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Review

Purification and Detection of Bacterial Endospores: Current Methods and Challenges

by
Souichirou Kawai
Toyo Institute of Food Technology, 23-2, 4-chome, Minami-Hanayashiki, Kawanishi 666-0026, Japan
Appl. Sci. 2026, 16(11), 5702; https://doi.org/10.3390/app16115702
Submission received: 23 April 2026 / Revised: 14 May 2026 / Accepted: 22 May 2026 / Published: 5 June 2026
(This article belongs to the Special Issue Innovative Perspectives on Food Microbiology and Biotechnology)
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This review provides practical guidance for selecting and validating endospore purification and detection workflows in food and environmental microbiology. An integrated view of sample preparation and detection can improve the comparability of spore resistance and enumeration data across laboratories and support the implementation of rapid methods (e.g., flow cytometry and molecular assays) in routine monitoring and process validation.

Abstract

Bacterial endospores are highly resistant, dormant forms that pose persistent challenges in food safety, environmental microbiology, and industrial hygiene. Accurate evaluation of endospore resistance, physiology, and inactivation depends on both purification and detection methods; however, these processes are typically examined independently, limiting methodological consistency and contributing to variability across studies. In this review, current approaches for endospore purification and detection are critically examined, including washing-based methods, density gradient centrifugation, enzymatic treatments, culture-based enumeration, molecular assays, flow cytometry, and emerging biosensor technologies. In addition, these methods are compared using metrics such as purity, recovery yield, sensitivity, and specificity, and their advantages and limitations are summarized to clarify performance. It is further proposed that endospore purification and detection should be considered as a single, end-to-end analytical workflow and optimized accordingly. Purification strategies influence sample cleanliness and aspects of endospore quality, including viability, structural integrity, and physiological state, which affect detection performance and quantitative accuracy. Based on this integrated perspective, a conceptual framework linking purification efficiency to detection outcomes is presented, along with practical considerations for method selection across relevant application contexts. Finally, gaps in standardization are identified, and future research directions are outlined to improve reproducibility and cross-study comparability in endospore-related studies.

1. Introduction

Bacterial endospores are highly resistant dormant structures produced primarily by Gram-positive genera such as Bacillus [1,2] and Clostridium [3,4]. Their remarkable tolerance to heat [5], desiccation [6], radiation [7], and chemical disinfectants [8] enables long-term persistence under adverse conditions [9], thereby posing persistent challenges in food safety [10], environmental microbiology [11], clinical settings [12], and industrial hygiene [13]. Spore-forming bacteria are frequently implicated in food spoilage [10], foodborne illness, and contamination of processing environments [14], where conventional sanitation procedures often fail to eliminate these dormant forms [15].
Reliable purification and detection of bacterial spores are essential for accurate characterization of their physiological properties [16]. However, laboratory preparations commonly contain substantial proportions of vegetative cells [17], cellular debris [18], and partially germinated spores [19]. Such heterogeneity can significantly distort viability assessments [20], resistance evaluation [21], and molecular analyses [22], leading to inconsistent or misleading results. Importantly, these biases represent a critical and often overlooked source of experimental variability in spore research [23,24].
A variety of purification strategies have been developed, including repeated washing and centrifugation [25], density gradient separation using media such as Percoll or Nycodenz [26], and enzymatic treatments such as lysozyme digestion [27]. While these approaches differ in efficiency, recovery yield, processing time, and cost, they may also exert distinct effects on spore integrity and physiological state [23,24]. Despite their widespread use, purification methods are typically selected based on empirical convention rather than systematic comparison, and no standardized framework currently exists to guide method selection.
In parallel, numerous detection methods have been established, including culture-based enumeration [28], microscopic staining [29], molecular techniques such as PCR [30,31], and emerging analytical platforms such as flow cytometry [32,33] and spectroscopy [34,35]. Although these methods offer varying degrees of sensitivity and specificity, their performance is inherently dependent on the quality of the spore preparation [24,36,37]. Nevertheless, purification and detection are often considered in isolation, thereby obscuring their fundamental interdependence [38,39,40]. This separation further obscures the extent to which purification-induced variability propagates into detection outcomes [41,42], affecting sensitivity, accuracy, and reproducibility. Despite the central role of both purification and detection in spore research [43,44,45], their combined impact on experimental outcomes has not been systematically examined, thereby limiting the ability to interpret data across studies and hindering the development of standardized methodologies.
This review comprehensively evaluates current approaches for bacterial spore purification and detection, with a particular focus on their interdependence. By linking purification strategies to spore quality attributes and subsequent detection performance, this work provides a conceptual framework for improving methodological consistency and supports the rational selection of appropriate techniques in food and environmental microbiology.

