Gene unstabilization induced by heat and by nitrous acid

Mild Heat Treatment and Biopreservatives for Artisanal Raw Milk Cheeses: Reducing Microbial Spoilage and Extending Shelf-Life through Thermisation, Plant Extracts and Lactic Acid Bacteria

The microbial quality of raw milk artisanal cheeses is not always guaranteed due to the possible presence of pathogens in raw milk that can survive during manufacture and maturation. In this work, an overview of the existing information concerning lactic acid bacteria and plant extracts as antimicrobial agents is provided, as well as thermisation as a strategy to avoid pasteurisation and its negative impact on the sensory characteristics of artisanal cheeses. The mechanisms of antimicrobial action, advantages, limitations and, when applicable, relevant commercial applications are discussed. Plant extracts and lactic acid bacteria appear to be effective approaches to reduce microbial contamination in artisanal raw milk cheeses as a result of their constituents (for example, phenolic compounds in plant extracts), production of antimicrobial substances (such as organic acids and bacteriocins, in the case of lactic acid bacteria), or other mechanisms and their combinations. Thermisation was also confirmed as an effective heat inactivation strategy, causing the impairment of cellular structures and functions. This review also provides insight into the potential constraints of each of the approaches, hence pointing towards the direction of future research.

Recent Advancements on Photothermal Conversion and Antibacterial Applications over MXenes-Based Materials

HIGHLIGHTS

Fabrication, characterizations and photothermal properties of MXenes are systematically described. Photothermal-derived antibacterial performances and mechanisms of MXenes-based materials are summarized and reviewed. Recent advances in the derivative applications relying on antibacterial properties of MXenes-based materials, including in vitro and in vivo sterilization, solar water evaporation and purification, and flexible antibacterial fabrics, are investigated.

ABSTRACT

The pernicious bacterial proliferation and emergence of super-resistant bacteria have already posed a great threat to public health, which drives researchers to develop antibiotic-free strategies to eradicate these fierce microbes. Although enormous achievements have already been achieved, it remains an arduous challenge to realize efficient sterilization to cut off the drug resistance generation. Recently, photothermal therapy (PTT) has emerged as a promising solution to efficiently damage the integrity of pathogenic bacteria based on hyperthermia beyond their tolerance. Until now, numerous photothermal agents have been studied for antimicrobial PTT. Among them, MXenes (a type of two-dimensional transition metal carbides or nitrides) are extensively investigated as one of the most promising candidates due to their high aspect ratio, atomic-thin thickness, excellent photothermal performance, low cytotoxicity, and ultrahigh dispersibility in aqueous systems. Besides, the enormous application scenarios using their antibacterial properties can be tailored via elaborated designs of MXenes-based materials. In this review, the synthetic approaches and textural properties of MXenes have been systematically presented first, and then the photothermal properties and sterilization mechanisms using MXenes-based materials are documented. Subsequently, recent progress in diverse fields making use of the photothermal and antibacterial performances of MXenes-based materials are well summarized to reveal the potential applications of these materials for various purposes, including in vitro and in vivo sterilization, solar water evaporation and purification, and flexible antibacterial fabrics. Last but not least, the current challenges and future perspectives are discussed to provide theoretical guidance for the fabrication of efficient antimicrobial systems using MXenes. [Image: see text]

Physiology of the Inactivation of Vegetative Bacteria by Thermal Treatments: Mode of Action, Influence of Environmental Factors and Inactivation Kinetics

Heat has been used extensively in the food industry as a preservation method, especially due to its ability to inactivate microorganisms present in foods. However, many aspects regarding the mechanisms of bacterial inactivation by heat and the factors affecting this process are still not fully understood. The purpose of this review is to offer a general overview of the most important aspects of the physiology of the inactivation or survival of microorganisms, particularly vegetative bacteria, submitted to heat treatments. This could help improve the design of current heat processes methods in order to apply milder and/or more effective treatments that could fulfill consumer requirements for fresh-like foods while maintaining the advantages of traditional heat treatments.

