Abstract
Bacterial spores are specialized cells that are exceptionally resistant to environmental stress. Spores convert back to actively growing cells, a process called germination, upon nutrient detection. The most common, initial step in the germination process is the recognition of small molecule germinants by germination (Ger) receptors. Ger receptors are inner-membrane heterocomplexes formed by three distinct protein products, the A-, B-, and C-subunits. In this review, we discuss and contrast published reports on representative Ger receptors from different Bacilli and Clostridia. We also present evidence for unrecognized germination pathways independent of Ger receptors. We further emphasize the function of L-alanine as a universal germinant. We also comment on biochemical aspects of germinant recognition and interaction between Ger receptor proteins. We propose that there are six general strategies used by Bacilli and Clostridia to integrate multiple germination signals. The use of different germinant recognition strategies results in germination response flexibility. Consequently, sporulating bacterial species that use the same biomolecules as germination signals can have different germination profiles. Finally, we discuss future directions for understanding the function of Ger receptors.
Highlights
The germination (Ger) receptor family Endospore-forming bacteria produce some of the most potent toxins known and are important pathogens in hospital-borne infections (Clostridium difficile) (Surowiec et al, 2006), food contamination (Bacillus cereus, Clostridium botulinum,)(Dembek et al, 2007 ; Ehling-Schulz et al, 2004), wound infestation (Clostridium perfringens, Clostridium tetani) (Edlich et al, 2003; Finsterer and Hess, 2007), and bioterrorism (Bacillus anthracis) (Mock and Fouet, 2001)
The spore’s durability is even more remarkable considering that dormant spores revert back to actively growing cells almost immediately after nutrients return to the environment (Moir, 2006)
Some Ger receptor subunits are encoded as orphan monocistronic genes (Peck and Robert, 2009) and yet other ger receptor operons encode duplicated subunit genes (Broussolle et al, 2002; Christie and Lowe, 2007)
Summary
The germination (Ger) receptor family Endospore-forming bacteria produce some of the most potent toxins known and are important pathogens in hospital-borne infections (Clostridium difficile) (Surowiec et al, 2006), food contamination (Bacillus cereus, Clostridium botulinum,)(Dembek et al, 2007 ; Ehling-Schulz et al, 2004), wound infestation (Clostridium perfringens, Clostridium tetani) (Edlich et al, 2003; Finsterer and Hess, 2007), and bioterrorism (Bacillus anthracis) (Mock and Fouet, 2001). These receptors interact in response to different germinant combinations to form four distinct amino acid and nucleoside dependent germination pathways (Fisher and Hanna, 2005). Structure activity relationship of nucleoside agonists and antagonists of B. cereus spore germination have shown that the GerI receptor is constrained to recognize a single germinant (inosine) and is inhibited in a predictable pattern by structurally related nucleosides.
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