Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL STRESS RESPONSE IN THE FOOD ENVIRONMENT 107
Figure 9.2 Genetic basis of coping stress with sigma factors by bacterial cells. See text for
explanations.
of normal cells. This situation can be both disadvantageous and advantageous, as
described next.
1. Pathogens and Spoilage Bacteria Surviving in Low-pH Foods
Most foodborne pathogens (especially the enteric pathogens) and spoilage bacteria
(especially Gram-negative) are susceptible to low pH and die off rapidly in highacid
foods (pH £ 4.5) during storage. Also, at low pH, normal cells are susceptible
to other antimicrobial treatments (such as hydrostatic pressure, pasteurization temperature,
or preservatives) at a much lower level. However, if they are first acidadapted,
they become relatively resistant to lower pH and other treatments at minimal
levels and survive in food. Recent occurrence of foodborne diseases from the consumption
of fruit juices, fermented sausages, and acidified foods containing viable
Salmonella, Esc. coli O157:H7, and Lis. monocytogenes are thought to be due to
acid-adapted pathogenic strains surviving low pH and low heat treatment.
To overcome the problem, it is necessary to avoid exposure of the food (containing
the target cells) to mild treatments; instead, several mild treatments can be
used simultaneously to destroy the strains (hurdle concept, see Chapter 40).
2. Stress-Adapted Pathogens Surviving Stomach pH
Cells of most enteric pathogens associated with foodborne infections (see Chapter
25) are highly susceptible to low pH of the stomach and die off. This is probably
one of the reasons why dose level is high (�10 6 viable cells) for many enteric
pathogens to cause infection in the gastrointestinal (GI) tract. Many of the cells
\
rpoH
Promoter
Regulon
(δ38 )
RNA Poly (Holo)
Provide Protection
Against Stress
RNA Poly (Core)
Heat Shock Gene Family
Heat Shock Proteins