112 ◾ Fundamental Food Microbiology
It was suggested at that time that in a repair medium, especially in a nutritionally rich medium,
the cells generate H 2 O 2 . The cells with greater levels or degrees of injury (see Figure 10.4) have
defective peroxidase or catalase systems and fail to degrade H 2 O 2 ; supplementation of a medium
with pyruvate or catalase helps degrade the H 2 O 2 and prevent cell death.
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.
Physiological Properties of VBNC Cells
The VBNC state is considered a survival strategy for many pathogens; thus bacteria undergo
genetic and physiologic changes. As a result, there are several noticeable changes in the appearance and physiology of cells under the VBNC state: (1) Bacteria become significantly smaller with
reduced amounts of macromolecular synthesis; (2) Bacteria lose virulence during the VBNC state,
however, they regain virulence immediately after resuscitation; (3) Cells are resistant to antibiotics. Antibiotics are effective primarily against growing cells, and VBNC cells are metabolically less
active; thus the antibiotics are less effective.
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 the 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 to safeguard the health of consumers and reduce waste of food from
spoilage.
Conclusion
Bacterial cells exposed to environmental stresses respond differently, depending on the level or degree
of a stress (Figure 10.6). 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 the 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 under any stress the level of
injury increases as the level of stress increases. For some cells, the injury can be repaired rapidly in a
nonselective or a nutritionally rich medium; these cells, usually designated as sublethally injured cells,
probably have a low level of injury. Some other cells, probably because of high level of injury, require a
more exact resuscitation environment for repair; these cells probably include VBNC cells (alternatively
designated as not normally culturable or NNC cells). Finally, some stressed cells fail to multiply under
any resuscitation conditions and are designated as lethally injured (or dead) cells.
Foodborne pathogens and spoilage bacteria can be stress adapted, sublethally injured, or
VBNC (or NNC) by the many processing, preservation, transportation, and storage conditions
used in food production. They may not be detected from a contaminated food by many of the
recommended methods used for their detection. Effective methods have to be developed and used
It was suggested at that time that in a repair medium, especially in a nutritionally rich medium,
the cells generate H 2 O 2 . The cells with greater levels or degrees of injury (see Figure 10.4) have
defective peroxidase or catalase systems and fail to degrade H 2 O 2 ; supplementation of a medium
with pyruvate or catalase helps degrade the H 2 O 2 and prevent cell death.
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.
Physiological Properties of VBNC Cells
The VBNC state is considered a survival strategy for many pathogens; thus bacteria undergo
genetic and physiologic changes. As a result, there are several noticeable changes in the appearance and physiology of cells under the VBNC state: (1) Bacteria become significantly smaller with
reduced amounts of macromolecular synthesis; (2) Bacteria lose virulence during the VBNC state,
however, they regain virulence immediately after resuscitation; (3) Cells are resistant to antibiotics. Antibiotics are effective primarily against growing cells, and VBNC cells are metabolically less
active; thus the antibiotics are less effective.
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 the 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 to safeguard the health of consumers and reduce waste of food from
spoilage.
Conclusion
Bacterial cells exposed to environmental stresses respond differently, depending on the level or degree
of a stress (Figure 10.6). 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 the 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 under any stress the level of
injury increases as the level of stress increases. For some cells, the injury can be repaired rapidly in a
nonselective or a nutritionally rich medium; these cells, usually designated as sublethally injured cells,
probably have a low level of injury. Some other cells, probably because of high level of injury, require a
more exact resuscitation environment for repair; these cells probably include VBNC cells (alternatively
designated as not normally culturable or NNC cells). Finally, some stressed cells fail to multiply under
any resuscitation conditions and are designated as lethally injured (or dead) cells.
Foodborne pathogens and spoilage bacteria can be stress adapted, sublethally injured, or
VBNC (or NNC) by the many processing, preservation, transportation, and storage conditions
used in food production. They may not be detected from a contaminated food by many of the
recommended methods used for their detection. Effective methods have to be developed and used