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Chapter 18 : Resistance of Bacterial Spores
Category: Microbial Genetics and Molecular Biology
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This chapter discusses the resistance of spores of Bacillales and Clostridiales, with focus Bacillus species, in which spore resistance mechanisms are best understood, with most specific knowledge from work with B. subtilis spores. The spore coat plays a major role in spore resistance. First, some protective enzymes that are loosely associated with or integral components of the coat can inactivate toxic chemicals; two such enzymes are superoxide dismutase and catalase. Second, coat protein appears to act like ‘’reactive armor’’ detoxifying damaging chemicals before they can react with components in the spore’s interior. Spore structure is important in spore resistance and because spore structure is different than that of growing cells, the major features of spore structure and how these features contribute to spore resistance is outlined in the chapter. Spores of Bacillales and Clostridiales are more resistant than growing cells to stress factors and studies have attempted to correlate differences in spore resistance with differences in spore structural or biochemical properties. It is reasonable to ask whether conclusions from work on one or two species are applicable to spores of other species, available evidence indicates that basic mechanisms of spore resistance are similar in spores of all Bacillales and Clostridiales. Some resistance of core proteins to damage, in particular to chemicals, is likely because of mechanisms that also protect DNA, including detoxification of reactive chemicals by enzymes in the coat/exosporium, inactivation of toxic chemicals by reaction with coat components, and the low permeability of the inner membrane.
Schematic structure of a Bacillus /Clostridium spore. Spore layers are not drawn to scale, the exosporium is not present on spores of some species, and spores of some species have large appendages arising from the exosporium or coat.
Amino acid sequences of α/β-type SASPs from various Bacillales and Clostridiales. Sequences are from one of these proteins in each species and data are from the NCBI database or Gold Tables Online of ongoing bacterial genome sequencing projects. Amino acid residues are shown in the one letter code and the regions highlighted in gray are the two large helical regions in the proteins, the initial region being helix 1 and the second region helix 2. The emboldened Met residue is protected from modification upon α/β-type SASP binding to DNA, the emboldened amino acid doublet with Glu as the first residue is the site cleaved by the SASP-specific protease following spore germination, and the Asn residue in the emboldened NG doublet deamidates rapidly and DNA binding blocks deamidation. The abbreviations for the species are as follows. Bacillales: Afl, Anoxybacillus flavithermus; Ali, Alicyclobacillus acidocaldarius, Bam, B. amyloliquefaciens; Ban, B. anthracis; Bbe, Brevibacillus brevis; Bce, B. cereus; Bcl, B. clausii; Bco, B. coahuilensis; Bfi, B. firmus; Bha, B. halodurans; Bli, B. licheniformis; Bme, B. megaterium; Bpu, B. pumilus; Bsu, B. subtilis; Bth, B. thuringiensis; Bwe, B. weihenstephanensis; Gka, Geobacillus kaustophilus; Gst, G. stearothermophilus; Gth, G. thermodenitrificans; Lsp, Lysinibacillus sphaericus; Oih, Oceanobacillus iheyensis; Pjd, Paenibacillus JDR-2; and Pla, P. larvae. Clostridium species are: Cac, C. acetobutylicum; Cba, C. bartletii; Cbe, C. beijerincki; Cbo, C. botulinum; Cbu, C. butylicum; Cce, C. cellulolyticum; Cdi, C. difficile; Ckl, C. kluyveri; Cle, C. leptum; Cno, C. novyi; Cpe, C. perfringens; Cph, C. phytofermentans; Cra, C. ramosum; Csp, C. spiroforme; Cso, C. sporogenes, Cte, C. tetani; and Cth, C. thermocellum. Note also that many α/β-type SASPs sequences available from Clostridiales are not shown here.
Structure of dipicolinic acid (DPA). At physiological pH, both carboxyl groups will be ionized, allowing DPA to chelate divalent metal ions, in particular Ca2+.
Essential spore components that are targets for lethal damage a
Factors important in protecting essential spore components a
Resistance properties of spores and growing cells of B. subtilis strains a
DNA photoproducts from UV irradiated spores of various B. subtilis strains a