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In-Depth Information
suggested that the low permeability of this membrane to small molecules may be an
important factor in spore resistance to chemical agents such as chlorine dioxide,
hypochlorite, hydrogen peroxide, and other oxidizing agents (Setlow 2006). Indeed,
increases in the permeability of this membrane result in increased sensitivity of spores
to a variety of chemical agents that kill spores by acting on the spore core (see below;
Cortezzo and Setlow 2005). To date the precise physical basis for the lipid immobility
in and low permeability of the spore's inner membrane are not known. However, it
seems likely that these two phenomena are related.
The fi nal spore layer is the central spore core, the site of the spore's DNA, tran-
scription and translation machinery, and most spore enzymes. Novel features of the
spore core include:
1. A relatively low water content. Growing cells have
80% (wet weight) as water,
but this can be as low as 25% in spores, although other spore layers have the normal
high water content.
2. An extremely high level (
∼
25% of dry weight) of pyridine - 2,6 - dicarboxylic acid
(common name:
dipicolinic acid, DPA
) in a 1 : 1 chelate with divalent cations,
primarily Ca
2+
(Gerhardt and Marquis 1989).
3. A pH 1-1.5 units below that of growing cells (Magill and others 1994, 1996). The
maximum removal of water from the core takes place late in spore formation,
largely concomitant with the accumulation of DPA by the spore, and
∼
∼
90 min after
the decrease in the developing spore's pH (Magill and others 1994).
The DPA and chelated cations are excreted early in spore germination, and the core
water content and pH also rise rapidly during this period to the level found in growing
cells (Setlow 2003). Available evidence indicates that the low core water content, low
pH, and high DPA and divalent cation contents play only a minor role in spore resis-
tance to chemicals, although more work is needed in this area (Setlow 2006).
Other unique components of the spore core are a group of novel small, acid-soluble
spore proteins (SASP) of the
-type that are named after the major proteins of this
type found in
B. subtilis
spores (Setlow 2006, 2007). The
α
/
β
- type SASP are the
products of a multigene family of up to 10 members, found in both
Bacillus
and
Clostridium
spores, synthesized only in the developing spore late in sporulation, and
the sequences of these proteins are highly conserved both within and across species,
but show no obvious sequence homology with other proteins. The
α
/
β
- type SASP are
DNA-binding proteins, and there is suffi cient protein of this type to saturate the spore's
DNA. This DNA binding provides tremendous protection to the DNA against heat,
desiccation and UV radiation, and to some genotoxic chemicals such as hydrogen
peroxide, formaldehyde, and sodium nitrite, although not against DNA alkylating
agents (Setlow 2006). Indeed, spores lacking the majority of their
α
/
β
α
/
β
- type SASP
−
spores) are much more sensitive to hydrogen peroxide, sodium nitrite, and
formaldehyde than are wild-type spores (Setlow 2006). The key role of the
−
(
α
β
- type
SASP in DNA protection in spores is shown most strongly by the fact that wild-type
B. subtilis
spores are not killed by hydrogen peroxide through damage to spore DNA,
although hydrogen peroxide treatment kills
α
/
β
−
−
spores by DNA damage (Setlow and
α
β
Setlow 1993 ).
