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Unformatted text preview: . The lysogenic
state can be disrupted and the phage can enter a lytic cycle; this is called lysogenic
induction. During lysogenic induction, the expression of phage replication and lysis genes
is activated, more phage particles are produced and the bacterial host cell is lysed.
Lysogenic induction is usually triggered when the bacterium encounters stressful
conditions and there is a chance that the bacterium - and thus the prophage - may die. For
example, exposure to UV light is one way to trigger lysogenic induction. Quorum sensing The expression of toxin genes in diverse bacteria is often regulated by population density.
Why? A pathogenic bacterium that secretes a toxin may have no effect as a single
organism so the production of the toxin by it alone would be a waste of resources.
However, if there is a sufficiently high population density of similar organisms present,
the coordinated expression of the toxin may successfully initiate disease. Another reason
for using a quorum-sensing mechanism may be to prevent alerting the host that it is under
attack before enough microbes can accumulate through reproduction. Tripping the host's
alarms too early would make eliminating infection easier. In contrast, releasing toxins
once a large number of bacteria have amassed could overwhelm the host. 9.4 Endospores and food-poisoning
Endospores are a type of resting cell (Chapter 3) very important in food-bourne
formation Endospores are formed almost exclusively by the Gm +ve soil bacteria Bacillus and
Clostridium. Endospores are elliptical in shape and derive their name from the fact
they are produced inside a normal vegetative cell and are released when this cell
ruptures. Formation of an endospore (sporulation) is triggered when bacteria are starved
for nutrients. This process involves the synthesis of new proteins in response to
environmental conditions and is thus an example of environmentally-regulated gene
expression (Chapter 3). FOOD-288 Figure 9-4: Endospore formation Sporulation is a complex process which involves a coordinated developmental
plan (Figure 9-4). It begins with duplication of the chromosome and cell fission directed by
a polar FtsZ ring yielding the forespore which will ultimately become the spore, and the
larger mother cell, from which it is derived (= asymmetric division).
Following forespore formation, the mother cell membrane engulfs the forespore creating
an additional membrane around the forespore. The outer forespore membrane, derived from
the mother cell’s membrane, has an opposite orientation to that of the forespore inner
membrane, so the outside surfaces of both membranes faces the space between them. This
allows both the forespore and the mother cell to use their membrane transport proteins to
transport materials into this space.
The forespore and mother cell “cooperate” to synthesize a cell wall made of PG, the
thick cortex made of modified PG, the spore coat made of tough layer of protein and the
exosporium. Simultaneously with cortex formation large amounts of sm...
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- Fall '12