Chemical, Biological, and Physical Methods
263
the most common is Pseudomonas syringae whose ina gene consists of about 3,600 base pairs (bp) of
DNA that will yield a single ice nucleation ina protein. These proteins facilitate the freezing of water
at warmer temperatures than would otherwise be the case, and they lead to frost damage to many field
plants since they lead to supercooling at temperatures of −6
◦ C or lower before nucleation becomes
active. This is an example of heterogeneous ice nucleation where supercooled water is bound to a
non-water material, and it can occur at temperatures as low as −2
◦ C.
224 The application of these ice
nucleators to food products such as egg white, salmon muscle, and others can lead to a reduction in
freezing time and energy savings.
126
The bacterial ice nucleation diagnostic (BIND) test, developed by scientists at the DNA Plant
Technology Corporation, was developed for the detection of salmonellae. In a nutshell, the ina gene
from P. syringae is cloned into genetically engineered bacteriophages that are specific for salmonellae.
If salmonellae are present, the phages infect and lead to the synthesis of the ice nucleation protein as
part of the outer cell membrane. This is evidenced by the formation of ice crystals at a temperature
around −9
◦ C. By coupling a fluorescent freeze indicator dye, a green color indicates freezing and thus
the presence of salmonellae while an orange color indicates no freezing. With salmonellae phage P22,
as few as 25 cells per gram can be detected within 24 hours.
FINGERPRINTING METHODS
A number of methods are in use for the purpose of characterizing (fingerprinting, differentiating,
typing) species and strains of organisms of interest in foods, including some of the molecular genetic
methods above, and most are listed below and briefly described.
Phage typing
Amplified-fragment length polymorphism (AFLP)
Multilocus enzyme electrophoresis (MEE)
Restriction enzyme analysis (REA)
Random amplification of polymorphic DNA (RAPD)
Pulsed field gel electrophoresis (PFGE)
Restriction fragment length polymorphism (RFLP)
Ribotyping
Microarrays
Bacteriophage Typing
Phage typing is based on the specificity of a given phage for its host bacterium, and this relationship
allows one to use known phages to identify their specific hosts. All foodborne pathogens can be phage
typed, but the practice is applied more to some than others. More on phage typing as it relates to
specific foodborne pathogens may be found in the respective chapters.
One of the earliest and perhaps most elaborate of phage typing schemes was that developed for
S. aureus in the 1950s. Although the routine use of staphylococcal phage typing has waned, it has
emerged as an important tool in studying the epidemiology of L. monocytogenes.
Since they were first described in 1945, bacteriophages specific for Listeria have been studied
by a number of investigators relative to their uses for species and strain differentiation, and their
263
the most common is Pseudomonas syringae whose ina gene consists of about 3,600 base pairs (bp) of
DNA that will yield a single ice nucleation ina protein. These proteins facilitate the freezing of water
at warmer temperatures than would otherwise be the case, and they lead to frost damage to many field
plants since they lead to supercooling at temperatures of −6
◦ C or lower before nucleation becomes
active. This is an example of heterogeneous ice nucleation where supercooled water is bound to a
non-water material, and it can occur at temperatures as low as −2
◦ C.
224 The application of these ice
nucleators to food products such as egg white, salmon muscle, and others can lead to a reduction in
freezing time and energy savings.
126
The bacterial ice nucleation diagnostic (BIND) test, developed by scientists at the DNA Plant
Technology Corporation, was developed for the detection of salmonellae. In a nutshell, the ina gene
from P. syringae is cloned into genetically engineered bacteriophages that are specific for salmonellae.
If salmonellae are present, the phages infect and lead to the synthesis of the ice nucleation protein as
part of the outer cell membrane. This is evidenced by the formation of ice crystals at a temperature
around −9
◦ C. By coupling a fluorescent freeze indicator dye, a green color indicates freezing and thus
the presence of salmonellae while an orange color indicates no freezing. With salmonellae phage P22,
as few as 25 cells per gram can be detected within 24 hours.
FINGERPRINTING METHODS
A number of methods are in use for the purpose of characterizing (fingerprinting, differentiating,
typing) species and strains of organisms of interest in foods, including some of the molecular genetic
methods above, and most are listed below and briefly described.
Phage typing
Amplified-fragment length polymorphism (AFLP)
Multilocus enzyme electrophoresis (MEE)
Restriction enzyme analysis (REA)
Random amplification of polymorphic DNA (RAPD)
Pulsed field gel electrophoresis (PFGE)
Restriction fragment length polymorphism (RFLP)
Ribotyping
Microarrays
Bacteriophage Typing
Phage typing is based on the specificity of a given phage for its host bacterium, and this relationship
allows one to use known phages to identify their specific hosts. All foodborne pathogens can be phage
typed, but the practice is applied more to some than others. More on phage typing as it relates to
specific foodborne pathogens may be found in the respective chapters.
One of the earliest and perhaps most elaborate of phage typing schemes was that developed for
S. aureus in the 1950s. Although the routine use of staphylococcal phage typing has waned, it has
emerged as an important tool in studying the epidemiology of L. monocytogenes.
Since they were first described in 1945, bacteriophages specific for Listeria have been studied
by a number of investigators relative to their uses for species and strain differentiation, and their