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Chapter 3 : Spores and Their Significance
Category: Applied and Industrial Microbiology; Food Microbiology
This chapter describes the fundamental basis of sporulation and problems that spores present to the food industry. The first obvious morphological event in sporulation is an unequal cell division. One purpose of the chapter is to highlight the state of knowledge of molecular mechanisms of sporulation, spore resistance and dormancy, and spore germination and outgrowth, and hopefully, provide a counterpoint to more applied aspects of this system. The chapter focuses on molecular mechanisms, most of which have been examined in Bacillus subtilis. The sporulating bacteria discussed in the chapter form heat-resistant endospores that contain dipicolinic acid (DPA) and are refractile or phase bright under phase-contrast microscopy. Most studies on sporulation, spores, and spore germination have been carried out with species of either the aerobic bacilli or the anaerobic clostridia. The discussion of gene expression control mechanisms has been simplified and concentrates on major regulatory gene products. As detailed mechanistic data is available for B. subtilis, the discussion on spore resistance is concentrated on B. subtilis. The most effective way to kill pressure-germinated spores is by heat, and thus pressure treatments are often carried out at elevated temperatures. In the anaerobic growth environment of clostridia, transition metals would be expected to have important roles in the sporulation and resistance properties of spores. The scientific investigation of sporeformers has greatly contributed to the development of microbiology for the enhancement of food safety and quality.
Key Concept Ranking
- Cell Wall Components
Structure of dipicolinic acid. Note that at physiological pH both carboxyl groups will be ionized.
Morphological, biochemical, and physiological changes during sporulation of a rod-shaped Bacillus cell. In stage 0, a cell with two nucleoids (N) is shown; in stage IIi the mother cell and forespore are designated MC and FS, respectively. Note that the forespore nucleoid is more condensed than that in the mother cell. Stage IIii is not shown in this scheme, and the forespore nucleoid is not shown after stage III for clarity. The time of some biochemical and physiological events, such as forespore dehydration and acquisition of types of resistance to different chemicals (all lumped together as “chemical resistance”), stretches over a number of stages. The data for this figure are taken from references 28 and 136 .
Regulation of gene expression during sporulation. The effect of Spo0A∼P on repressors is negative; other effects of regulatory molecules on reactions are generally positive, although the effect of signals may be positive or negative. The enclosure of the pro-σ factors and σ factors denotes that at this time these factors are inactive. This figure is adapted from that in reference 136 .
Structure of a dormant spore. The various structures are not drawn precisely to scale, especially the exosporium, whose size varies tremendously between spores of different species. The relative size of the germ cell wall is also generally smaller than that shown. The positions of the inner and outer forespore membranes, between the core and the germ cell wall and between the cortex and coats, respectively, are also noted.
Structures of (A) cyclobutane-type TT dimer and (B) 5-thyminyl-5,6-dihydrothymine adduct (spore photoproduct). The positions of the hydrogens noted by the asterisks are the locations of the glycosylic bond in DNA.
Correlation of spore heat resistance and protoplast (core) water content of lysozyme-sensitive spore types from seven Bacillus species that vary in thermal adaptation and mineralization. Reprinted from Gerhardt and Marquis ( 38 ) with permission. The numbers refer to spores of various species: 1, G. stearothermophilus; 2, “Bacillus caldolyticus”; 3, Bacillus coagulans; 4, B. subtilis; 5, B. thuringiensis; 6, B. cereus; and 7, Bacillus macquariensis. The letters denote the sporulation temperature or the mineralization of the spores of various species as described in the original publication.
Spore activation, germination, and outgrowth. The events in activation are not known, hence the question mark. The loss of the spore cortex and the hydration and swelling of the core are shown in the germinated spore. The figure is adapted from Fig. 3 in reference 134 .
Transmission electron micrograph (×50,000) of a longitudinal section through a spore and sporangium of C. botulinum type A, showing the characteristic club-shaped morphology.
Electron micrographs of C. botulinum type B (A) and type E (B) showing characteristic exosporium in types B and E and appendages in type E. Micrographs courtesy of Philipp Gerhardt from spores produced in E.A.J.’s laboratory.
Small molecules in cells and spores of Bacillus species
Killing and mutagenesis of spores and cells of B. subtilis by various treatments a
Heat resistance of B. subtilis spores prepared at different temperatures with different ions and with or without α/β-type SASP a
Heat resistance of sporeformers of importance in foods a
Growth requirements of sporeformers of public health significance
Spoilage of canned foods by sporeformers a