Fundamental Food Microbiology, Third Edition - Fuad Fathir

106 FUNDAMENTAL FOOD MICROBIOLOGY
preservation methods, results of studies developed with normal cells (not stress
adapted) may not be effective to control or kill stress-adapted foodborne pathogens
and spoilage bacteria. To overcome this problem, it is necessary to understand the
underlying mechanisms that confer resistance to stress-adapted cells and develop
methods to control them. 10
B. Mechanisms of Stress Adaptation 10,11
An earlier theory was that bacterial cells cope with stress (such as temperature stress)
by changing the lipid composition of the cytoplasmic or inner membrane so that the
fluid state is maintained. Thus, at lower or higher ranges of growth temperature,
membrane lipid accumulates more low-molecular-weight and unsaturated fatty acids
but at optimum growth temperature accumulates more high-molecular-weight saturated
fatty acids. In recent years, stress adaptation by bacterial cells (and other
microorganisms) is viewed to be mediated through the synthesis of many types of
shock proteins or stress proteins, some of which are specific for specific stress
whereas others are nonspecific and expressed against more than one stress. Stress
proteins provide protection to structures that could be otherwise adversely affected
by the stress, such as DNA and many enzymes. Synthesis of stress proteins in large
quantities is mediated through the expression of stress-related gene systems, some
of which are inducible whereas others are constitutive but expressed at a low level
when cells are not under stress. As some of the gene systems are global, gene
expression by one stress can also help cells to adapt to other related stresses.
Expression of stress-related genes is initiated by specific polypeptides or sigma
factor (d) synthesized by specific genes. Some of these, such as d B or d 37 (encoded
by gene sig B), help cope with general stress in Gram-positive bacteria; d 32
(encoded by rpoH gene) and d 24 (encoded by rpoE gene) help cope with heat
response; and d 38 (encoded by rpoS gene) helps cope with general stress and
starvation in Gram-negative bacteria. Under a specific stress (such as under heatshock
condition), rpoH is turned on to effect synthesis of Rpo H or d 32 protein in
high amounts (Figure 9.2). This sigma factor (also called a regulon) then combines
with the core RNA polymerase (consisting of four subunits, aabb¢) to form the
complete RNA polymerase enzyme or holoenzyme. This holoenzyme then binds
to the promoter of a heat-shock gene family, leading to synthesis of heat-shock
proteins (e.g., in Esc. coli), which then protect the structural and functional units
of stressed cells susceptible to heat damage (e.g., DNA and proteins). They can
also protect against other stresses. Involvement of different sigma factors in protecting
against stresses (such as cold, heat, low pH, UV) has been studied with
several species of foodborne bacteria.
C. Importance of Stress-Adapted Microorganisms in Food 5,11
As indicated before, during the handling of food and food ingredients from the farm
to table, foodborne bacteria are exposed to different suboptimal physical and chemical
environments. This can enable foodborne pathogens and spoilage bacteria, as
well as beneficial bacteria, to develop characteristics that are different from those