Heat Activation and Inactivation of Bacterial Spores: Is There an Overlap?

Heat activation at a sublethal temperature is widely applied to promote Bacillus species spore germination. This treatment also has the potential to be employed in food processing to eliminate undesired bacterial spores by enhancing their germination and then inactivating the less-heat-resistant germinated spores at a milder temperature. However, incorrect heat treatment could also generate heat damage in spores and lead to more heterogeneous spore germination. Here, the heat activation and heat damage profile of Bacillus subtilis spores was determined by testing spore germination and outgrowth at both population and single-spore levels. The heat treatments used were 40 to 80°C and for 0 to 300 min. The results were as follows. (i) Heat activation at 40 to 70°C promoted l-valine- and l-asparagine-glucose-fructose-potassium (AGFK)-induced germination in a time-dependent manner. (ii) The optimal heat activation temperatures for AGFK and l-valine germination via the GerB plus GerK or GerA germinant receptors were 65°C and 50 to 65°C, respectively. (iii) Heat inactivation of dormant spores appeared at 70°C, and the heat damage of molecules essential for germination and growth began at 70 and 65°C, respectively. (iv) Heat treatment at 75°C resulted in both activation of germination and damage to the germination apparatus, and 80°C treatment caused more pronounced heat damage. (v) For the spores that should withstand adverse environmental temperatures in nature, heat activation seemed functional for a subsequent optimal germination process, while heat damage affected both germination and outgrowth. IMPORTANCE Bacterial spores are thermal-stress-resistant structures that can thus survive food preservation strategies and revive through the process of spore germination. The more heat resistant spores are, the more heterogeneous their germination upon the addition of germinants. Upon germination, spores can cause food spoilage and food intoxication. Here, we provide new information on both heat activation and inactivation regimes and their effects on the (heterogeneity of) spore germination.

Intracellular pH Response to Weak Acid Stress in Individual Vegetative Bacillus subtilis Cells.

Intracellular pH (pHi) critically affects bacterial cell physiology. Hence, a variety of food preservation strategies are aimed at perturbing pHi homeostasis. Unfortunately, accurate pHi quantification with existing methods is suboptimal, since measurements are averages across populations of cells, not taking into account interindividual heterogeneity. Yet, physiological heterogeneity in isogenic populations is well known to be responsible for differences in growth and division kinetics of cells in response to external stressors. To assess in this context the behavior of intracellular acidity, we have developed a robust method to quantify pHi at single-cell levels in Bacillus subtilis Bacilli spoil food, cause disease, and are well known for their ability to form highly stress-resistant spores. Using an improved version of the genetically encoded ratiometric pHluorin (IpHluorin), we have quantified pHi in individual B. subtilis cells, cultured at an external pH of 6.4, in the absence or presence of weak acid stresses. In the presence of 3 mM potassium sorbate, a decrease in pHi and an increase in the generation time of growing cells were observed. Similar effects were observed when cells were stressed with 25 mM potassium acetate. Time-resolved analysis of individual bacteria in growing colonies shows that after a transient pH decrease, long-term pH evolution is highly cell dependent. The heterogeneity at the single-cell level shows the existence of subpopulations that might be more resistant and contribute to population survival. Our approach contributes to an understanding of pHi regulation in individual bacteria and may help scrutinizing effects of existing and novel food preservation strategies. IMPORTANCE: This study shows how the physiological response to commonly used weak organic acid food preservatives, such as sorbic and acetic acids, can be measured at the single-cell level. These data are key to coupling often-observed single-cell heterogeneous growth behavior upon the addition of weak organic acid food preservatives. Generally, these data are gathered in the form of plate counting of samples incubated with the acids. Here, we visualize the underlying heterogeneity in cellular pH homeostasis, opening up avenues for mechanistic analyses of the heterogeneity in the weak acid stress response. Thus, microbial risk assessment can become more robust, widening the scope of use of these well-known weak organic acid food preservatives.

Quantifying the effect of sorbic acid, heat and combination of both on germination and outgrowth of Bacillus subtilis spores at single cell resolution.

Bacillus subtilis spores are a problem for the food industry as they are able to survive preservation processes. The spores often reside in food products, where their inherent protection against various stress treatments causes food spoilage. Sorbic acid is widely used as a weak acid preservative in the food industry. Its effect on spore germination and outgrowth in a combined, 'hurdle', preservation setting has gained limited attention. Therefore, the effects of mild sorbic acid (3 mM), heat-treatment (85 °C for 10 min) and a combination of both mild stresses on germination and outgrowth of B. subtilis 1A700 spores were analysed at single spore level. The heat-treatment of the spore population resulted in a germination efficiency of 46.8% and an outgrowth efficiency of 32.9%. In the presence of sorbic acid (3 mM), the germination and outgrowth efficiency was 93.3% and 80.4% respectively whereas the combined heat and sorbic acid stress led to germination and outgrowth efficiencies of 52.7% and 27.0% respectively. The heat treatment clearly primarily affected the germination process, while sorbic acid affected the outgrowth and generation time. In addition a new 'burst' time-point was defined as the time-point at which the spore coat visibly breaks and/or is shed. The combined stresses had a synergistic effect on the time of the end of germination to the burst time-point, increasing both the mean and its variation more than either of the single stresses did.

Quantitative analysis of the effect of specific tea compounds on germination and outgrowth of Bacillus subtilis spores at single cell resolution.