2. Spore Purification Methods

Before purification, the preparation of spore-rich cultures is a critical step that can significantly influence downstream purification efficiency and analytical reproducibility. Traditional sporulation approaches based on nutrient depletion and metabolite accumulation in closed culture systems remain widely used for laboratory-scale spore production. Although these methods generally require longer incubation periods (typically 72–96 h), they may promote more synchronous sporulation in heterogeneous cultures and reduce contamination from incompletely sporulated cells [23]. In addition, specialized sporulation media such as Difco Sporulation Medium (DSM) may improve sporulation efficiency and experimental reproducibility under controlled laboratory conditions [46,47]. Therefore, both conventional nutrient depletion approaches and DSM-based methods remain useful depending on the target species, experimental objectives, and required sample throughput.
Preparation of highly purified spore suspensions is a fundamental prerequisite for the accurate evaluation of spore resistance, physiology, and detection sensitivity [48,49,50]. However, complete separation of dormant spores from vegetative cells, cellular debris, and partially germinated forms remain technically challenging [1,51,52]. In practice, spore preparations often exhibit substantial heterogeneity, which can introduce considerable variability into downstream analyses [23,24]. Importantly, spore purification is a critical determinant of experimental outcomes [53,54,55,56]. Different purification strategies can selectively remove, retain, or alter specific subpopulations of spores, thereby influencing apparent resistance profiles (e.g., response to heat, chemical disinfectants, UV irradiation, and desiccation), viability measurements, and detection performance [23,24,57]. Despite its central role, purification is frequently treated as a routine procedure, and its impact on analytical reliability is often underappreciated.
A variety of purification strategies have been developed, differing in complexity, efficiency, and potential effects on spore integrity. However, these methods are rarely evaluated within a unified framework, making it difficult to directly compare their relative performance or select appropriate approaches for specific analytical objectives.

2.1. Washing and Centrifugation

The most widely used approach for spore purification relies on repeated washing and low-speed centrifugation cycles following sporulation [25], often combined with a preliminary heat treatment to inactivate residual vegetative cells. Because optimal heat-treatment conditions vary substantially among species and strains of Bacillus and Clostridium, no single universal temperature can be recommended; however, mild heat treatments around 65–80 °C for 10–30 min are commonly used to reduce vegetative-cell carryover and/or to activate spores prior to downstream analyses. Repeated heating/cooling cycles have been explored in some studies, but their effects appear inconsistent and depend strongly on the physiological state and pre-culture conditions of the spores [58,59,60]. In this method, spores are pelleted by centrifugation and subjected to multiple resuspension and washing steps to remove loosely associated cellular debris and soluble contaminants.
The effectiveness of this approach is primarily based on differences in sedimentation behavior among spores, vegetative cells, and cellular debris [61,62]. Due to their dense and compact structure, mature spores are preferentially recovered in the pellet fraction, whereas lighter debris is progressively removed during washing steps. This principle enables simple physical enrichment without requiring specialized reagents [63,64].
Owing to its simplicity, low cost, and accessibility, this method remains a standard procedure in many laboratories [23,24,63,64]. However, its performance is highly dependent on the efficiency of sporulation and the initial composition of the sample. In cases of incomplete sporulation, substantial contamination with vegetative cells, cell fragments, or partially germinated spores may persist even after repeated washing cycles [23,65,66]. In addition, repeated centrifugation and resuspension steps may impose mechanical stress on spores, potentially affecting recovery yield and, in some cases, altering surface properties relevant to downstream analyses [67,68,69]. Importantly, residual contaminants not completely removed by washing can interfere with detection methods by contributing to background signals in optical or cytometric analyses or introducing nontarget DNA in molecular assays [70].
Therefore, while washing and centrifugation are suitable for routine laboratory applications and preliminary enrichment, this approach alone is often insufficient when high purity is required for precise quantitative measurements or advanced detection platforms. In such cases, additional purification steps, such as density gradient centrifugation or enzymatic treatment, are typically necessary to achieve the desired level of spore quality [23,33].

