Mechanisms of Spore Heat Resistance. Advances in Microbial Physiology Volume 11. Elsevier; 1974. pp. 137-64. [DOI: 10.1016/s0065-2911(08)60271-4]

Fate of Clostridia and other spore-forming Firmicute bacteria during feedstock anaerobic digestion and aerobic composting

Pathogenic spore-forming Firmicutes are commonly present in animal and human wastes that are used as fertilizers in crop production. Pre-treatments of organic waste prior to land application offer the potential to abate enteric microorganisms, and therefore reduce the risk of contamination of crops or adjacent water resources with pathogens carried in these materials. The inactivation and reduction of gram-positive spore formers such as Clostridium spp., Clostridioides spp. and Bacillus spp. from animal and human waste can be challenging given the recalcitrance of the spores these bacteria produce. Given the significance of these organisms to human and animal health, information concerning spore-forming bacteria inactivation during anaerobic digestion (AD) and aerobic composting (AC) is required as the basis for recommending safe organic waste management practices. In this review, an assessment of the inactivation of spore-forming Firmicutes during AD and AC was conducted to provide guidance for practical management of organic matrices of animal or human origin. Temperature and pH may be the main factors contributing to the inactivation of spore-forming Firmicutes during batch lab-scale AD (log reduction <0.5-5 log). In continuous digesters, wet AD systems do not effectively inactivate spore-forming Firmicutes even under thermopholic conditions (log reduction -1.09 - 0.98), but dry AD systems could be a feasible management practice to inactivate spore-forming Firmicutes from organic materials with high solid content (log reduction 1.77-3.1). In contrast, composting is an effective treatment to abate spore-forming Firmicutes (log reduction 1.7-6.5) when thermophilic conditions last at least six consecutive days. Temperature, moisture content and composting scale are the key operating conditions influencing the inactivation of spore-forming Firmicutes during composting. Where possible, undertaking AD with subsequent composting to ensure the biosafety of digestate before its downstream processing and recycling is recommended to abate recalcitrant bacteria in digestate.

Inactivation of Bacillus cereus Spores on Stainless Steel by Combined Superheated Steam and UV-C Irradiation Treatment

Bacillus cereus spore contamination on food contact surfaces is of great concern in the food industry. Thus, in the present study, superheated steam (SHS) was used alone or combined with UV-C irradiation for inactivation of B. cereus spores inoculated on stainless steel coupons. Temperatures higher than 250°C were needed to effectively inactivate B. cereus spores by SHS treatment alone, while a synergistic bactericidal effect resulted from the sequential treatment of SHS before or after UV-C irradiation. The increased dipicolinic acid ratio obtained by the combined treatment had a significant role in the synergistic bactericidal effect. Therefore, the combined treatment of SHS and UV-C could be used effectively to inactivate B. cereus on stainless steel. It is recommended to use hurdle technology with reduced energy consumption to ensure microbiological safety on food contact surfaces.

Bacillus spore wet heat resistance and evidence for the role of an expanded osmoregulatory spore cortex

Previous work reported that decoated Bacillus cereus spores incubated in 4 mol l(-1) CaCl2 are killed at lower temperatures than spores in water. This wet heat sensitization was suggested to support a role for an osmoregulatory peptidoglycan cortex in spore cores' low water content, and their wet heat resistance. Current work has replicated this finding with spores of B. cereus, Bacillus megaterium and Bacillus subtilis. However, this work found that decoated spores apparently killed at 80°C in 4 mol l(-1) CaCl2 : (i) were recovered on plates containing lysozyme; (ii) lost no dipicolinic acid (DPA) and their inner membrane remained impermeable; (iii) released no DPA upon stimulation with nutrient germinants and could not complete germination; and (iv) released DPA relatively normally upon stimulation with dodecylamine. These results indicate that decoated spores treated with 80°C- 4 mol l(-1) CaCl2 are not dead, but some protein(s) essential for spore germination, most likely germinant receptors, are inactivated by this treatment. Thus, the original finding does not support a role for an osmoregulatory cortex in spore wet heat resistance.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacillus spores' low core water content is a major factor in their wet heat resistance. One suggested mechanism for achieving low spore core water content is osmoregulated expansion of spores' peptidoglycan cortex. Evidence for this mechanism includes a report that decoated Bacillus cereus spores incubated in 4 mol l(-1) CaCl2 exhibit drastically reduced heat resistance. The current work shows that this heat sensitization of decoated spores of three Bacillus species is most likely due to inactivation of some crucial spore germination protein(s), since while treated spores appear dead, their apparent low viability is rescued by triggering spore germination with lysozyme.

Two-dimensional multiwavelength fluorescence spectra of dipicolinic acid and calcium dipicolinate

Dipicolinic acid (DPA) and the Ca2+ complex of DPA (CaDPA) are major chemical components of bacterial spores. With fluorescence being considered for the detection and identification of spores, it is important to understand the optical properties of the major components of the spores. We report in some detail on the room-temperature fluorescence excitation and emission spectra of DPA and its calcium ion complex and provide a comparison of the excitation-emission spectrum in a dry, wet paste and aqueous form. DPA solutions have weak, if any, fluorescence, with increased fluorescence when the DPA is dry. After exposure to a broad source UV light of the DPA, wet or dry, we observe a large increase in fluorescence with a maximum intensity emission peak at around 440 nm for excitation light with a wavelength of around 360 nm. There is a slight blueshift in the absorption spectra of UV-exposed DPA from the unexposed DPA solution. CaDPA in solution shows a slight fluorescence with increased fluorescence in the dry form, and a substantial increase of fluorescence was observed after UV exposure with an emission peak of around 410 nm for excitation around 305 nm. The detailed excitation-emission spectra are necessary for better interpretation of the fluorescence spectra of bacterial spores where DPA is a major chemical component.

