Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL STRESS RESPONSE IN THE FOOD ENVIRONMENT 117
D. Current Views
The results of several recent studies have provided a better insight of bacterial cells
exposed to an unfavorable environment for a prolonged period. These results show
that the so-called VBNC cells are really viable as well as culturable but are not
recovered unless they are exposed to appropriate culture conditions. 24–27 Many of
the starved or stressed cells have imbalanced metabolic pathways. When these cells
are exposed to a nutritionally rich medium, they start metabolism and generate large
quantities of superoxides and free radicals. However, because of defective metabolic
processes, they cannot detoxify these products and are killed. This death can be
prevented or greatly reduced by exposing the cells in a medium that either reduces
the production of the superoxides and free radicals or increases their degradation.
Supplementation of pyruvate or catalase to resuscitation media was thus found to
transform the cells to a culturable state very rapidly, as determined by an almost
immediate increase in CFUs without any change in TMCs. Several studies in the
1970s and 1980s on bacterial sublethal injury have shown that following environmental
stress of a bacterial population, the recovery of the survivors can be increased
either by incubating the cells in a minimal medium or in a nutritionally rich medium
supplemented with pyruvate or catalase. It was suggested at that time that in a repair
medium, especially in a nutritionally rich medium, the cells generate H 2O 2. The cells
with greater levels or degrees of injury (see Figure 9.3) have defective peroxidase
or catalase systems and fail to degrade H 2O 2; supplementation of a medium with
pyruvate or catalse helps degrade the H 2O 2 and prevent cell death. 14,15
It appears from the similarities of these observations that there are many similarities
between the VBNC phenomenon and sublethal injury of bacterial cells. The
differences are more in the study methods used and interpretation of the results.
E. Importance of VBNC Microorganisms in Food
The existence of bacterial cells that cannot be cultured by many currently recommended
bacteriological methods cannot be disputed. They can be present in food
and food environment and can potentially cause foodborne diseases and food spoilage.
Food microbiologists have to recognize this and develop proper resuscitation
and detection procedures for effective identification of their presence in food and
safeguard the health of consumers and reduce waste of food from spoilage.
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V. CONCLUSION
Bacterial cells exposed to environmental stresses respond differently, depending on
the level or degree of a stress (Figure 9.5). In the suboptimal growth range, away
from the optimal growth range, the cells can have a stress adaptation as manifested
by an increased resistance to same and several other stresses at higher levels. An
exposure beyond the growth range will cause injury in different structural and
functional components of some cells without affecting some others. The levels or
degrees of injury in the cells differ with the nature and duration of a stress, and