Die another day: Fate of heat-treated Geobacillus stearothermophilus ATCC 12980 spores during storage under growth-preventing conditions

New Thoughts on an Old Topic: Secrets of Bacterial Spore Resistance Slowly Being Revealed

The quest for bacterial survival is exemplified by spores formed by some Firmicutes members. They turn up everywhere one looks, and their ubiquity reflects adaptations to the stresses bacteria face. Spores are impactful in public health, food safety, and biowarfare. Heat resistance is the hallmark of spores and is countered principally by a mineralized gel-like protoplast, termed the spore core, with reduced water which minimizes macromolecular movement/denaturation/aggregation. Dry heat, however, introduces mutations into spore DNA. Spores have countermeasures to extreme conditions that are multifactorial, but the fact that spore DNA is in a crystalline-like nucleoid in the spore core, likely due to DNA saturation with small acid-soluble spore proteins (SASPs), suggests that reduced macromolecular motion is also critical in spore dry heat resistance. SASPs are also central in the radiation resistance characteristic of spores, where the contributions of four spore features-SASP; Ca2+, with pyridine-2,6-dicarboxylic acid (CaDPA); photoproduct lyase; and low water content-minimize DNA damage. Notably, the spore environment steers UV photochemistry toward a product that germinated spores can repair without significant mutagenesis. This resistance extends to chemicals and macromolecules that could damage spores. Macromolecules are excluded by the spore coat which impedes the passage of moieties of ≥10 kDa. Additionally, damaging chemicals may be degraded or neutralized by coat enzymes/proteins. However, the principal protective mechanism here is the inner membrane, a compressed structure lacking lipid fluidity and presenting a barrier to the diffusion of chemicals into the spore core; SASP saturation of DNA also protects against genotoxic chemicals. Spores are also resistant to other stresses, including high pressure and abrasion. Regardless, overarching mechanisms associated with resistance seem to revolve around reduced molecular motion, a fine balance between rigidity and flexibility, and perhaps efficient repair.

Viability of bacterial spores surviving heat-treatment is lost by further incubation at temperature and pH not suitable for growth

Spores from 21 strains from different genera were heat-treated and stored in different sets of process conditions (4 temperatures and 3 pH levels) defined to prevent growth. In these conditions, spores surviving the heat treatment progressively lost viability during storage. Different inactivation curve shapes (linear, shoulder and tailing) and different sensitivities to storage were observed. B. coagulans showed the fastest inactivation kinetics, with more than 4-log reduction of spore population within 24 h after heating and G. stearothermophilus displayed slower inactivation kinetics, whereas all the anaerobic strains studied (M. thermoacetica and Thermoanaerobacterium spp.) proved resistant to storage conditions, with no destruction detected during 90 days in most cases. Inactivation rates were relatively unaffected by sub-lethal pH but sharply accelerated by temperature: Inactivation became faster as temperature increased (in the 8 °C-55 °C temperature range), with growth blocked by low pH in sub-lethal temperatures. There were changes in surviving spore numbers after the heat-treatment phase. This has implications and applications in canned food industries, as the probability of a retorted sample testing as non-stable, meaning possible spoilage, may decrease with time. In simple terms, a batch of low-acid canned food that tests as non-shelf-stable after an incubation test i.e. positive growth conditions, may later become negative if stored at room temperature (below the minimal growth temperature for thermophilic spores), which may change the marketability of the batch.

Text mining tools for extracting information about microbial biodiversity in food

Information on food microbial diversity is scattered across millions of scientific papers. Researchers need tools to assist their bibliographic search in such large collections. Text mining and knowledge engineering methods are useful to automatically and efficiently find relevant information in Life Science. This work describes how the Alvis text mining platform has been applied to a large collection of PubMed abstracts of scientific papers in the food microbiology domain. The information targeted by our work is microorganisms, their habitats and phenotypes. Two knowledge resources, the NCBI taxonomy and the OntoBiotope ontology were used to detect this information in texts. The result of the text mining process was indexed and is presented through the AlvisIR Food on-line semantic search engine. In this paper, we also show through two illustrative examples the great potential of this new tool to assist in studies on ecological diversity and the origin of microbial presence in food.

•

We present new text-mining tools to extract information in food microbiology.

•

The results of the extraction are available in an on-line semantic search engine.

•

Taxa, habitats, phenotypes and links between them can be queried in PubMed abstracts.

•

Text-mining tools could assist to browse past and recent scientific literature.

•

Two use-cases are presented: fruit microbiota and spore-forming bacteria in food.