Effects of hydration on molecular mobility in phase-bright Bacillus subtilis spores

The molecular mobility of 31P and 13C in dormant Bacillus subtilis spore samples with different water concentrations was investigated by high-resolution solid-state NMR. Lowest molecular mobility was observed in freeze-dried preparations. Rehydration to a 10% weight increase resulted in increases in molecular motions and addition of excess water furthered this effect. A spore slurry which had been freeze-dried displayed after addition of excess water similar NMR spectra to native wet preparations. Dipicolinic acid (DPA), which is mainly located in the core, was detected at all hydration levels in 13C cross-polarization magic angle spinning (CPMAS) but not in single-pulse magic angle spinning (SPMAS) spectra, indicating that hydration had no effect on its mobility. The molecular mobility of 31P, present mainly in core-specific components, was strongly dependent on hydration. This result suggests reversible water migration between inner spore compartments and the environment, whereas 13C spectra of DPA indicate that it is immobilized in a water-insoluble network in the core. Scanning transmission electron microscopy revealed that freeze-dried spores were significantly longer and narrower than fully hydrated spores and had a 3% smaller volume.

Interaction of Bacillus subtilis spores with sodium hypochlorite, sodium dichloroisocyanurate and chloramine-T

Solutions of chlorine-releasing agents (CRAs) show varying activity against Bacillus subtilis spores; sodium hypochlorite (NaOCl) shows higher activity than sodium dichloroisocyanurate (NaDCC) which is more active than chloramine-T. Investigations with coat- and cortex-extracted spores indicate that resistance to CRAs depends not only on the spore coat but also the cortex. Whereas extraction of alkali-soluble coat protein increased sensitivity to NaOCl and NaDCC, degradation of coat and cortex material was required to achieve significant activity with chloramine-T. NaOCl (in the presence and absence of NaOH) and NaDCC (in the presence of NaOH only) produced degradation of spore coat and cortex material which may be related to their rapid sporicidal action at low concentrations under these conditions. By contrast, chloramine-T produced no degradation of cortex peptidoglycan and was only effective against normal and alkali-treated spores at high concentrations, requiring extraction of peptidoglycan with urea/dithiothreitol/sodium lauryl sulphate (UDS) or UDS/lysozyme to achieve significant activity at low concentrations. Results suggest that the sporicidal action of CRAs is associated with spore coat and cortex degradation causing rehydration of the protoplast allowing diffusion to the site of action on the underlying protoplast.

Relation of the heat resistance of bacterial spores to chemical composition and structure. II. Relation to cortex and structure

The relation between the amount of cortex, measured as total hexosamine, as diaminopimelic acid and as muramic lactam, and the heat resistance of spores of five different strains of Bacillus stearothermophilus was studied. Electron micrographs of thin sections of the spores were made to relate the structure of the spores to chemical and thermal characteristics. It was found that the amount of the cortex was significantly related to heat resistance of the spores. Strains with more electron-dense and better organized cortices were found to express higher heat resistance.

Properties of Hg- and Cd-spores of Bacillus megaterium QM B1551

Hg- and Cd-spores of Bacillus megaterium QM B1551 were produced in Schaeffer's medium containing mercuric chloride and cadmium chloride respectively. Metals were added to the medium at 9 hr of incubation (Stage V) to give a final concentration of 50 microM. It was found by electron microscopic and biochemical studies that the coats of both Hg- and Cd-spores were thinner than those of control spores. Of the total Hg and Cd in the spores, 77% of the Hg and 63% of the Cd were detected in the spore coats. Hg- and Cd-spores were less resistant to heat and more sensitive to germinants than control spores. Other properties of Hg- and Cd-spores were similar to those of control spores. These results suggest that the spore coat has some relationship to the heat resistance and germinability of spores.

Cortex content of asporogenous mutants of Bacillus subtilis

A method for the measurement of muramic lactam, which is specifically located in the cortical peptidoglycan of bacterial spores, was developed as a quantitative assay method for spore cortex content. During sporulation of Bacillus subtilis 168, muramic lactam (i.e., spore cortex) began to appear at state IV of sporulation and continued to increase over most of the late stages of sporulation. Spore cortex contents of various spo mutants of B. subitils were surveyed. Cortex was not detected in mutants in which sporulation was blocked earlier than stage II sporulation. Spores of spo IV mutant had about 40% of the cortex content of the wild-type spores. One spo III mutant had a low amount of cortex, but four others had none.

Relationship between cortex content and properties of Bacillus sphaericus spores

The muramic lactam content of spores of Bacillus sphaericus mutants defective in meso-diaminopimelic acid synthesis increases almost linearly with an increase of meso-diaminopimelic acid concentration in the medium. Since muramic lactam content is a measure of cortex content, the amount of cortex in spores of the mutants can be easily varied by changing the meso-diaminopimelic acid concentration in the medium. Characteristic properties were tested in spores containing different amounts of cortex. Critical amounts of cortex were associated with different spore properties. Refractility and dipicolinic acid accumulation in the spores both required about 20% of the maximum cortex content (although refractility is independent of dipicolinic acid content). For xylene octanol resistance, about 25% of the maximum cortex content was required.

Heat resistance of bacterial endospores and concept of an expanded osmoregulatory cortex

The extreme resistance of bacterial endospores to heat may result from dehydration of the central protoplast brought about and maintained by osmotic activity of expanded electronegative peptidoglycan polymer, and positively charged counterions associated with it, in the surrounding cortex. The cortex may thus act as a specialised osmoregulatory organelle. Changes in the environment which would be expected reversibly to affect osmotic properties alter the heat resistance of spores.