Kinetics of germination of individual spores of Geobacillus stearothermophilus as measured by raman spectroscopy and differential interference contrast microscopy

Elucidating Bacterial Spore Dynamics through Lanthanide-Enhanced Live Imaging

Identifying and distinguishing dormant and active bacterial spores are vital for biosecurity, food safety, and space exploration. Yet, there is a lack of simple, quick, and nondestructive methods to achieve this. The common Schaeffer-Fulton method is both sample-destructive and requires significant operator involvement. In this study, we employed lanthanide-beta-diketonate complexes to directly observe both dormant and germinated single spores. Staining is instantaneous and requires minimal sample processing. The complex stains areas outside the core of dormant spores, leaving the core hollow and nonfluorescent. However, upon germination, the complex enters the core, making it brightly fluorescent. This difference was noted in five bacterial species including Bacillus, Clostridium, and Clostridioides. Various lanthanides and beta-diketonates can be mixed to form a range of spore-visualizing complexes. Due to their low toxicity, these complexes allow for live imaging of single germinating spores. We demonstrate low-cost imaging using a USB microscope as well as imaging of spores in milk matrices. This method provides a valuable tool for studying bacterial spores.

Properties of spores of Bacillus subtilis with or without a transposon that decreases spore germination and increases spore wet heat resistance

AIMS

This work aimed to determine how genes on transposon Tn1546 slow Bacillus subtilis spore germination and increase wet heat resistance, and to clarify the transposon's 3 gene spoVA operon's role in spore properties, since the seven wild-type SpoVA proteins form a channel transporting Ca²⁺ -dipicolinic acid (DPA) in spore formation and germination.

METHODS AND RESULTS

Deletion of the wild-type spoVA operon from a strain with Tn1546 gave spores with slightly reduced wet heat resistance but some large decreases in germination rate. Spore water content and CaDPA analyses found no significant differences in contents of either component in spores with different Tn1546 components or lacking the wild-type spoVA operon.

CONCLUSIONS

This work indicates that the SpoVA channel encoded by Tn1546 functions like the wild-type SpoVA channel in CaDPA uptake into developing spores, but not as well in germination. The essentially identical CaDPA and water contents of spores with and without Tn1546 indicate that low core water content does not cause elevated wet heat resistance of spores with Tn1546.

SIGNIFICANCE AND IMPACT OF THE STUDY

Since wet heat resistance of spores of Bacillus species poses problems in the food industry, understanding mechanisms of spores' wet heat resistance is of significant applied interest.

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.

Molecular Engineering of the Geobacillus stearothermophilus α-Amylase and Cel5E from Chlostridium thermocellim; In Silico Approach

Background

Considering natural thermal stability, Geobacillus stearothermophilus amylase and Cel5E from Clostridium thermocellum are good candidates for industrial applications. To be compatible with the industrial applications, this enzyme should be stable in the high temperatures, so any improvement in their thermal stability is valuable.

Objectives

Using in silico approach and identifying point mutations in the structure amylase of G. stearothermophilus and Cel5E from C. termocellum we tried to increase thermal stability of the enzymes along with their catalytic activity to reach a new industrial amylase with higher thermostability and an improved function.

Materials and Methods

In this study we predicted the 3D structure of the enzymes, then simulated the molecular docking study using MolDock, PLANTS, and Lamarkian genetic algorithm as scoring functions for the docking and in silico engineering of the protein aiming to increase the thermal stability and catalytic activity.

Results

A series of thermal stability increasing point mutations were exerted around the active site of the enzyme, then by docking procedure, the binding affinity was measured and finally a list of mutations which theoretically improved the increased thermal stability as well as catalytic activity were proposed.

Conclusions

Based on the in silico results obtained the modified enzymes seems to be suitable candidates for considering in both laboratory and industrial scales.

Effect of storage temperature on the lag time of Geobacillus stearothermophilus individual spores

The lag times (λ) of Geobacillus stearothermophilus single spores were studied at different storage temperatures ranging from 45 to 59 °C using the Bioscreen C method. A significant variability of λ was observed among individual spores at all temperatures tested. The storage temperature affected both the position and the spread of the λ distributions. The minimum mean value of λ (i.e. 10.87 h) was observed at 55 °C, while moving away from this temperature resulted in an increase for both the mean and standard deviation of λ. A Cardinal Model with Inflection (CMI) was fitted to the reverse mean λ, and the estimated values for the cardinal parameters T_min, T_max, T_opt and the optimum mean λ of G. stearothermophilus were found to be 38.1, 64.2, 53.6 °C and 10.3 h, respectively. To interpret the observations, a probabilistic growth model for G. stearothermophilus individual spores, taking into account λ variability, was developed. The model describes the growth of a population, initially consisting of N₀ spores, over time as the sum of cells in each of the N₀ imminent subpopulations originating from a single spore. Growth simulations for different initial contamination levels showed that for low N₀ the number of cells in the population at any time is highly variable. An increase in N₀ to levels exceeding 100 spores results in a significant decrease of the above variability and a shorter λ of the population. Considering that the number of G. stearothermophilus surviving spores in the final product is usually very low, the data provided in this work can be used to evaluate the probability distribution of the time-to-spoilage and enable decision-making based on the "acceptable level of risk".

