When the availability of carbon, ni

When the availability of carbon, nitrogen, and phosphorus necessary for
growth is limited, many species of bacteria (e .g., aerobic Bacillus, the
anaerobic Clostridium, as well as a few Sporosarcina and Actinomycetes)
produce dehydrated elements, or endospores that contain the essential
cellular components (Leive and Davis, 1980). Bacterial endospores show no
metabolic activity and are well adapted for prolonged survival under adverse
conditions.
In the case of B. subtilis, the metamorphosis starts with the formation of
a "sporangium" whose cellular volume contains the mother cell and the
prespore. An asymmetric septation creates two unequal compartments, with
each one receiving a copy of the bacterial chromosome, but evolving
differently in accordance with the differential expression of their genomes.
The prespore behaves as a germinal cellule that will assure the survival of
a clone when the mother cell whose function is to assure the maturation of
the endospore has completed its role . A chromosomic rearrangement
(Stragier et al., 1989) produces a new sequence and is one of the major
determinants of the metamorphosis in the mother cell. The product of this
new composite gene is a protein called a'27 (or sigma K) and is one of the
subunits of RNA polymerase, regulating protein, at the transcribing stage in
cellular development.
TIle structure of an endospore is represented in Figure 4.1. TIle
bacterial DNA is bicatenary, circular, not rearranged, folded in loops more
than 800 times on itself, and lies in the dense cytoplasm, slightly hydrated,
THERMOBACTERIOLOGY 119
A
Prespore
Mother cell
Endospore
DNA Replication
B
Condensed nucleus
Plasmalella
Internal wall
Cortex
Envelope
'-----Exosporium
Figure 4.1. Stages in the maturation of a clostridial spore (A) and schematic view
of a mature bacterial endospore (B). (From: La Conserve appertisee:
aspects scientifiques, techniques et economiques by Larousse, Copyright
1991 by Technique et Documentati on - Lavoisier, Paris. Used with
permission.)
rich in ribosomes and transfer RNA, but deprived of messenger RNA (Leive
and Davis, 1980). Amino acids are virtually absent. The residual energy is
sto red in the form of mono- and diphosphate ribonucleosides, but especially
3-phosphoglyce rol. Due to a cyclic activation and reciprocal repression of
a number of operons, only 20% of the genes are expressed in the prespore,
which results in the absence of the Krebs cycle enzymes.
In the bacterial cell, the two genomes are separated by invagination of
the plasmalemma (Ryter, 1982). The oval prespore is isolated in the center
of a double membrane within which th e spore wall will be developed. The
120 FOOD CANNING TECHNOLOGY
wall provides an irreversible osmotic stabilization to the endospore and the
elements necessary for the germination. The cortex, formed around the
fourth hour of the transformation and representing 15% of the spore mass,
contains very elevated calcium dipicolinate levels , a substance that activates
the dehydration of the prespore cytoplasm in the course of its maturation
(Wainwright, 1988). The muramic acid of the cortex is bridged by dipeptide
links (Leu-Ala) instead of a vegetative tetrapeptide. The resulting threedimensional
structure is very serrated. The very thick coat is rich in keratin,
which shows a number of disulfide bridges and represents 80% of the total
protein content of the spores.
The mother cell is responsible for the formation and placement of the
most external coat (exosporium), which provides the extraordinary resistance
of these endospores to a number of chemical products. The exosporium is
a complex of lipoproteins that comprises 20% of glucosidic compounds and
requires 90 minutes for completion. While not essential for survival, it
appears to be implicated in the germination process.
Spore germination is initiated under the influence of a stress (e.g., heat
shock, pH deviation, SH compounds, etc.). A storage protein of low
molecular mass in the cytoplasm is hydrolyzed in 10 minutes and charges the
transfer RNA. The coat starts to autolyze and becomes more and more
permeable to water and to various activating solutes such as alanine or
manganese ions (Leive and Davis, 1980). The peptiglucides are hydrolyzed
and the cortical barrier collapses. The spore cell rapidly absorbs water, and
some ions and simultaneously loses its refractivity and its thermal resistance.
The phosphoglyceric acid is then converted to ATP.
The final stage, if the appropriate environmental physicochemical
conditions permit, involves the return to the normal vegetative form in which
synthesis of messenger RNA is reestablished, promoting active protein
synthesis. DNA replication is initiated, and the cellular volume is doubled
I hour later