2.2. Density Gradient Centrifugation

Density gradient centrifugation was developed to improve spore purification efficiency by exploiting differences in buoyant density between spores and other cellular components [71,72]. In this approach, samples are layered onto discontinuous or continuous density gradients generated using media such as Percoll or Nycodenz [24,26,73]. During centrifugation, particles migrate to positions corresponding to their equilibrium densities, enabling the effective separation of spores from vegetative cells, cellular debris, and partially degraded materials.
The high density and compact structure of mature spores generally result in their localization in the lower regions of the gradient, whereas vegetative cells and lighter debris remain in the upper fractions [74,75,76,77]. This density-based partitioning enables more precise and reproducible separation than simple washing procedures. In addition, gradient systems can partially resolve heterogeneity within spore populations, including differences in maturation state or structural integrity, which are not easily distinguishable by conventional centrifugation alone [78]. As a result, density gradient centrifugation is widely employed in studies requiring highly purified and homogeneous spore preparations, such as resistance assays, structural characterization, and advanced analytical applications [79,80]. However, the method involves several practical and technical limitations. Preparation of gradients, optimization of centrifugation parameters, and fraction collection increase procedural complexity and overall processing time [81,82]. Furthermore, recovery yield can be reduced because of losses during fractionation and incomplete recovery from gradient media.
Importantly, separation efficiency is highly sensitive to experimental conditions, including gradient composition, density range, and centrifugation speed and duration. Small variations in these parameters can shift particle distribution within the gradient, leading to inconsistent recovery or incomplete separation when density differences between components are minimal [83,84]. Such variability complicates reproducibility and cross-study comparisons, particularly in the absence of standardized protocols [23,24].

2.3. Enzymatic Treatments

Enzymatic strategies, particularly lysozyme digestion, are widely employed to eliminate residual vegetative cells following initial washing steps [27,85]. Lysozyme specifically hydrolyzes the β-(1,4)-glycosidic linkages between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycan, leading to rapid weakening and lysis of vegetative cell walls [86,87]. In contrast, mature spores are largely resistant to this treatment due to the presence of multiple protective layers, including the cortex, coat, and, in some species, an exosporium, which limits enzyme accessibility [88,89]. The efficiency of lysozyme treatment is strongly influenced by experimental parameters such as enzyme concentration, incubation time, temperature, and buffer composition (e.g., ionic strength, pH), as well as the physiological state of the cells [90,91,92]. For example, sublethally damaged or germinating spores can exhibit increased susceptibility to enzymatic degradation, potentially leading to underestimation of spore recovery [93]. Therefore, careful optimization is required to maximize selective lysis of vegetative cells while preserving spore integrity.
When combined with physical purification steps such as repeated washing or density gradient centrifugation, lysozyme treatment can substantially improve the apparent purity of spore preparations [23,24]. However, excessive enzymatic exposure or inappropriate conditions may partially disrupt spore surface structures, including coat proteins, or alter key physicochemical properties such as hydrophobicity and surface charge. These changes can consequently affect downstream analyses, including antibody binding, flow cytometry, or germination assays [32,33]. Accordingly, enzymatic purification should be applied under controlled conditions and validated for compatibility with the intended analytical techniques.

