Combining spore germination and heat inactivation to decontaminate materials contaminated with Bacillus anthracis spores

Bacillus spore germination: mechanisms, identification, and antibacterial strategies

Bacterial spores are metabolically inactive and highly resistant to harsh environmental conditions in nature and during decontamination processes in food and related industries. However, inducing germination using specific germinants in dormant spores can convert them into vegetative cells which are metabolically active and fragile. The potential utility of a "germinate to eradicate" strategy, also known as germination-inactivation, has been validated in foods. Meanwhile, the strategy has sparked much interest in triggering and maximizing spore germination. Although many details of the spore germination process have been identified over the past decades, there remain many uncertainties, including some signal transduction mechanisms involved in germination. In addition, the successful implementation of the germination-inactivation strategy relies on the sensitive detection of germinative biomarkers within minutes of germination initiation and the optimal timing for the subsequent inactivation step. Meanwhile, the emergence of biomarkers has renewed attention to the practical application of the spore germination process. Here, this review presents the current knowledge of the germination mechanisms of Bacillus spore, influencing factors, and germination biomarkers. It also covers a detailed discussion on the development of germination-inactivation as a spore eradication strategy.

Dose-response analysis of Bacillus thuringiensis HD-1 cry- spore reduction on surfaces using formaldehyde with pre-germination

AIM

To establish a basis for rapid remediation of large areas contaminated with Bacillus anthracis spores.

METHODS AND RESULTS

Representative surfaces of wood, steel and cement were coated by nebulization with B. thuringiensis HD-1 cry- (a simulant for B. anthracis) at 5.9 ± 0.2, 6.3 ± 0.2 and 5.8 ± 0.2 log10 CFU per cm² , respectively. These were sprayed with formaldehyde, either with or without pre-germination. Low volume (equivalent to ≤2500 L ha^-1 ) applications of formaldehyde at 30 g l^-1 to steel or cement surfaces resulted in ≥4 or ≤2 log10 CFU per cm² reductions respectively, after 2 h exposure. Pre-germinating spores (500 mmol l^-1 l-alanine and 25 mmol l^-1 inosine, pH 7) followed by formaldehyde application showed higher levels of spore inactivation than formaldehyde alone with gains of up to 3.4 log10 CFU per cm² for a given dose. No loss in B. thuringiensis cry- viability was measured after the 2 h germination period, however, a pre-heat shock log10 reduction was seen for B. anthracis strains: LSU149 (1.7 log10), Vollum and LSU465 (both 0.9 log10), LSU442 (0.2 log10), Sterne (0.8 log10) and Ames (0.6 log10).

CONCLUSIONS

A methodology was developed to produce representative spore contamination of surfaces along with a laboratory-based technique to measure the efficacy of decontamination. Dose-response analysis was used to optimize decontamination. Pre-germinating spores was found to increase effectiveness of decontamination but requires careful consideration of total volume used (germinant and decontaminant) by surface type.

SIGNIFICANCE AND IMPACT OF THE STUDY

To be practically achievable, decontamination of a wide area contaminated with B. anthracis spores must be effective, timely and minimize the amount of materials required. This study uses systematic dose-response methodology to demonstrate that such an approach is feasible.

Visualizing germination of microbiota endospores in the mammalian gut

Transmission of bacterial endospores between the environment and people and the following germination in vivo play critical roles in both the deadly infections of some bacterial pathogens and the stabilization of the commensal microbiotas in humans. Our knowledge about the germination process of different bacteria in the mammalian gut, however, is still very limited due to the lack of suitable tools to visually monitor this process. We proposed a two-step labeling strategy that can image and quantify the endospores' germination in the recipient's intestines. Endospores collected from donor's gut microbiota were first labeled with fluorescein isothiocyanate and transplanted to mice via gavage. The recipient mice were then administered with Cyanine5-tagged D-amino acid to label all the viable bacteria, including the germinated endospores, in their intestines in situ. The germinated donor endospores could be distinguished by presenting two types of fluorescent signals simultaneously. The integrative use of cell-sorting, 16S rDNA sequencing, and fluorescence in situ hybridization (FISH) staining of the two-colored bacteria unveiled the taxonomic information of the donor endospores that germinated in the recipient's gut. Using this strategy, we investigated effects of different germinants and pre-treatment interventions on their germination, and found that germination of different commensal bacterial genera was distinctly affected by various types of germinants. This two-color labeling strategy shows its potential as a versatile tool for visually monitoring endospore germination in the hosts and screening for new interventions to improve endospore-based therapeutics.

What's new and notable in bacterial spore killing!

