Agriculture Reference
In-Depth Information
There is one other group of proteins in the spore core that could also possibly
contribute to spore resistance. These are enzymes that can inactivate toxic chemicals,
and include catalases and superoxide dismutases that can inactivate hydrogen peroxide
and superoxide, respectively. However, while these enzymes are present in spores,
including at least one spore-specifi c catalase, it appears most likely that these enzymes
do not play a role in spore resistance (Casillas-Martinez and Setlow 1997; Setlow
2006). The most likely reason for the lack of effect of these enzymes in dormant spore
resistance is that all spore core enzymes appear to be inactive, probably due to the
low level of core water (Setlow 2006). Indeed, a normally soluble protein is immobi-
lized in the dormant spore core, although it becomes freely mobile early in spore
germination when the spore core's water content rises to that of a growing cell (Cowan
and others 2003 ).
Mechanisms of Spore Killing by Chemical Agents
Just as there are multiple factors that contribute to spore resistance against different
chemical agents, there are also different mechanisms of spore killing by different
chemicals as well, including DNA damage, inactivation of spore germination proteins,
damage to the spore's inner membrane, and, perhaps, inactivation of spore core
enzymes.
DNA Damage
One group of chemicals, including formaldehyde, nitrous acid, and DNA alkylating
agents, clearly kills spores by DNA damage, because survivors of treatments with
these chemicals have a high level of mutations, and spore resistance to these agents
is greatly decreased by the loss of DNA repair proteins (Setlow 2006). For the chemi-
cals that kill spores by DNA damage, the spore coats provide minimal protection,
while the low permeability of the spore's inner membrane appears to be important in
protection (Cortezzo and Setlow 2005; Setlow 2006). For some DNA-damaging
chemicals (hydrogen peroxide, nitrous acid, formaldehyde), the
α
/
β
- type SASP are
extremely important in spore resistance, but
-type SASP are not important in pro-
tection against DNA alkylating agents (Setlow 2006). The analysis of a recently
determined high-resolution structure of a DNA saturated with
α
/
β
- type SASP has now
provided the structural rationale for the differences in DNA protection against the
effects of different DNA reactive chemicals by
α
/
β
-type SASP binding (Lee and others
2008). However, in some cases the specifi c DNA damage leading to spore death is
not known.
α
/
β
Spore Germination Disruption
There are also at least a few chemicals that cause spore death by destroying one or
more essential proteins of the spore germination apparatus (Setlow 2006). The best-
studied example of such a chemical is strong alkali. Incubation of
B. subtilis
spores
in 1 M NaOH quickly causes loss in spore viability on nutrient plates (Setlow and
others 2002). This loss in spore viability is due to the inability of the treated spores
to complete the process of spore germination, since the alkali treatment inactivates
the enzymes required to degrade the spore's PG cortex, an essential event in spore
germination (Setlow and others 2002; Setlow 2006). Consequently these spores appear