2.4. Comparative Evaluation of Purification Strategies

Each purification strategy involves inherent trade-offs among purity, recovery yield, processing time, cost, and practical applicability. For instance, simplified isolation procedures incorporating enzymatic digestion (e.g., lysozyme treatment) can effectively remove residual vegetative cells and yield highly purified preparations [27,85]. However, such approaches might reduce overall recovery because of partial loss of spores during repeated washing steps or unintended damage to germination-sensitive subpopulations [90,91,92].
Systematic comparisons of purification workflows have demonstrated that protocols optimized for maximal purity, such as those combining enzymatic digestion with density gradient centrifugation, typically require additional processing steps, specialized reagents, and longer handling times, thereby increasing cost and experimental complexity [79,80,81,83,84].
A major limitation in the current literature is the lack of standardized criteria for assessing spore purity. Reported metrics vary widely, including phase-contrast microscopy, colony-forming unit (CFU) ratios, staining-based methods, and flow cytometric analyses, which makes direct comparison across studies challenging [28,29,30,31,32,33,34,35]. Furthermore, few studies have comprehensively reported purity and recovery yield according to standardized evaluation criteria, further hindering robust methodological evaluation. Therefore, the selection of an appropriate purification strategy should be guided by the intended downstream application. High-precision applications, such as resistance profiling, structural analysis, or antibody-based detection, often require highly purified spores obtained through density gradient centrifugation or combined approaches [23,24]. In contrast, routine enumeration or screening assays may tolerate lower purity levels and benefit from simpler, higher-yield washing-based methods [45,94]. Establishing standardized benchmarks that integrate purity, yield, and practical performance metrics would substantially improve reproducibility and comparability in spore research (Table 1).

3. Bacterial Spore Detection Methods

Reliable detection of bacterial spores is critical for food safety monitoring [13], environmental surveillance [11], and validation of spore purification procedures [23,24]. Spores are highly resistant to heat [5], desiccation [6], and chemical treatment [8], allowing them to persist in food processing environments and evade conventional sanitation strategies. This persistence not only increases the risk of food spoilage [10] and foodborne illness [95] but also complicates accurate microbial risk assessment.
Detection strategies vary widely in sensitivity, specificity, processing time, and ability to distinguish dormant spores from vegetative cells or nonviable particles [96,97,98]. Many conventional culture-based methods require germination prior to detection, which can lead to underestimation because of incomplete or heterogeneous germination [99,100]. Conversely, molecular and staining-based approaches can detect both viable and nonviable cells, thereby potentially overestimating the actual risk [101,102]. Furthermore, the structural similarity between spores and cellular debris presents an additional challenge for accurate discrimination [45,103]. Consequently, there remains a strong demand for rapid, reliable detection methods capable of selectively identifying intact and physiologically relevant spores across diverse sample matrices.