The nature of mutations induced by replication–transcription collisions

The DNA replication and transcription machineries share a common DNA template and thus can collide with each other co-directionally or head-on^1,2. Replication-transcription collisions can cause replication fork arrest, premature transcription termination, DNA breaks, and recombination intermediates threatening genome integrity^1–10. Collisions may also trigger mutations, which are major contributors of genetic disease and evolution^5,7,11. However, the nature and mechanisms of collision-induced mutagenesis remain poorly understood. Here we reveal the genetic consequence of replication-transcription collisions in actively dividing bacteria to be two classes of mutations: duplications/deletions and base substitutions in promoters. Both signatures are highly deleterious but are distinct from the well-characterized base substitutions in coding sequence. Duplications/deletions are likely caused by replication stalling events that are triggered by collisions; their distribution patterns are consistent with where the fork first encounters a transcription complex upon entering a transcription unit. Promoter substitutions result mostly from head-on collisions and frequently occur at a nucleotide conserved in promoters recognized by the major sigma factor in bacteria. This substitution is generated via adenine deamination on the template strand in the promoter open complex, as a consequence of head-on replication perturbing transcription initiation. We conclude that replication-transcription collisions induce distinct mutation signatures by antagonizing replication and transcription, not only in coding sequences but also in gene regulatory elements.

Genetic Factors in Radiation Resistance of Bacillus subtilis

Zamenhof, Stephen (University of California, Los Angeles), Hela Bursztyn, T. K. Ramachandra Reddy, and Patrice J. Zamenhof. Genetic factors in radiation resistance of Bacillus subtilis. J. Bacteriol. 90:108-115. 1965.-A study of several wild cross-transformable strains of Bacillus subtilis revealed differences in the resistance of their spores to X rays. Closer study of two such strains revealed differences of the same type when vegetative cells were exposed to X rays or to ultraviolet light (UV). Cell cultures repeatedly exposed to sublethal doses of UV (with cultivation between exposures) became more resistant to UV, presumably by enrichment in a more UV-resistant mutant. A sulfanilamide-resistant mutant of one strain (vegetative cells and spores) was less resistant to ionizing radiation; this sensitivity was transferable by transformation. No difference in radiation-induced mutability could be demonstrated in any of the strains studied. It is concluded that, at least in the cases studied, (i) the differences in radiation resistance of spores of different strains are not just a result of a superimposition of a common spore resistance mechanism(s) but rather are an amplification of genetically determined resistance differences in vegetative cells of these strains; (ii) sulfanilamide-resistance locus (p-aminobenzoic acid overproduction locus) is one of the loci of radiation sensitivity; (iii) no evidence was obtained that the differences in radiation resistance of cells or spores can be ascribed to differences in radiation resistance of their deoxyribonucleic acid.

EVOLUTION OF A CATABOLIC PATHWAY IN BACTERIA

Genetic derepression of ribitol dehydrogenase in a mutant of Aerobacter aerogenes, strain 1033, enabled it to grow on xylitol, a substrate of the enzyme, but not its inducer. A derivative strain with an improved rate of growth on xylitol was obtained from the first mutant. The faster growth rate was made possible by the production of an altered ribitol dehydrogenase, as demonstrated by an increase in its activity on xylitol relative to ribitol and by its decreased heat stability.

Evaluation and re-evaluation of genetic radiation hazards in man. I. Interspecific comparison of estimates of mutation rates

A detailed presentation is made of the experimental data from the various systems used by Abrahamson et al. [2] to conclude that the per locus per rad (low LET) radiation-induced forward mutation rates in organisms, whose DNA content varies by a factor of about 1000, is proportional to genome size. Additional information pertinent in this context is also reviewed. It is emphasized that the mutation rates cited by Abrahamson et al. [2], although considered as pertaining to mutations at specific loci, actually derive from a broad variety of genetic end-points. It is argued that an initial (if not sufficient) condition for sound inter-specific mutation rate comparisions, covering a wide range of organisms and detecting systems of various sensitivities, requires a reasonalbly consistent biological definition of a specific locus mutation, namely, a transmissible intra-locus change. Granting the differences between systems in their resolving power to detect intragenic change, the data cited in this paper do not support the existence of a simple proportionality between radiotion-induced intra-locus mutation rate and genome size for the different species reviewed here. Furthermore, in Drosophila melanogaster, where individual salivary gland chromosome bands (that can differ greatly in DNA content) are usually associated with individual loci or at least distinct complementation groups, radiation-induced intra-locus mutation rates are not correlated with apparent differences in the DNA content of bands. This result is incompatible with the notion that most of the DNA in a band represents a radiation-mutable target capable of eliciting the kind of mutation observed in mutation rate experiments. All these considerations argue against the validity of the hypothesis of Abrahamson et al. [2] and their generalization that, for the evaluation of genetic radiation hazards in man, we can now "extrapolate from mutation rates obtained in lower organisms to man with greater confidence" on the basis of DNA content (italics are ours).

