Insights into the mechanisms of L. salivarius CECT5713 resistance to freeze-dried storage

Synergistic effect of high-pressure thermal sterilization and muramidase on Bacillus subtilis spores: alterations in intrasporal components, inner membrane permeability, and structural integrity

The sporicidal mechanism of high-pressure thermal sterilization (HPTS) combined with muramidase against Bacillus subtilis spores was investigated. Results demonstrated that HPTS at 600 MPa/75°C with 0.3% muramidase achieved a 6.09 log reduction in Bacillus subtilis spores. The combined processing significantly increased the leakage of protein, nucleic acid, and dipicolinic acid, while significantly reducing Na+/K+-ATPase activity (P < 0.05). Scanning electron microscopy revealed notable morphological changes in spores after combined processing. A significant increase in propidium iodide (PI)-infiltrated spores indicated enhanced spore inner membrane permeability (P < 0.05). molecular composition analysis further showed disordered arrangement of fatty acid acyl chains, structural alterations in nucleic acids and proteins, and increased the peptidoglycan layer flexibility. These findings provided insights into the sporicidal mechanism of HPTS combined with muramidase.

Methionine affects the freeze-drying resistance of Lactiplantibacillus plantarum LIP-1 by improving its antioxidant capacity

BACKGROUND

Lactic acid bacteria is an essential industrial strain, and improving its freeze-drying survival rate is the key challenge to ensuring the activity and stability of bacterial powder. Although medium optimization has been shown to strengthen strain freeze-drying tolerance, the mechanism by which amino acids repair freeze-drying damage in lactic acid bacteria remains unclear. This study investigated the effects of methionine on the freeze-drying survival rate of Lactiplantibacillus plantarum LIP-1 and explored the underlying protective mechanisms.

RESULTS

The study demonstrates that supplementing the medium with 0.06 g/L methionine significantly improved the freeze-drying survival rate of Lactiplantibacillus plantarum LIP-1 (P < 0.05). Further analysis revealed that the strain significantly reduced intracellular reactive oxygen species levels through metabolizing methionine (P < 0.05), decreased the oxidation degree of unsaturated fatty acids in the cell membrane, and reduced cell membrane damage, thereby strengthening the freeze-drying resistance of the strain.

CONCLUSION

Methionine can enhance the freeze-drying resistance of Lactiplantibacillus plantarum LIP-1 by enhancing antioxidant capacity and maintaining the stability of the subcellular structure. This study provides a specific reference value for improving the freeze-drying survival rate of lactic acid bacteria by modifying the medium conditions. © 2025 Society of Chemical Industry.

A Closer Look at the Potential Mechanisms of Action of Protective Agents Used in the Drying of Microorganisms: A Review

Yeast, bacteria and sourdough are widely used in our daily lives, yet their drying and storage remains a significant challenge. A variety of techniques are used to improve the resistance of cells to thermal, dehydration, oxidative and osmotic stresses, which can occur at different stages of the process. The addition of protective agents prior to drying is a commonly used method, but the mechanisms that may lead to a change in viability following the addition of these agents, or more generally, the interaction between a protective agent and the drying process, are not yet fully understood. This review outlines seven main potential mechanisms, as highlighted in the literature, which can lead to internal or external modifications of the cells. The mechanisms in question are change of membrane fluidity, accumulation of compounds for osmoregulation, prior osmotic dehydration, prevention of oxidation, coating or encapsulation, enhancement in thermal resistance and change in drying kinetics. A comprehensive explanation of these mechanisms is provided. This review also highlights the connection between the mechanisms and the influence of the stresses occurring during drying and storage, which depend on the drying technique used and the operating conditions, the strains and the protective agents involved, on the importance of the different protection mechanisms. By gaining a deeper understanding of the mechanisms of action of protective agents, strategies to improve the quality of the microorganisms obtained after drying can be developed. One such strategy would be to combine several agents to achieve a synergistic effect.

