Microbial Stress Response in the Food Environment ◾ 103
Enhancing Viability of Starter Cultures and Probiotic Bacteria 7
Commercial starter cultures are normally frozen or freeze-dried before their use by food processors with the intention of having high levels of survivors. Viability of these cultures, especially
freeze-dried cultures, is generally low. Similarly, many probiotic bacteria are normally susceptible to stomach pH and low-pH food products (e.g., yogurt containing Lactobacillus acidophilus).
However, first exposing the cultures to a mild stress to release stress proteins may enable the cells
to survive subsequent freezing, freeze-drying, or exposure to low pH in the stomach or in food
products. Genetic engineering techniques can also be used to develop new strains capable of producing cryoproteins and different stress proteins and surviving better.
Sublethal Stress and injury 1–3,6
Definition and Observations
Sublethal injury occurs following exposure of bacterial cells to unfavorable physical and chemical
environments (beyond the growth range but not in the lethal range) that cause reversible alterations in the functional and structural organizations of the cells. As early as 1932, Rahn 15 suggested
that death of microbes exposed to sublethal stresses is a gradual process that can be reversed under
the proper conditions if the reactions have not progressed too far. In 1959, Strake and Stokes 16
showed that bacterial cells exposed to cold temperatures developed characteristics that were different from those of the normal population; the cells also temporarily lost the ability to multiply.
However, in an appropriate environment, they repaired their injury and initiated multiplication.
Later studies revealed that cells of yeasts and molds and spores of bacteria also incur reversible
injury following exposure to sublethal stresses. Stress has been shown to alter morphology of cells
because of down-regulation of the gene products that are responsible for cell division, such as
muramidase, autolysin amidase, and so forth. In Lis. monocytogenes, exposure to a high temperature (45°C) or 5.5% NaCl caused cells to elongate or appear in chains (Figure 10.3). 17
Many of the treatments include those used directly during food processing and storage as well as
microbial detection from foods. Treatments include low heat (such as pasteurization), low temperature (freezing, refrigeration, and chilling), low A W (different types of drying or adding high solutes,
such as sugar or salt), radiation (UV or X-ray), high hydrostatic pressure, electric pulse, low pH (both
organic and inorganic acids), preservatives (sorbates or benzoates), sanitizers (chlorine or quaternary
ammonium compounds), hot microbiological media (especially selective agar media above 48°C), and
different selective enumeration and detection methods. This phenomenon is observed in many species
of bacterial cells and spores, yeasts, and molds that are important in foodborne diseases, food spoilage,
and food bioprocessing, and as indicators of sanitation (Table 10.1). From this list, it becomes apparent that other microorganisms that have not been studied will most likely also be injured by sublethal
stresses. In general, Gram-negative bacteria are more susceptible to injury than Gram-positive bacteria, and bacterial spores are much more resistant than vegetative cells to a particular stress.
Microbial injury and growth of injured cells have been studied with both bacterial cells and
spores. The material discussed here mainly covers bacterial cell and spore injury.
Manifestation of Bacterial Sublethal Injury
A bacterial population, following exposure to a sublethal stress, contains three physiologically
different subpopulations: the uninjured (normal) cells, reversibly injured (injured) cells, and
Enhancing Viability of Starter Cultures and Probiotic Bacteria 7
Commercial starter cultures are normally frozen or freeze-dried before their use by food processors with the intention of having high levels of survivors. Viability of these cultures, especially
freeze-dried cultures, is generally low. Similarly, many probiotic bacteria are normally susceptible to stomach pH and low-pH food products (e.g., yogurt containing Lactobacillus acidophilus).
However, first exposing the cultures to a mild stress to release stress proteins may enable the cells
to survive subsequent freezing, freeze-drying, or exposure to low pH in the stomach or in food
products. Genetic engineering techniques can also be used to develop new strains capable of producing cryoproteins and different stress proteins and surviving better.
Sublethal Stress and injury 1–3,6
Definition and Observations
Sublethal injury occurs following exposure of bacterial cells to unfavorable physical and chemical
environments (beyond the growth range but not in the lethal range) that cause reversible alterations in the functional and structural organizations of the cells. As early as 1932, Rahn 15 suggested
that death of microbes exposed to sublethal stresses is a gradual process that can be reversed under
the proper conditions if the reactions have not progressed too far. In 1959, Strake and Stokes 16
showed that bacterial cells exposed to cold temperatures developed characteristics that were different from those of the normal population; the cells also temporarily lost the ability to multiply.
However, in an appropriate environment, they repaired their injury and initiated multiplication.
Later studies revealed that cells of yeasts and molds and spores of bacteria also incur reversible
injury following exposure to sublethal stresses. Stress has been shown to alter morphology of cells
because of down-regulation of the gene products that are responsible for cell division, such as
muramidase, autolysin amidase, and so forth. In Lis. monocytogenes, exposure to a high temperature (45°C) or 5.5% NaCl caused cells to elongate or appear in chains (Figure 10.3). 17
Many of the treatments include those used directly during food processing and storage as well as
microbial detection from foods. Treatments include low heat (such as pasteurization), low temperature (freezing, refrigeration, and chilling), low A W (different types of drying or adding high solutes,
such as sugar or salt), radiation (UV or X-ray), high hydrostatic pressure, electric pulse, low pH (both
organic and inorganic acids), preservatives (sorbates or benzoates), sanitizers (chlorine or quaternary
ammonium compounds), hot microbiological media (especially selective agar media above 48°C), and
different selective enumeration and detection methods. This phenomenon is observed in many species
of bacterial cells and spores, yeasts, and molds that are important in foodborne diseases, food spoilage,
and food bioprocessing, and as indicators of sanitation (Table 10.1). From this list, it becomes apparent that other microorganisms that have not been studied will most likely also be injured by sublethal
stresses. In general, Gram-negative bacteria are more susceptible to injury than Gram-positive bacteria, and bacterial spores are much more resistant than vegetative cells to a particular stress.
Microbial injury and growth of injured cells have been studied with both bacterial cells and
spores. The material discussed here mainly covers bacterial cell and spore injury.
Manifestation of Bacterial Sublethal Injury
A bacterial population, following exposure to a sublethal stress, contains three physiologically
different subpopulations: the uninjured (normal) cells, reversibly injured (injured) cells, and