Agriculture Reference
In-Depth Information
of total spore protein) may simply act as “reactive armor,” detoxifying reactive chemi-
cals before they penetrate to more sensitive and essential targets further within the
spore (Setlow 2006). Notwithstanding the important role of the spore coat in spore
resistance to some chemical agents, the coat plays a much smaller role in spore resis-
tance with other chemical agents, such as hydrogen peroxide, sodium nitrite, and DNA
alkylating agents, (Cortezzo and Setlow 2005; Riesenman and Nicholson 2000). Even
in the presence of chemicals for which the coat is known to play a large role in spore
resistance, coat-defective spores generally remain much more resistant to these chemi-
cals than are growing cells (Cortezzo and Setlow 2005; Riesenman and Nicholson
2000). Thus, either residual coat protein in coat-defective spores or some other spore
component plays major roles in spore resistance to such chemicals.
Underlying the coat layer is the spore's outer membrane. Insofar as is known this
membrane plays no major role in spore chemical resistance, and this membrane may
not even be an intact permeability barrier in mature spores (Setlow 2006). However,
there are no mutants that specifi cally lack the outer spore membrane, and there is also
no method for selectively removing this membrane without removing the spore coat.
Indeed, methods for chemically removing the spore coat also remove many compo-
nents of the outer membrane (Buchanan and Neyman 1986). Consequently, it is
theoretically possible that the outer membrane could play some role, although
presently not defi ned, in spore resistance to chemicals such as chlorine dioxide and
hypochlorite.
Beneath the outer membrane are two layers of peptidoglycan (PG), fi rst the cortex
and then the germ cell wall. The cortex contains the great majority of spore PG, and
cortical PG has a number of structural features different from the PG in growing cells
(Popham 2002). Although the cortex is almost certainly largely responsible for the
reduction in the water content of the spore core that takes place late in spore formation
(see below), it is not thought to play a major role in spore chemical resistance. The
cortex is degraded in the fi rst minutes of spore germination, and this degradation is
essential for the germinated spore to develop into a growing cell (Setlow 2003). The
second PG layer in spores is the germ cell wall (Popham 2002). The PG in this layer
comprises only a small fraction of total spore PG and its structure appears identical
to that of growing cell PG. The germ cell wall is not degraded during spore germina-
tion, and it becomes the cell wall of the outgrowing spore (Popham 2002).
The second spore membrane, the inner membrane, is under the germ cell wall.
Although the lipid composition of this membrane does not appear to be especially
novel, it contains a number of proteins not found in growing cells, including a number
of proteins needed for spore germination events (Setlow 2003). This membrane also
has some novel properties. First, lipids in this membrane appear to be largely immobile
as determined by measurement of the fl uorescence redistribution after photobleaching
of lipid probes in this membrane (Cowan and others 2004). However, shortly after
initiation of spore germination, these lipid probes become fully mobile in the inner
membrane. Second, this membrane appears to have an extremely low permeability in
the dormant spore, not only to small charged molecules, but also to small hydrophobic
molecules such as methylamine, and perhaps even to water (Cortezzo and others 2004;
Cortezzo and Setlow 2005; Westphal and others 2003). The low permeability of this
membrane is also lost shortly after the initiation of spore germination. It has been