Lactose permeation via the arabinose transport system in Escherichia coli K-12

This paper describes the isolation and characterization of a mutant of Escherichia coli that transports lactose and its analog thiomethylgalactoside via the arabinose permeation system. Unlike transport via the lactose permease, this transport is not inhibited by thiodigalactoside, but was inhibited by arabinose, xylose, and fucose. The site of the mutation was in the arabinose C gene and confers constitutivity on the entire arabinose operon. Furthermore, this transport was found in 24 independently isolated arabinose-constitutive strains, and in strains which had been induced with arabinose and then starved to remove all traces of it. It was therefore concluded that lactose and thiomethylgalactoside are low-affinity substrates of at least one component of the normal arabinose permeation system.

Evolution of a second gene for beta-galactosidase in Escherichia coli

Mutants of E. coli K12 with deletions of the beta-galactosidase gene (lacZ) can reacquire the ability to hydrolyze beta-galactosides during prolonged intense selection for growth on lactose. Full lactose competence is restored through a sequence of at least five mutations. Cell extracts of these derived strains hydrolyze o-nitrophenyl-beta-D-galactoside, the standard substrate for assay of beta-galactosidase. The enzyme responsible for this activity differs in its immunological, kinetic, and sedimentation characteristics from the lacZ beta-galactosidase of wild-type E. coli. Its genetic determinant, designated ebg-5, maps at 59 min on the E. coli chromosome, whereas the lac operon maps at 10 min. We suggest that a gene not involved in lactose utilization has been progressively changed into a form capable of specifying a beta-galactosidase and that this process is similar to that whereby genes with new functions are evolved by natural selection.

Transduction mechanisms of bacteriophage epsilon 15. I. General properties of the system

Bacteriophageεγis capable of transduction both by replacement of a genetic segment of the recipient by the homologous genetic material from the donor strain and by the formation of defective transducing particles capable of lysogenizing the recipient strain ofS. anatum.

The isolation of strains carrying such prophages, which have incorporated the lactose or arabinose operons, is reported. Lysogenic strains, carrying both normal and defective transducing prophage, form high-frequency transducing lysates. Other strains, carrying only defective prophage, show evidence that the association of prophage genes and transduced materials is stable since the loss of one frequently entails loss of the other.

Variants of Serratia marcescens induced by freeze-drying

Unusually high numbers of pigmentless variants and sectored colonies in cultures of lyophylized Serratia marcescens are reported. Clonal analyses of sectored colonies show the presence of unstable bacteria that continue to sector again when plated. Further analysis of pigmentless variants suggest that pigment formation, oxygen uptake, and the production of an inducible protease are affected.

Studies on induction of mutations by heat in spores of Bacillus subtilis

Previous studies have demonstrated that heating dry spores induced the formation of a high proportion of mutants. In this paper, a study was made of the correlation between the temperature of the heat treatment and the proportion of auxotrophic mutants induced. This proportion was found to increase between 105° and 115°, and then to decline (115°–155°). The proportions of auxotrophs for individual amino acids reached maxima at various temperatures.

Cross-resistance relationships in Escherichia coli between ultraviolet radiation and nitrous acid

Zampieri, Antonio (Palo Alto Medical Research Foundation, Palo Alto, Calif.), and Joseph Greenberg. Cross-resistance relationships in Escherichia coli between ultraviolet radiation and nitrous acid. J. Bacteriol. 87:1094-1099. 1964.-A number of radiosensitive and radioresistant strains of Escherichia coli were tested for sensitivity to injury by nitrous acid. All the radioresistant strains, including 13 radioresistant mutants of strain S, B/r, Bpr5, and K-12, were found to be significantly more resistant to nitrous acid than were the radiosensitive strains S and B. The radioresistant mutants of strain S, Bpr5, and K-12 displayed similar responses to nitrous acid and were less resistant than was strain B/r. Strains B and S were indistinguishable on the basis of nitrous acid sensitivity. The survival curves of all strains examined were similar in shape to corresponding survival curves after ultraviolet radiation. The sensitivity to nitrous acid of the radiosensitive strains S and B, but not that of the radioresistant strains, was found to be greater on Tryptone medium than on Penassay medium, and greater on Penassay medium than on glucose-salts medium. Between 2 and 3% of the strain S survivors of nitrous acid treatment were radioresistant; 46 such radioresistant mutants were isolated and found to be identical in cross-resistance pattern with radioresistant types (R(3), R(4), or R(6)) previously described. The proportions in which these radioresistant types were found to occur were similar to those observed after selection by other radiomimetic agents.