3.1. Culture-Based Enumeration

Culture-based methods remain the conventional standard for spore detection and quantification. Typically, samples undergo heat treatment to inactivate vegetative cells, followed by plating on suitable growth media and enumeration of CFU/mL−1. This approach is widely used because of its simplicity, low cost, and ability to selectively quantify viable spores capable of germination and outgrowth [97,104]. It provides a direct measure of microbiologically relevant viability, which is critical for food safety risk assessment and process validation. Nevertheless, several limitations must be considered. Heat treatment protocols vary widely in temperature, duration, and matrix conditions, which can significantly influence apparent spore counts and hinder comparability across studies. In addition, spores in different physiological states, such as superdormant spores or those subjected to sublethal stress, might exhibit delayed or incomplete germination, resulting in underestimation [58,105,106,107]. Lethally injured or environmentally stressed spores may also fail to form colonies under standard cultivation conditions, further contributing to bias [108,109,110]. Moreover, the efficiency of recovery is strongly affected by the choice of culture medium, germinant, and incubation parameters, all of which can introduce methodological variability. The presence of competing microbiota or inhibitory substances in complex samples can further suppress colony formation. In biofilm-forming species such as Bacillus subtilis, extracellular polymeric substances (EPS) may promote spore aggregation and interfere with accurate enumeration methods. In such cases, repeated washing, mild enzymatic treatment, or mechanical dispersion (e.g., vortexing or sonication) may improve particle separation. In microscopic analyses, the use of an eyepiece reticle or counting grid may further improve counting consistency and reduce operator-dependent variability [111,112]. From a practical perspective, colony development requires extended incubation times, typically ranging from 24 h to several days, making this approach less suitable for rapid or high-throughput applications [113,114,115]. Additionally, in sporicidal efficacy testing, carrier-based methods such as porcelain carrier-based assays have been incorporated into standardized protocols by organizations such as AOAC INTERNATIONAL to evaluate disinfectant efficacy against bacterial spores [116,117,118].

3.2. Microscopic and Staining Techniques

Phase-contrast microscopy provides a rapid and non-destructive means for the qualitative assessment of spore morphology, refractility, and overall preparation purity. Mature endospores typically appear as highly refractile bodies because of their dehydrated core and dense cortex structure, allowing preliminary discrimination from vegetative cells without staining [119,120,121]. In addition, classical endospore staining techniques, such as malachite green staining, Schaeffer–Fulton staining, and related differential staining protocols, facilitate clearer visualization by selectively retaining dye within the spore structure while counterstaining vegetative cells [29,34,35,122]. These microscopy-based methods are widely applied during spore purification workflows to monitor the progression of sporulation, assess the efficiency of purification steps, and confirm the absence of residual vegetative cells [23,24,123]. Furthermore, they can provide insights into morphological heterogeneity, including variations in spore size, shape, and structural integrity, which might reflect differences in sporulation conditions or physiological states. Despite their utility, microscopy-based approaches have several inherent limitations. First, quantitative accuracy is limited, as enumeration is typically based on manual counting or semiquantitative estimation, which is subject to sampling bias and low throughput [124]. Second, differentiation among mature, immature, and structurally compromised spores remains challenging, particularly in heterogeneous preparations, in which intermediate forms may coexist [125]. Third, interpretation is highly operator-dependent and can be influenced by microscope settings, staining efficiency, and observer experience. In addition, residual debris, including cell fragments and extracellular matrix components from incompletely purified samples, can interfere with accurate identification and lead to overestimation of spore content [126].
Recent advances, such as fluorescence microscopy combined with nucleic acid dyes or membrane integrity probes, alongside automated image analysis and machine learning-based classification, have been explored to improve objectivity and throughput. Nevertheless, these approaches are not yet standardized, and they often require specialized equipment and advanced data processing pipelines. Consequently, microscopy is most appropriately employed as a complementary tool for morphological validation and quality control, rather than as a standalone quantitative method [127,128].

3.3. Molecular Detection Methods

PCR-based methodologies, including conventional PCR and quantitative PCR (qPCR), have been extensively employed for the detection of spores due to their high sensitivity and specificity. By targeting species-specific genetic markers, these approaches enable more rapid detection than traditional culture-based methods [30,31].
Nevertheless, a fundamental limitation of molecular detection lies in its inability to discriminate between DNA derived from dormant spores, metabolically active vegetative cells, and nonviable cellular remnants.
DNA released from lysed cells may persist in samples and give rise to false-positive signals, thereby compromising detection accuracy [129]. To address this issue, various pretreatment strategies, such as the use of DNA-intercalating dyes designed to inhibit amplification of extracellular or membrane-compromised DNA, have been developed [130,131,132]. However, complete discrimination between viable and nonviable sources remains challenging to achieve. Moreover, the presence of inhibitory substances in complex food and environmental matrices can adversely affect amplification efficiency, leading to reduced sensitivity and potential underestimation of target organisms. These challenges underscore the critical importance of rigorous sample preparation and appropriate pretreatment procedures to ensure reliable molecular detection [133,134].