Spores of many species of the orders Bacillales and Clostridiales can be vectors for food spoilage, human diseases and intoxications, and biological warfare. Many agents are used for spore killing, including moist heat in an autoclave, dry heat at elevated temperatures, UV radiation at 254 and more recently 222 and 400 nm, ionizing radiation of various types, high hydrostatic pressures and a host of chemical decontaminants. An alternative strategy is to trigger spore germination, as germinated spores are much easier to kill than the highly resistant dormant spores—the so called “germinate to eradicate” strategy. Factors important to consider in choosing methods for spore killing include the: (1) cost; (2) killing efficacy and kinetics; (3) ability to decontaminate large areas in buildings or outside; and (4) compatibility of killing regimens with the: (i) presence of people; (ii) food quality; (iii) presence of significant amounts of organic matter; and (iv) minimal damage to equipment in the decontamination zone. This review will summarize research on spore killing and point out some common flaws which can make results from spore killing research questionable.

Dodecylamine rapidly kills of spores of multiple Firmicute species: properties of the killed spores and the mechanism of the killing

AIMS

Previous work showed that Bacillus subtilis dormant spore killing and germination by dodecylamine take place by different mechanisms. This new work aimed to optimize killing of B. subtilis and other Firmicutes spores and to determine the mechanism of the killing.

METHODS AND RESULTS

Spores of seven Firmicute species were killed rapidly by dodecylamine under optimal conditions and more slowly by decylamine or tetradecylamine. The killed spores were not recovered by additions to recovery media, and some of the killed spores subsequently germinated, all indicating that dodecylamine-killed spores truly are dead. Spores of two species treated with dodecylamine were more sensitive to killing by a subsequent heat treatment, and spore killing of at least one species was faster with chemically decoated spores. The cores of dodecylamine-killed spores were stained by the nucleic acid stain propidium iodide, and dodecylamine-killed wild-type and germination-deficient spores released their stores of phosphate-containing small molecules.

CONCLUSIONS

This work indicates that dodecylamine is likely a universal sporicide for Firmicute species, and it kills spores by damaging their inner membrane, with attendant loss of this membrane as a permeability barrier.

SIGNIFICANCE AND IMPACT OF THE STUDY

There is a significant need for agents that can effectively kill spores of a number of Firmicute species, especially in wide area decontamination. Dodecylamine appears to be a universal sporicide with a novel mechanism of action, and this or some comparable molecule could be useful in wide area spore decontamination.

Bacterial Spore mRNA - What's Up With That?

Bacteria belonging to the orders Bacillales and Clostridiales form spores in response to nutrient starvation. From a simplified morphological perspective, the spore can be considered as comprising a central protoplast or core, that is, enveloped sequentially by an inner membrane (IM), a peptidoglycan cortex, an outer membrane, and a proteinaceous coat. All of these structures are characterized by unique morphological and/or structural features, which collectively confer metabolic dormancy and properties of environmental resistance to the quiescent spore. These properties are maintained until the spore is stimulated to germinate, outgrow and form a new vegetative cell. Spore germination comprises a series of partially overlapping biochemical and biophysical events - efflux of ions from the core, rehydration and IM reorganization, disassembly of cortex and coat - all of which appear to take place in the absence of de novo ATP and protein synthesis. If the latter points are correct, why then do spores of all species examined to date contain a diverse range of mRNA molecules deposited within the spore core? Are some of these molecules "functional," serving as translationally active units that are required for efficient spore germination and outgrowth, or are they just remnants from sporulation whose sole purpose is to provide a reservoir of ribonucleotides for the newly outgrowing cell? What is the fate of these molecules during spore senescence, and indeed, are conditions within the spore core likely to provide any opportunity for changes in the transcriptional profile of the spore during dormancy? This review encompasses a historical perspective of spore ribonucleotide biology, from the earliest biochemical led analyses - some of which in hindsight have proved to be remarkably prescient - through the transcriptomic era at the turn of this century, to the latest next generation sequencing derived insights. We provide an overview of the key literature to facilitate reasoned responses to the aforementioned questions, and many others, prior to concluding by identifying the major outstanding issues in this crucial area of spore biology.

Bacillus spore germination: Knowns, unknowns and what we need to learn

How might a microbial cell that is entirely metabolically dormant - and which has the ability to remain so for extended periods of time - irreversibly commit itself to resuming vegetative growth within seconds of being exposed to certain amino acids or sugars? That this process takes place in the absence of any detectable ATP or de novo protein synthesis, and relies upon a pre-formed apparatus that is immobilised, respectively, in a semi-crystalline membrane or multi-layered proteinaceous coat, only exacerbates the challenge facing spores of Bacillales species when stimulated to germinate. Whereas the process by which spores are formed in response to nutrient starvation - sporulation - involves the orchestrated interplay between hundreds of distinct proteins, the process by which spores return to life - germination - is a much simpler affair, requiring a handful of receptor and channel proteins complemented with specialized peptidoglycan lysins. Despite this relative simplicity, and research effort spanning many decades, comprehensive understanding of key molecular and biochemical details and, in particular signal transduction mechanisms associated with spore germination, has remained elusive. In this review we provide an up to date overview of the field while identifying what we consider to be the key gaps in knowledge associated with germination of Bacillales spores, suggesting also technical approaches that may provide fresh insight to this unique biological process.