Spore Heat Activation Requirements and Germination Responses Correlate with Sequences of Germinant Receptors and with the Presence of a Specific spoVA2mob Operon in Foodborne Strains of Bacillus subtilis

Spore heat resistance, germination, and outgrowth are problematic bacterial properties compromising food safety and quality. Large interstrain variation in these properties makes prediction and control of spore behavior challenging. High-level heat resistance and slow germination of spores of some natural Bacillus subtilis isolates, encountered in foods, have been attributed to the occurrence of the spoVA^2mob operon carried on the Tn1546 transposon. In this study, we further investigate the correlation between the presence of this operon in high-level-heat-resistant spores and their germination efficiencies before and after exposure to various sublethal heat treatments (heat activation, or HA), which are known to significantly improve spore responses to nutrient germinants. We show that high-level-heat-resistant spores harboring spoVA^2mob required higher HA temperatures for efficient germination than spores lacking spoVA^2mob The optimal spore HA requirements additionally depended on the nutrients used to trigger germination, l-alanine (l-Ala), or a mixture of l-asparagine, d-glucose, d-fructose, and K⁺ (AGFK). The distinct HA requirements of these two spore germination pathways are likely related to differences in properties of specific germinant receptors. Moreover, spores that germinated inefficiently in AGFK contained specific changes in sequences of the GerB and GerK germinant receptors, which are involved in this germination response. In contrast, no relation was found between transcription levels of main germination genes and spore germination phenotypes. The findings presented in this study have great implications for practices in the food industry, where heat treatments are commonly used to inactivate pathogenic and spoilage microbes, including bacterial spore formers.IMPORTANCE This study describes a strong variation in spore germination capacities and requirements for a heat activation treatment, i.e., an exposure to sublethal heat that increases spore responsiveness to nutrient germination triggers, among 17 strains of B. subtilis, including 9 isolates from spoiled food products. Spores of industrial foodborne isolates exhibited, on average, less efficient and slower germination responses and required more severe heat activation than spores from other sources. High heat activation requirements and inefficient, slow germination correlated with elevated resistance of spores to heat and with specific genetic features, indicating a common genetic basis of these three phenotypic traits. Clearly, interstrain variation and numerous factors that shape spore germination behavior challenge standardization of methods to recover highly heat-resistant spores from the environment and have an impact on the efficacy of preservation techniques used by the food industry to control spores.

Effects of steam autoclave treatment on Geobacillus stearothermophilus spores

AIMS

To determine the mechanism of autoclave killing of Geobacillus stearothermophilus spores used in biological indicators (BIs) for steam autoclave sterilization, and rates of loss of spore viability and a spore enzyme used in BIs.

METHODS AND RESULTS

Spore viability, dipicolinic acid (DPA) release, nucleic acid staining, α-glucosidase activity, protein structure and mutagenesis were measured during autoclaving of G. stearothermophilus spores. Loss of DPA and increases in spore core nucleic acid staining were slower than loss of spore viability. Spore core α-glucosidase was also lost more slowly than spore viability, although soluble α-glucosidase in spore preparations was lost more rapidly. However, spores exposed to an effective autoclave sterilization lost all viability and α-glucosidase activity. Apparently killed autoclaved spores were not recovered by artificial germination in supportive media, much spore protein was denatured during autoclaving, and partially killed autoclave-treated spore preparations did not acquire mutations.

CONCLUSIONS

These results indicate that autoclave-killed spores cannot be revived, spore killing by autoclaving is likely by protein damage, and spore core α-glucosidase activity is lost more slowly than spore viability.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides insight into the mechanism of autoclave killing of spores of an organism used in BIs, and that a spore enzyme in a BI is more stable to autoclaving than spore viability.

Raman Spectroscopy of Optically Trapped Single Biological Micro-Particles

The combination of optical trapping with Raman spectroscopy provides a powerful method for the study, characterization, and identification of biological micro-particles. In essence, optical trapping helps to overcome the limitation imposed by the relative inefficiency of the Raman scattering process. This allows Raman spectroscopy to be applied to individual biological particles in air and in liquid, providing the potential for particle identification with high specificity, longitudinal studies of changes in particle composition, and characterization of the heterogeneity of individual particles in a population. In this review, we introduce the techniques used to integrate Raman spectroscopy with optical trapping in order to study individual biological particles in liquid and air. We then provide an overview of some of the most promising applications of this technique, highlighting the unique types of measurements enabled by the combination of Raman spectroscopy with optical trapping. Finally, we present a brief discussion of future research directions in the field.

