Chemical, Biological, and Physical Methods
269
is used, the identity of the unknown species can be confirmed. A DNA microarray is, in essence, a
dot blot set-up with the capacity to obtain and process large amounts of data. A DNA microarray for
microorganisms begins with the construction of oligonucleotides (primers), probes, hybridization, and
data analysis. The overall process has been presented and reviewed by Ye et al.
230
As currently used, several hundred to several thousand specimens or probes may be applied to a solid
surface. In a study of Xanthomonas pathovars, a 47-probe microarray was employed to fingerprint
14 closely related strains, and the fingerprints showed clear differences between the test strains.
111
To determine the relative numbers and genera of bacteria in the microbial consortium of ready-to-eat
vegetable salads, a study was undertaken when the salads were fresh and after storage at 4 and 10
◦ C for
up to 12 days. While stored in modified-atmosphere packages, specific probes from the 16S sequences
were used to identify bacterial genera in the salads without cultural isolations.
175 The investigators
concluded that the DNA array-based method gave an accurate picture of the heterogeneous bacterial
community that was dominated by pseudomonads after 4
◦ C storage and by enteric bacteria after
10
◦ C storage. A fiber optic DNA microarray has been developed for the detection of some foodborne
pathogens, with as few as 100 cfu being detectable in <1 hour.
It appears that microarrays represent great potential for identifying microbial species and strains
in foods and for the fingerprinting of biotypes. Array platforms are available from a number of
commercial companies. In addition to DNA, RNA and protein microarrays are in use, and protein
chips are available for proteomics research.
PHYSICAL METHODS
Biosensors
In a broad sense, a biosensor is a device, method, or procedure that can be used to detect the presence
or activity of an organism—living or dead. A more concise definition is “. . . a device containing a
biological sensing element connected to a transducer.” In this definition, the transducer is the unit
that converts the change into a measurable signal. Not included are biochemical or immunological
methods that are used primarily to measure enzyme–substrate or antigen–antibody reactions although
these reactions may be components of a biosensor. Some biosensors are based on principles of physics
(e.g., fiber optics) while others are based on biological principles (e.g., lux gene luminescence). Those
that have been demonstrated to be of value for foodborne microorganisms are listed and briefly
described below.
Piezoelectric Crystals (Accoustical Biosensors)
Piezoelectric is electricity or electric polarity due to pressure in a crystalline substance such as quartz.
A vibrating quartz is an extremely sensitive weight indicator. If a crystal is coated with an antibody,
a flow injection analysis (FIA) system can be used to detect the addition of its homologous antigen.
The working principle of a piezoelectric biosensor is depicted in Figure 11–4. With the quartz coated
with an antibody, the target analyte is the homologous antigen which, when it binds to the antibody,
changes the mass with a resulting decrease in frequency.
In one study, gold-coated quartz crystal surfaces were used to develop a FIA system to detect S. Typhimurium.
229 To mobilize the antibody, crystals were first coated with DSP (dithiobissuccinimidly-propionate), then the S. Typhimurium antibody, and finally with S. Typhimurium cells. It
can be seen from Figure 11–5 that the F (Hz) response for DSP alone was 9.0; 123 for DSP+antibody;
269
is used, the identity of the unknown species can be confirmed. A DNA microarray is, in essence, a
dot blot set-up with the capacity to obtain and process large amounts of data. A DNA microarray for
microorganisms begins with the construction of oligonucleotides (primers), probes, hybridization, and
data analysis. The overall process has been presented and reviewed by Ye et al.
230
As currently used, several hundred to several thousand specimens or probes may be applied to a solid
surface. In a study of Xanthomonas pathovars, a 47-probe microarray was employed to fingerprint
14 closely related strains, and the fingerprints showed clear differences between the test strains.
111
To determine the relative numbers and genera of bacteria in the microbial consortium of ready-to-eat
vegetable salads, a study was undertaken when the salads were fresh and after storage at 4 and 10
◦ C for
up to 12 days. While stored in modified-atmosphere packages, specific probes from the 16S sequences
were used to identify bacterial genera in the salads without cultural isolations.
175 The investigators
concluded that the DNA array-based method gave an accurate picture of the heterogeneous bacterial
community that was dominated by pseudomonads after 4
◦ C storage and by enteric bacteria after
10
◦ C storage. A fiber optic DNA microarray has been developed for the detection of some foodborne
pathogens, with as few as 100 cfu being detectable in <1 hour.
It appears that microarrays represent great potential for identifying microbial species and strains
in foods and for the fingerprinting of biotypes. Array platforms are available from a number of
commercial companies. In addition to DNA, RNA and protein microarrays are in use, and protein
chips are available for proteomics research.
PHYSICAL METHODS
Biosensors
In a broad sense, a biosensor is a device, method, or procedure that can be used to detect the presence
or activity of an organism—living or dead. A more concise definition is “. . . a device containing a
biological sensing element connected to a transducer.” In this definition, the transducer is the unit
that converts the change into a measurable signal. Not included are biochemical or immunological
methods that are used primarily to measure enzyme–substrate or antigen–antibody reactions although
these reactions may be components of a biosensor. Some biosensors are based on principles of physics
(e.g., fiber optics) while others are based on biological principles (e.g., lux gene luminescence). Those
that have been demonstrated to be of value for foodborne microorganisms are listed and briefly
described below.
Piezoelectric Crystals (Accoustical Biosensors)
Piezoelectric is electricity or electric polarity due to pressure in a crystalline substance such as quartz.
A vibrating quartz is an extremely sensitive weight indicator. If a crystal is coated with an antibody,
a flow injection analysis (FIA) system can be used to detect the addition of its homologous antigen.
The working principle of a piezoelectric biosensor is depicted in Figure 11–4. With the quartz coated
with an antibody, the target analyte is the homologous antigen which, when it binds to the antibody,
changes the mass with a resulting decrease in frequency.
In one study, gold-coated quartz crystal surfaces were used to develop a FIA system to detect S. Typhimurium.
229 To mobilize the antibody, crystals were first coated with DSP (dithiobissuccinimidly-propionate), then the S. Typhimurium antibody, and finally with S. Typhimurium cells. It
can be seen from Figure 11–5 that the F (Hz) response for DSP alone was 9.0; 123 for DSP+antibody;