Tea is one of the most widely consumed beverages in the world and known for its antimicrobial activity against many microorganisms. Preliminary studies have shown that tea polyphenols can inhibit the growth of a wide range of Gram-positive bacteria. However, the effect of these compounds on germination and outgrowth of bacterial spores is unclear. Spore-forming bacteria are an aggravating problem for the food industry due to spore formation and their subsequent returning to vegetative state during food storage, thus posing spoilage and food safety challenges. Here we analysed the effect of tea compounds: gallic acid, gallocatechin gallate, Teavigo (>90% epigallocatechin gallate), and theaflavin 3,3'-digallate on spore germination and outgrowth and subsequent growth of vegetative cells of Bacillus subtilis. To quantitatively analyse the effect of these compounds, live cell images were tracked from single phase-bright spores up to microcolony formation and analysed with the automated image analysis tool "SporeTracker". In general, the tested compounds had a significant effect on most stages of germination and outgrowth. However, germination efficiency (ability of spores to become phase-dark) was not affected. Gallic acid most strongly reduced the ability to grow out. Additionally, all compounds, in particular theaflavin 3,3'-digallate, clearly affected the growth of emerging vegetative cells.

Live cell imaging of germination and outgrowth of individual bacillus subtilis spores; the effect of heat stress quantitatively analyzed with SporeTracker.

Spore-forming bacteria are a special problem for the food industry as some of them are able to survive preservation processes. Bacillus spp. spores can remain in a dormant, stress resistant state for a long period of time. Vegetative cells are formed by germination of spores followed by a more extended outgrowth phase. Spore germination and outgrowth progression are often very heterogeneous and therefore, predictions of microbial stability of food products are exceedingly difficult. Mechanistic details of the cause of this heterogeneity are necessary. In order to examine spore heterogeneity we made a novel closed air-containing chamber for live imaging. This chamber was used to analyze Bacillus subtilis spore germination, outgrowth, as well as subsequent vegetative growth. Typically, we examined around 90 starting spores/cells for ≥4 hours per experiment. Image analysis with the purposely built program "SporeTracker" allows for automated data processing from germination to outgrowth and vegetative doubling. In order to check the efficiency of the chamber, growth and division of B. subtilis vegetative cells were monitored. The observed generation times of vegetative cells were comparable to those obtained in well-aerated shake flask cultures. The influence of a heat stress of 85°C for 10 min on germination, outgrowth, and subsequent vegetative growth was investigated in detail. Compared to control samples fewer spores germinated (41.1% less) and fewer grew out (48.4% less) after the treatment. The heat treatment had a significant influence on the average time to the start of germination (increased) and the distribution and average of the duration of germination itself (increased). However, the distribution and the mean outgrowth time and the generation time of vegetative cells, emerging from untreated and thermally injured spores, were similar.

Models of the behaviour of (thermally stressed) microbial spores in foods: tools to study mechanisms of damage and repair.

The 'Omics' revolution has brought a wealth of new mechanistic insights in many fields of biology. It offers options to base predictions of microbial behaviour on mechanistic insight. As the cellular mechanisms involved often turn out to be highly intertwined it is crucial that model development aims at identifying the level of complexity that is relevant to work at. For the prediction of microbiologically stable foods insight in the behaviour of bacterial spore formers is crucial. Their chances of germination and likelihood of outgrowth are major food stability indicators, as well as the transition from outgrowth to first cell division and vegetative growth. Current available technology to assess these parameters in a time-resolved manner at the single spore level will be discussed. Tools to study molecular processes operative in heat induced damage will be highlighted.

Physiological and mathematical aspects in setting criteria for decontamination of foods by physical means.

In heat processing, microbial inactivation is traditionally described as log-linear. As a general rule, the relation between rate of inactivation and temperature is also described as a log-linear relation. The model is also sometimes applied in pressure and in pulsed electric field (PEF) processing. The model has proven its value by the excellent safety record of the last 80 years, but there are many deviations from log-linearity. This could lead to either over-processing or under-processing resulting in safety problems or, more likely, spoilage problems. As there is a need for minimal processing, accurate information of the inactivation kinetics is badly needed. To predict inactivation more precisely, models have been developed that can cope with deviations of linearity. As extremely low probabilities of survival must be predicted, extrapolation is almost always necessary. However, extrapolation is hardly possible without knowledge of the nature of nonlinearity. Therefore, knowledge of the physiology of inactivation is necessary. This paper discusses the physiology of denaturation by heat, high pressure and pulse electric field. After discussion of the physiological aspects, the various aspects of the development of inactivation models will be addressed. Both general and more specific aspects are discussed such as choice of test strains, effect of the culture conditions, conditions during processing and recovery conditions and mathematical modelling of inactivation. In addition to lethal inactivation, attention will be paid to sublethal inactivation because of its relevance to food preservation. Finally, the principles of quantitative microbiological risk assessment are briefly mentioned to show how appropriate inactivation criteria can be set.

Modelling the effect of sublethal injury on the distribution of the lag times of individual cells of Lactobacillus plantarum.

The effect of heat stress on subsequent duration of the lag time of individual cells of Lactobacillus plantarum was analysed by flow cytometry. The results show clearly that both the mean and the standard deviation of the distribution of the lag time increased after sublethal heat treatment. The distributions of the lag times or the log lag times of untreated and treated cells, respectively, could be described as extreme value distributions. From these distributions, the distribution of the minimum lag times could be calculated and thus the effect of inoculum size on the apparent lag could be deduced. The results show clearly that the apparent lag time is dependent on the size of the inoculum, especially when the inoculum is sublethally injured.