High-precision fitting measurements of the kinetics of size changes during germination of individual Bacillus spores

High-precision measurements of size changes of individual bacterial spores based on ellipse fitting to bright-field images recorded with a digital camera were employed to monitor the germination of Bacillus spores with a precision of ~5 nm. To characterize the germination of individual spores, we recorded bright-field and phase-contrast images and found that the timing of changes in their normalized intensities coincided, so the bright-field images can be used to characterize spore size and refractility changes during germination. The major conclusions from this work were as follows. (i) The sizes of germinating B. cereus spores were nearly unchanged until Trelease, the time of the completion of CaDPA (a 1:1 chelate of Ca(2+) and dipicolinic acid [DPA]) release after addition of nutrient germinants. (ii) The minor axis of germinating B. cereus spores rapidly increased by ~50 nm in a few seconds right after Trelease, while the major axis was slightly decreased or unchanged. Both the minor and major axes remained unchanged for a further 30 to 45 s and then increased by 100 to 200 nm by Tlys, the time of completion of cortex lysis. (iii) Individual spores in a population showed significant heterogeneity in the timing of germination events, such as Trelease and Tlys, but also variation in size changes during germination. (iv) Bacillus subtilis wild-type spores, B. subtilis spores lacking the cortex-lytic enzyme CwlJ, and wild-type Bacillus megaterium spores showed similar kinetics of size changes during nutrient germination. The size increases in germinating spores probably result from uptake of water and cortex lysis after completion of CaDPA release.

Characterization of the Dynamic Germination of Individual Clostridium difficile Spores Using Raman Spectroscopy and Differential Interference Contrast Microscopy

The Gram-positive spore-forming anaerobe Clostridium difficile is a leading cause of nosocomial diarrhea. Spores of C. difficile initiate infection when triggered to germinate by bile salts in the gastrointestinal tract. We analyzed germination kinetics of individual C. difficile spores using Raman spectroscopy and differential interference contrast (DIC) microscopy. Similar to Bacillus spores, individual C. difficile spores germinating with taurocholate plus glycine began slow leakage of a ∼15% concentration of a chelate of Ca(2+) and dipicolinic acid (CaDPA) at a heterogeneous time T1, rapidly released CaDPA at Tlag, completed CaDPA release at Trelease, and finished peptidoglycan cortex hydrolysis at Tlysis. T1 and Tlag values for individual spores were heterogeneous, but ΔTrelease periods (Trelease - Tlag) were relatively constant. In contrast to Bacillus spores, heat treatment did not stimulate spore germination in the two C. difficile strains tested. C. difficile spores did not germinate with taurocholate or glycine alone, and different bile salts differentially promoted spore germination, with taurocholate and taurodeoxycholate being best. Transient exposure of spores to taurocholate plus glycine was sufficient to commit individual spores to germinate. C. difficile spores did not germinate with CaDPA, in contrast to B. subtilis and C. perfringens spores. However, the detergent dodecylamine induced C. difficile spore germination, and rates were increased by spore coat removal although cortex hydrolysis did not follow Trelease, in contrast with B. subtilis. C. difficile spores lacking the cortex-lytic enzyme, SleC, germinated extremely poorly, and cortex hydrolysis was not observed in the few sleC spores that partially germinated. Overall, these findings indicate that C. difficile and B. subtilis spore germination exhibit key differences.

IMPORTANCE

Spores of the Gram-positive anaerobe Clostridium difficile are responsible for initiating infection by this important nosocomial pathogen. When exposed to germinants such as bile salts, C. difficile spores return to life through germination in the gastrointestinal tract and cause disease, but their germination has been studied only with population-wide measurements. In this work we used Raman spectroscopy and DIC microscopy to monitor the kinetics of germination of individual C. difficile spores, the commitment of spores to germination, and the effect of germinant type and concentration, sublethal heat shock, and spore decoating on germination. Our data suggest that the order of germination events in C. difficile spores differs from that in Bacillus spores and provide new insights into C. difficile spore germination.

The effects of heat activation on Bacillus spore germination, with nutrients or under high pressure, with or without various germination proteins

Nutrient germination of spores of Bacillus species occurs through germinant receptors (GRs) in spores' inner membrane (IM) in a process stimulated by sublethal heat activation. Bacillus subtilis spores maximum germination rates via different GRs required different 75 °C heat activation times: 15 min for l-valine germination via the GerA GR and 4 h for germination with the L-asparagine-glucose-fructose-K(+) mixture via the GerB and GerK GRs, with GerK requiring the most heat activation. In some cases, optimal heat activation decreased nutrient concentrations for half-maximal germination rates. Germination of spores via various GRs by high pressure (HP) of 150 MPa exhibited heat activation requirements similar to those of nutrient germination, and the loss of the GerD protein, required for optimal GR function, did not eliminate heat activation requirements for maximal germination rates. These results are consistent with heat activation acting primarily on GRs. However, (i) heat activation had no effects on GR or GerD protein conformation, as probed by biotinylation by an external reagent; (ii) spores prepared at low and high temperatures that affect spores' IM properties exhibited large differences in heat activation requirements for nutrient germination; and (iii) spore germination by 550 MPa of HP was also affected by heat activation, but the effects were relatively GR independent. The last results are consistent with heat activation affecting spores' IM and only indirectly affecting GRs. The 150- and 550-MPa HP germinations of Bacillus amyloliquefaciens spores, a potential surrogate for Clostridium botulinum spores in HP treatments of foods, were also stimulated by heat activation.