3.4. Flow Cytometry and Advanced Analytical Approaches

Flow cytometry has emerged as a powerful high-throughput platform for the rapid enumeration and multiparametric characterization of bacterial spores. By integrating fluorescent staining strategies with intrinsic light-scattering properties, this technique enables discrimination of distinct subpopulations and provides insight into the heterogeneity of spore preparations at the single-particle level [33,135,136,137,138]. Differences in refractive index, size, and internal complexity can be exploited to distinguish dormant spores from vegetative cells or partially germinated forms. However, the reliability of flow cytometry is highly dependent on staining protocols, instrument settings, and gating strategies, which may vary considerably between studies [139,140]. Furthermore, the presence of debris, cell fragments, or structurally compromised spores can lead to misclassification and overestimation of target populations [141,142]. In addition, the requirement for specialized instrumentation, standardized calibration procedures, and experienced operators limits its widespread adoption in routine laboratory or industrial settings.
Spectroscopic approaches, including Raman spectroscopy and other label-free analytical techniques, have attracted increasing attention as complementary tools for spore detection. These methods provide molecular and structural fingerprints based on intrinsic biochemical composition, enabling rapid, nondestructive analysis without requiring exogenous labeling [34,35]. Notably, Raman-based detection can capture signatures associated with dipicolinic acid and other spore-specific components, potentially facilitating highly specific identification [143]. Despite these advantages, challenges remain in terms of sensitivity, reproducibility, and data interpretation, particularly in complex matrices. Consequently, further efforts toward methodological standardization, spectral database development, and integration with advanced data analysis frameworks are required before these techniques can be translated into robust, field-deployable detection systems.

3.5. Emerging Rapid Detection Technologies

In recent years, significant progress in biosensor-based detection technologies has led to notable improvements in analytical sensitivity and substantial reductions in detection times. A diverse array of platforms, including electrochemical, optical, and microfluidic systems, has been actively explored for the detection of bacterial spores in complex food and environmental matrices [144,145,146]. Despite their technological sophistication and growing applicability, these systems generally require highly purified samples to achieve optimal performance. The presence of interfering substances, such as matrix-derived components, particulate debris, and residual vegetative cells, can adversely affect signal integrity, leading to diminished sensitivity and reduced specificity [133,134]. Therefore, it is crucial to carefully evaluate the characteristics and limitations of each detection approach and to select the most suitable method based on the specific analytical objectives and the type of data required (Table 2). In addition, many emerging spore detection platforms rely on specialized instrumentation, such as flow cytometry systems, fluorescence imaging devices, or microfluidic detection platforms. The high initial cost of these instruments, together with maintenance requirements and technical training, may limit their widespread implementation in routine laboratories, industrial quality control settings, or resource-limited environments.

