Fundamental Food Microbiology, Third Edition - Fuad Fathir

114 FUNDAMENTAL FOOD MICROBIOLOGY
3. Enhancing Viability of Starter Cultures
In the bioprocessing of foods, starter cultures are used as frozen concentrates or
freeze-dried preparations. However, both these conditions are known to cause death
and injury to the cells. By studying the mechanisms responsible for cell death or
cell injury, it may be possible to stop the events and reduce death and injury. This
will help produce starter concentrates that can be stored for a long time without a
reduction in their viability or desirable characteristics.
A. Definition and Terminologies
IV. VIABLE-BUT-NONCULTURABLE
Under unfavorable environments, some cells in a bacterial population remain viable
but are not able to multiply in many recommended bacteriological media, unless
they are subjected to a prior resuscitation treatment. This phenomenon has been
reported with starved cells of Vibrio vulnificus, Vib. cholerae, Vib. parahaemolyticus,
Salmonella serovars, Esc. coli O157:H7, Campylobacter jejuni, Enterococcus faecalis,
Shigella dysenterie, Helicobacter pylori, Pseudomonas fluorescens, and others
exposed to low temperature, seawater, water, saliva, phosphate buffer, and salted
salmon roe. Because this phenomenon is observed with many foodborne pathogens,
a concern was raised that the pathogens can be present in a food in a viable-butnonculturable
(VBNC or VNC) state but could not be detected by the recommended
methods. However, these cells can resuscitate under a favorable environment, multiply,
and, following consumption of the food, cause foodborne diseases. 4,17,18
In recent years, the existence of the VBNC state of bacterial cells has been
questioned because of the study methods used by researchers. In addition, the
scientific validity of the VBNC terminology, especially in food microbiology, has
been questioned by others. The operational definition of some terminologies related
to this phenomenon is listed here: 19,21
• Viability. Cells that are metabolically active and able to multiply in an appropriate
environment. In bacteriology, this term is analogous with the term culturability.
• Metabolically Active. Cells capable of carrying out at least some metabolic activity
but may not necessarily multiply.
• Nonculturability. Inability to multiply under any conditions.
• Dead Cells. Cells unable to divide in an appropriate environment.
• Resuscitation. Metabolically active cells changed from a state of inability to
multiply in one environment to a state of ability to multiply in another environment.
These definitions suggest the term viable-but-nonculturable may be contradictory.
The cells included under this term could fail to multiply in one selective environment
but multiply in another environment. Also, following resuscitation, they can multiply
in the selective environment (some similarities with sublethally injured cells).