Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL STRESS RESPONSE IN THE FOOD ENVIRONMENT 105
A. Definition and Observations
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II. STRESS ADAPTATION
Stress adaptation or stress response has been explained as a situation whereby a
brief exposure of a bacterial population to a suboptimal physical or chemical
(growth) environment enables the cells to resist subsequent exposure to the same or
other types of harsher treatment to which the species is normally susceptible. This
phenomenon has been observed among many foodborne pathogens and spoilage
bacteria following exposure of cells to various suboptimal physical and chemical
environments, such as cold and warm temperature, low A W, low hydrostatic pressure,
UV light, high salt concentrations, bacteriocins, preservatives, detergents, several
dyes, and antibiotics. It is assumed that a brief exposure to a suboptimal environment
triggers some cellular mechanisms that enables them to resist subsequent exposure
to harsher treatment. 5 However, once the cells are removed and allowed to grow for
several generations in the optimum conditions, they do not remain resistant; rather,
they revert to the original state. Several confusing and ambiguous terms have been
used by different research groups to describe the stress adaptation phenomenon in
bacteria. Some terms for suboptimal pH are included here. 6–8
• Acid Resistance or Acid Adaptation. An exposure of cells for an extended period
to mild acidic environment (e.g., pH 5.0 to 5.8), enables them to develop resistance
to subsequent exposure to pH £ 2.5.
• Acid Tolerance or Acid Tolerance Response (ATR). A brief exposure of cells to
mild acidic environment enables them to survive subsequent exposure to pH 2.4
to 4.0.
• Acid Shock Response (ASR). The response of bacterial cells to a low pH without
previous adaptations to a mild pH.
Many studies have been conducted to determine stress adaptation of foodborne
bacteria. Escherichia coli cells exposed for one or two generations at pH 5.0 survive
better subsequently at pH 3 to 4 (but not pH 72�C). 6,7
Similarly, acid-adapted Listeria monocytogenes cells survive well when exposed to
pH 3.5. Acid-adapted Lis. monocytogenes cells also develop resistance to nisin. Lis.
monocytogenes cells briefly exposed to 0.1% H 2O 2 also developed cross resistance
against subsequent exposure to 0.5% H 2O 2, 5% ethanol, 7% NaCl, pH 5.0, or 45�C
as compared with unadapted control cells. 8 Heat resistance (increase in D-value) of
Lis. monocytogenes, Salmonella serovars, and Esc. coli O157:H7 suspended in lowpH
fruit juices has been recently reported. 9 The cells of the pathogens were initially
acid adapted by exposing to pH 5.0. They were then suspended in orange, apple, or
grape juice (pH 3.5 to 3.9) and heated at 56�C. In all instances, the D-values of
acid-adapted cells as compared with those of control cells increased significantly.
For Lis. monocytogenes, D-values for control and acid-adapted cells were 2.1 and
3.8 min in orange juice, 1.6 and 5.0 min in apple juice, and 2.3 and 4.6 min in grape
juice, respectively. These results suggest that stress adaptation of microorganisms
can occur both in culture broth and in food systems. In food processing and