4. Interdependence Between Purification and Detection

To highlight this interdependence, Figure 1 illustrates how purification choices shape spore quality attributes and subsequently propagate into detection outcomes. Although purification and detection strategies are often described independently, accumulating evidence indicates that these processes are inherently interconnected and should be considered within a unified analytical framework. Variations in purification efficiency can markedly influence downstream detection outcomes, including sensitivity, apparent resistance profiles, and quantitative accuracy. For instance, insufficient removal of vegetative cells may lead to overestimation of viable counts following heat treatment [113,114,115], while residual cellular debris can interfere with optical measurements and flow cytometric discrimination [135,136,137,138]. Furthermore, nucleic acids released from lysed cells may contribute to false-positive signals in molecular assays, thereby inflating estimates of spore abundance [30,31].
Purification strategies influence multiple aspects of spore quality, including purity, viability, structural integrity, and physiological state. These factors collectively determine detection performance across analytical methods. Residual contaminants and excessive purification can introduce bias, potentially leading to inaccurate quantification and misinterpretation of results.
Conversely, overly rigorous purification procedures may compromise spore integrity and viability, resulting in underestimation in culture-based enumeration methods [33]. These bidirectional effects highlight a fundamental trade-off between purity and biological representativeness, emphasizing the need for careful methodological optimization. From a broader perspective, the lack of standardized criteria for evaluating purification efficiency and its impact on detection performance remains a critical limitation across studies.
Future efforts should therefore focus on systematically linking purification conditions with detection outputs across multiple analytical platforms, thereby enabling the development of harmonized protocols and comparable datasets. Establishing such integrative frameworks will be essential for improving reproducibility and ensuring accurate interpretation of spore-related measurements in both research and applied contexts [23,24].

5. Discussion

Despite substantial advances in spore research, methodological standardization remains limited, and this represents a significant barrier to reproducibility and cross-study comparability. One of the most critical challenges lies in the absence of universally accepted criteria for defining and evaluating spore purity. Current studies employ diverse approaches, including microscopic assessment, heat resistance assays, and molecular confirmation—leading to inconsistencies that hinder direct comparison and meta-analytical integration. In addition, intrinsic physiological heterogeneity within spore populations further complicates both purification and detection. Even within a single preparation, spores may differ in maturation state, resistance properties, and germination potential. Such variability can significantly influence purification efficiency and detection sensitivity, particularly when analytical methods assume population uniformity and fail to account for functionally relevant subpopulations.
Matrix-associated effects constitute another persistent challenge. Complex sample matrices, such as food and environmental materials, often contain organic matter, particulate debris, and inhibitory compounds that interfere with both purification efficiency and detection accuracy. Although rapid detection technologies offer considerable promise, their performance frequently depends on highly purified samples and may decline under realistic sample conditions, thereby limiting their practical applicability. For example, in dairy matrices, fats and proteins can co-purify with spores and increase background signals in optical or flow-cytometric analyses, and they may also reduce DNA extraction efficiency and/or inhibit PCR-based assays [30,31,127,128]. In soil and other environmental samples, humic substances and fine particulates can inhibit amplification and complicate discrimination due to overlapping particle characteristics, unless additional cleanup or density-based separation steps are implemented [60,102].

6. Future Directions

Addressing these challenges will require coordinated efforts toward methodological harmonization. Priority should be given to establishing standardized criteria for evaluating spore purity and recovery efficiency, alongside systematic studies that clarify how different purification strategies impact downstream detection performance across analytical platforms. Furthermore, the integration of complementary detection approaches may improve overall robustness and reliability, particularly for rapid and quantitative applications. Advances in analytical technologies, including automation and high-throughput systems, are expected to improve reproducibility; however, their effectiveness will depend on rigorous validation across diverse sample types. Finally, the establishment of consensus benchmarks, such as defined contamination thresholds and viability criteria, will be critical for enabling meaningful cross-study comparisons and driving progress toward greater methodological consistency in spore research.

7. Conclusions

The reliability of bacterial spore research is fundamentally dependent on the accuracy and consistency of purification and detection methodologies. Despite the availability of diverse analytical approaches, methodological variability and the lack of standardization continue to limit reproducibility and hinder comparability across studies. Importantly, purification and detection are not independent processes but are closely interconnected, with each step exerting a direct influence on overall analytical outcomes.
Accordingly, future progress in the field will depend on the adoption of integrated experimental frameworks that consider purification and detection as interdependent components of a unified workflow. The establishment of clear evaluation criteria, coupled with increased methodological transparency, will be essential for improving data reliability. Such efforts will ultimately support more robust and reproducible investigations of bacterial spores, thereby advancing research and applications in food safety, environmental microbiology, and related applied disciplines.