Investigation of Freezing and Freeze-Drying for Preserving and Re-Using a Whole Microbial Cheese Community

Preserving microbial ecosystems obtained from traditional cheese-making processes is crucial to safeguarding the biodiversity of microbial cheese communities and thus ensuring that the high flavor quality of traditional cheeses is maintained. Few protocols have been proposed for the long-term storage of microbial consortia. This work aimed to develop preservation methods to stabilize the entire microbial community in smear-ripened cheese without multiplication or isolation. A simplified microbial community, capable of reproducing the metabolic pattern of cheese maturation, was used in three independent cheese productions. Cheese samples were taken before and after the ripening step, mixed with maltodextrin or saline solution, and subjected to different stabilization conditions including freezing and freeze-drying, followed by 1 month of storage. Microbial survival was quantified using the colony-forming unit assay. Differential scanning calorimetry was used to relate the physical events occurring within the samples to the microbial storage stability. Freezing at -80 °C resulted in the lowest loss of culturability (<0.8 log unit), followed by freezing at -20 °C and freeze-drying. The ripening bacteria appeared as the most sensitive microorganisms within the community. Moreover, a successful cheese production using the best-stabilized community showed the possibility of preserving and re-using an entire microbial community of interest.

Mechanistic study of the differences in lactic acid bacteria resistance to freeze- or spray-drying and storage

Lactobacillus delbrueckii subsp. bulgaricus and Lactiplantibacillus plantarum are two lactic acid bacteria (LAB) widely used in the food industry. The objective of this work was to assess the resistance of these bacteria to freeze- and spray-drying and study the mechanisms involved in their loss of activity. The culturability and acidifying activity were measured to determine the specific acidifying activity, while membrane integrity was studied by flow cytometry. The glass transitions temperature and the water activity of the dried bacterial suspensions were also determined. Fourier transform infrared (FTIR) micro-spectroscopy was used to study the biochemical composition of cells in an aqueous environment. All experiments were performed after freezing, drying and storage at 4, 23 and 37 °C. The results showed that Lb. bulgaricus CFL1 was sensitive to osmotic, mechanical, and thermal stresses, while Lpb. plantarum WCFS1 tolerated better the first two types of stress but was more sensitive to thermal stress. Moreover, FTIR results suggested that the sensitivity of Lb. bulgaricus CFL1 to freeze-drying could be attributed to membrane and cell wall degradation, whereas changes in nucleic acids and proteins would be responsible of heat inactivation of both strains associated with spray-drying. According to the activation energy values (47-85 kJ/mol), the functionality loss during storage is a chemically limited reaction. Still, the physical properties of the glassy matrix played a fundamental role in the rates of loss of activity and showed that a glass transition temperature 40 °C above the storage temperature is needed to reach good preservation during storage. KEY POINTS: • Specific FTIR bands are proposed as markers of osmotic, mechanic and thermal stress • Lb. bulgaricus CFL1 was sensitive to all three stresses, Lpb. plantarum WCFS1 to thermal stress only • Activation energy revealed chemically limited reactions ruled the activity loss in storage.

Heterotypic stress-induced adaptive evolution enhances freeze-drying tolerance and storage stability of Leuconostoc mesenteroides WiKim33

Lactic acid bacteria (LAB) are currently being investigated for their potential use as probiotics and starter cultures. Researchers have developed powdering processes for the commercialization of LAB. Previous studies have focused on identifying innovative cryoprotective agents and freeze-drying (FD) techniques to enhance the stability of LAB. In this study, adaptive laboratory evolution (ALE) was employed to develop a strain with high FD tolerance and enhanced storage stability. Leuconostoc mesenteroids WiKim33 was subjected to heterotypic shock (heat and osmosis shock) to induce the desired phenotype and genotype. An FD-tolerant enhanced Leu. mesenteroides WiKim33 strain (ALE50) was obtained, which harbored a modified fatty acid composition and cell envelope characteristics. Specifically, ALE50 showed a lower unsaturated fatty acid (UFA)/saturated fatty acid (SFA) ratio and a higher cyclic fatty acid (CFA) composition. Moreover, the exopolysaccharide (EPS) thickness increased significantly by 331% compared to that of the wild type (WT). FD tolerance, which was evaluated using viability testing after FD, was enhanced by 33.4%. Overall, we demonstrated the feasibility of ALE to achieve desirable characteristics and provided insights into the mechanisms underlying increased FD tolerance.