Artificial Spores - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 144

Thus, the shells of spores have a hierarchy of layers, and each stratum has an
orthogonal function.
Dormant endospores sense the condition of the external environments,
and when circumstances become favorable for growth, the spores are con-
verted back into growing cells through the process called germination. 9
Germination is usually initiated by the presence of nutrients that bind to
receptors in the spore's inner membrane. Although the precise mechanism
has not been known so far, the following events take place: (1) release of
small molecules including ions (H 1 ,Zn 21 , and Ca 21 ) and dipicolinic acid
from the spore core; (2) hydrolysis of the peptidoglycan cortex; (3) water
uptake and expansion of the shrunk spore core.
Sporulation represents an intelligent strategy that protects cells tem-
porarily from nutritionally unfavorable local conditions using dormancy.
Spores can also relocate themselves using various hosts, wind, and water,
which can coincidentally move them to fruitful environments for their ger-
mination and the resumption of vegetative growth. 10
Similar protective shells are observed in the resting cyst of the
ciliate Maryna umbrellata whose hierarchical shells composed of glass
(i.e. silica) granules protect the cell from environmental stress, and also
play a significant role in initiating the excystment process for their
reproduction. 11
On the other hand, the Nature-inspired, cytocompatible formation of
ultrathin ( o 100 nm), tough artificial shells on the surfaces of nonspore-
forming cells shows the spore-like behavior that enhances the protection of
the encapsulated cells from stresses as well as chemically controlling their
metabolic activities. 12 These synthetic ''cell-in-nanoshell'' structures, desig-
nated ''artificial spores'', emulate the essential features of natural endo-
spores, as follows: (1) the growth or division of living cells is controlled to
some extent by tuning the thickness or physicochemical properties of the
artificial shells; (2) resistance to external stressors is enhanced, with in-
creased tolerance against malnutrition, osmotic pressure, enzymatic attack,
chemical stressors, or UV, which is lethal to native, uncoated cells. In this
chapter, the current status of artificial spores is described, and their appli-
cations are discussed.

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8.2 Criteria and Approaches for Fabrication of
Artificial Spores
Artificial spores are usually constructed by introducing robust and per-
meable nanometric shells onto the surfaces of individual living cells. The
minimum requirements for the artificial shells are as follows. (1) Mechanical
durability: an artificial shell requires some degree of mechanical durability
to withstand the force of cell division; to resist external physical stresses,
such as osmotic pressure and dehydration; and to maintain the original cell
structure under these stresses. (2) Selective permeability: permselectivity is
desirable to block foreign aggressors (e.g., lytic enzymes and macrophages),