Funding

This research was funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI, grant number 25K24227. The APC was funded by the Toyo Institute of Food Technology.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

The author thanks the members of Toyo Institute of Food Technology for their helpful advice and suggestions throughout the study.

Conflicts of Interest

The author declares no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
APCArticle Processing Charge
CFUColony-Forming Unit
DSMDifco Sporulation Medium
DNADeoxyribonucleic Acid
EPSExtracellular Polymeric Substances
PCRPolymerase Chain Reaction
qPCRQuantitative Polymerase Chain Reaction

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Applsci 16 05702 g001
Figure 1. Conceptual framework illustrating the interdependence between spore purification and detection.
Figure 1. Conceptual framework illustrating the interdependence between spore purification and detection.
Applsci 16 05702 g001
Table 1. Semiquantitative comparison of representative spore purification methods based on commonly reported characteristics in the literature.
Table 1. Semiquantitative comparison of representative spore purification methods based on commonly reported characteristics in the literature.
MethodsPurityYieldTimeCostConditionsSpore
Damage
Wash/
centrifuge		+++++++++++*Minimal
Density
gradient		++++++**Minimal
Enzymatic+++++++++***High
Abbreviations: Wash/centrifuge, repeated washing and centrifugation; Density gradient, density gradient centrifugation (e.g., Percoll or Nycodenz); Enzymatic, enzymatic treatment (e.g., lysozyme). * Sterile distilled water or PBS; centrifugation at 3000–10,000× g for 5–15 min at 4 °C or room temperature; repeated 3–10 times. This step was performed at 4 °C to minimize protein denaturation and autolysis. ** Density gradient centrifugation using Percoll or Nycodenz; 5000–20,000× g for 15–30 min. *** Lysozyme treatment (0.1–1.0 mg/mL), followed by centrifugation at 5000–20,000× g for 15–30 min at 37 °C. Note: Purity indicates the relative proportion of spores to residual vegetative cells or debris. Yield refers to spore recovery efficiency following purification. Time represents the relative duration of the procedure, and cost reflects the relative requirement for reagents and specialized equipment. Scoring was assigned as follows: +++, high; ++, moderate; and +, low.
Table 2. Comparison of methods for the detection and enumeration of bacterial spores.
Table 2. Comparison of methods for the detection and enumeration of bacterial spores.
MethodsSensitivitySpecificityTimeQuantitative
Capability		AdvantagesLimitations
Culture-based enumeration +++++++++Low costTime-consuming
Microscopy +++++++RapidLow quant accuracy
Endospore staining +++++++Visual discriminationDebris interference
PCR/qPCR+++++++++++RapidViable not distinguished
Flow
cytometry		++++++++++++High throughputInstrument- dependent
Notes: Methods were compared semiquantitatively in terms of sensitivity, specificity, time, and quantification capability. Symbols indicate relative performance: +++, high/excellent; ++, moderate; and +, low/limited. Reported “advantages” and “limitations” summarize typical practical considerations.
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Kawai, S. Purification and Detection of Bacterial Endospores: Current Methods and Challenges. Appl. Sci. 2026, 16, 5702. https://doi.org/10.3390/app16115702

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Kawai S. Purification and Detection of Bacterial Endospores: Current Methods and Challenges. Applied Sciences. 2026; 16(11):5702. https://doi.org/10.3390/app16115702

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Kawai, Souichirou. 2026. "Purification and Detection of Bacterial Endospores: Current Methods and Challenges" Applied Sciences 16, no. 11: 5702. https://doi.org/10.3390/app16115702

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Kawai, S. (2026). Purification and Detection of Bacterial Endospores: Current Methods and Challenges. Applied Sciences, 16(11), 5702. https://doi.org/10.3390/app16115702

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