation. Post-irradiation synthesis of DNA in the wild-type strain DB21 was reduced after a dose of 20 krad and totally inhibited after exposure to 200 krad. With radiation-resistant strain, D21R6008, on the other hand, DNA synthesis was delayed after a dose of 200 krad but not inhibited. Doses of 20 and 200 krad produced a similar number of single-strand breaks in the DNA of both strains as determined by zone sedimentation analysis in alkaline sucrose gradients. The radiation-resistant strain D21R6008, on the other hand, DNA synthesis was strand breaks in its DNA and repairs these damages more rapidly than wild-type Salmonella.