31 Emerging Bacterial Foodborne Pathogens and Methods of Detection 737the angle of refraction of a beam of light over the
coated chip. When the protein of interest binds to
the antibody coated on the chip, the mass changes,
resulting in a change of the angle of light refraction.
The results are displayed in real time as the optical
response, measured in resonance units (RU), plotted
against time (Bamdad 1997, Nice and Catimel
1999). The number of RUs is proportional to the
amount of protein binding to the coated chip (Bam-
dad 1997). The data generated can be used to calcu-
late the antigen concentration and equilibrium disso-
ciation binding constants.
Using SPR, each binding experiment takes only 5
to 10 minutes (Bamdad 1997). Once the conditions
of the experiment are established, this offers consid-
erable time saving over traditional methods of deter-
mining kinetic rates. However, determining the right
conditions for running SPR experiments as well as
regenerating the chip can take from days to months,
and this represents a major limitation of the system
if many different proteins are being analyzed.
SPR is good for measuring antibody affinity for
toxins secreted from pathogens. The conditions for
different antibodies against toxins are about the
same, making optimization of the conditions less
time consuming than may otherwise be the case.
Samples containing the antigen can be passed over
the antibody-coated chip and the change in reso-
nance units compared to a standard curve in order to
quantify the number of pathogens present. Figure
31.6B illustrates a typical SPR profile of the binding
of llama heavy-chain antibodies to the L. monocyto-
genestoxin LLO.
Given the correct antigen, such as a secreted toxin,
SPR can be a useful method for screening samples
for the presence of specific pathogens. However, the
cost of the instrument and the time required for opti-
mizing sample purification severely limit the applica-
tion of this technology as a routine screening method.
Latex Agglutination Assays
One simple assay that has been developed for the
detection of foodborne pathogens is the latex agglu-
tination assay. The assay makes use of latex bead–
bound antibodies specific to the antigen of choice.
Antibodies on each bead bind to the antigen. Each
antigen can bind to more than one antibody bead,
causing the beads to agglutinate.
Matar et al. (1997) used antibodies specific to the
L. monocytogenesLLO toxin to detect the pathogen
in foods and pure cultures. The toxin was used
instead of cells because antibodies to cell surface
proteins tend to be more genus specific. Addi-
tionally, LLO is a secreted protein, so that the super-
natants of enrichment cultures can be used directly
without further extraction. After culture enrichment
following the USDA method, the latex agglutination
assay was able to detect LLO at concentrations indi-
cating contamination of the original food sample
with between 0.3 and 220 cfu/g food sample (Matar
et al. 1997). The agglutination test itself gives quali-
tative (such as positive or negative) results (Matar et
al. 1997) rather than quantitative results.
Although this test still requires culture enrich-
ment and then 48 hours to give results, it does have
one major advantage over most of the molecular
methods available, that is, no specialized equipment
is required. Only the bare eye and a light source are
needed to see the agglutination result, and there is
no need for any expensive reagents, making the test
suitable for use in the field or for use in third world
countries (Matar et al. 1997).CONCLUSIONS
Despite considerable efforts made to prevent con-
tamination of food, new pathogens often emerge as
a result of changing demography, food consumption
habits, food technology, commerce, changes in water
sources and environmental factors (Gugnani 1999),
and microbial adaptation (Altekruse et al. 1997).
Because of this, the food industry must have the
ability to detect a number of different pathogens, at
a cost that is affordable. They must also have the
ability to easily incorporate new pathogen tests into
regimes to ensure that any emerging pathogen is not
overlooked.
For the food industry, the ideal pathogen detec-
tion test should have at least some of the following
characteristics: the optimal test for pathogen detec-
tion should be simple to perform by nonspecialized
personnel. It should be sensitive enough to detect
low levels of pathogens, and yet specific for detec-
tion of the pathogenic species of interest. The test
should be rapid, suitable for automation, and inex-
pensive (Ingianni et al. 2001). Technologies are
becoming available that satisfy a number of these
criteria, but none so far has been able to satisfy all,
especially for determination of multiple pathogens.
As this article has illustrated, a number of tech-
nologies are available or being developed to detect