Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL STRESS RESPONSE IN THE FOOD ENVIRONMENT 111
In Gram-positive and -negative bacteria, freezing and drying cause changes in
cell surface hydrophobicity and the inability to form compact pellets and to adsorb
some phages. In Gram-positive bacteria, surface layer proteins are also lost. In
sublethally stressed Gram-negative bacteria, the lipopolysaccharide (LPS) layer
undergoes conformational alteration and loses its barrier property to many chemicals
(such as SDS, bile salts, antibiotics, lysozyme, and RNase), which can easily enter
the injured cells and kill them. Very little LPS is lost from the cells into the
environment. The alteration in conformation of LPS is due to loss of divalent cations,
which are necessary for the normal stability of LPS. In both Gram-positive and -
negative cells, the cytoplasmic membrane (or IM) remains intact in injured cells,
but they lose their permeability barrier function. The cells become sensitive to NaCl
and also lose different cellular materials. It is suggested that protein molecules in
this structure probably undergo conformational changes in the injured cells. rRNA
is extensively degraded by the activated RNase. DNA can undergo single- and
double-strand breaks. In some strains, autolytic enzymes can be activated by stress,
causing lysis of the cells. 1–3,14
In bacterial spores, depending on the type of sublethal stress, different structural
and functional components can be injured. High heat causes damage to the lytic
enzymes necessary for the lysis of cortex before spore germination and to the spore
membrane structures, causing loss of permeability barrier functions. Damages by
irradiation (UV and g) are mainly confined to DNA in the form of single-strand
breaks, and by some chemicals (H 2O 2, antibiotics, or chlorine) to the lytic enzymes
of the germination system. Hydrostatic pressure, in combination with heat, damages
cortex, whereas g-irradiation damages both cortex and DNA. Milder to strong acid
treatments can cause the spores to become dormant by removing Ca 2+ from the
spores and making them sensitive to heat. 2,3
D. Repair of Reversible Injury
One of the most important characteristics of injured bacterial cells is the ability to
repair the injury in a suitable environment and become similar to normal cells. The
repair process and the rate of repair can be measured by specific methods. One of
them is to measure regain in resistance of injured cells to the surface-active agents
by repair in the cell wall or the OM. Suspending a sublethally stressed population
in a repair medium and simultaneously enumerating the colony-forming units
(CFUs) during incubation in nonselective and selective plating media help determine
the rate of repair (Figure 9.3). Initially, the injured survivors fail to form colonies
in the selective, but not in the nonselective, media. However, as they repair and
regain resistance to the selective agent, they form colonies on both media, as indicated
by the increase in counts in the selective media only. Injured cells differ in
the levels of injury (from low to high), as manifested from the differences in recovery
time in a suitable medium. 1–3,14,15
The injured cells can repair in a medium devoid of selective compounds but
containing the necessary nutrients during incubation at optimum pH and temperature.
In general, the cells repair well in a medium rich in metabolizable carbon and
nitrogen sources and several vitamins. Supplementation with catalase and pyruvate
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