31 Emerging Bacterial Foodborne Pathogens and Methods of Detection 717temperatures are warm, and after the holiday season
in the winter. In both cases, illness is associated with
improper storage of foods (Monday and Bohach
1999). Outbreaks of Staphylococcus aureushave
been decreasing since 1973 (Mead et al. 1999).
It is the enterotoxins secreted by the S. aureus,
rather than the organism itself, that are responsible
for causing the symptoms of infection. These symp-
toms include vomiting, nausea, diarrhea, and ab-
dominal pain. The incubation period between ingest-
ing contaminated food and the onset of symptoms is
short, that is, between 2 and 6 hours. Because the ill-
ness is acute and short-lived, it tends to be underre-
ported (Balaban and Rasooly 2000, Monday and
Bohach 1999).
S. aureussecretes a number of toxins. Of these,
the nine known enterotoxins (A–E and G–J) are prob-
ably the best researched (Balaban and Rasooly 2000,
Monday and Bohach 1999). These toxins facilitate
the ability of the bacteria to colonize and persist
within a broad range of hosts. They include mem-
brane-active hemolysins, nucleases, proteases, leu-
cocidins, collagenases, cell-surface proteins, and
superantigens (Monday and Bohach 1999). Staphyl-
ococcusenterotoxin A is the most commonly found
of the enterotoxins in foodborne illness at 77.8%
(Balaban and Rasooly 2000).
Another well-researched toxin of S. aureusis -
hemolysin, which is a member of the thiol-activated
cytolysin family and has a sequence and structure
related to LLO and PLO (Menestrina et al. 2001).
Like LLO, S. aureus-hemolysin is a pore-forming
protein that inserts itself into membranes, oligomer-
izes, and forms channels in the membrane. The tox-
in causes the death of phagocytic cells and necrotic
lesions in hosts when the bacteria are injected sub-
cutaneously; it also allows replication of S. aureus
cells inside macrophages. In mouse, -hemolysin
preferentially attacks endothelial cells and thrombo-
cytes, leading to pulmonary edema and eventual
death (Menestrina et al. 2001). Mutation of the gene
to inactivate -hemolysin reduces the ability of S.
aureusto kill mice (Menestrina et al. 2001).
METHODS TO DETECT
FOODBORNE PATHOGENS
The ideal pathogen detection test for the food indus-
try should have a number of characteristics. The
optimal test for pathogen detection should be simple
to perform. It should be sensitive enough to detect
pathogens at levels as low as 1 cell/g of food materi-
al; some pathogens such as E. coliO157:H7 can
infect at doses of less than 10 cells. The test should
be specific to the pathogenic species within the
genus; in Listeria,for example, there are six species,
of which only L. monocytogenesis an important
human pathogen. The test should also be rapid, giv-
ing results in less than a day. Finally, the test should
be amenable to automation and inexpensive (Ingi-
anni et al. 2001).
In this section are described some of the tech-
niques commonly used to detect foodborne patho-
gens. It should be noted that none of the tests alone
satisfies all of the criteria listed above. Where need-
ed, the detection of L. monocytogenesis used to
illustrate how the methods can be applied, but the
methods described below can be used or adapted for
use to detect all the foodborne pathogens described
above.CULTURE-DEPENDENTENRICHMENT
METHODS FORPATHOGENDETECTIONMost conventional methods for detecting foodborne
bacterial pathogens in food and other substrates rely
on the use of microbiological media to selectively
grow and enumerate bacterial species. The methods
are sensitive and inexpensive, and provide qualita-
tive as well as quantitative results. Unfortunately for
the food industry, where time and cost are issues, the
preparation of media and plates, colony counting,
and biochemical characterization of the isolated
colonies make for a time-consuming and labor-
intensive process (de Boer and Beumer 1999). The
success of culturing methods depends on the num-
ber and state of the bacteria in the sample, the selec-
tivity of the media (balance between inhibition of
competitors and inhibition of the target organism),
the conditions of the incubation (time, temperature,
O 2 ), and the selectivity of the isolation medium (dis-
tinction between the target organism and competi-
tive microflora) (Beumer and Hazeleger 2003).
The most common methods used to detect L.
monocytogenes are those developed by the U.S.
Department of Agriculture—Food Safety and In-
spection Service (USDA-FSIS) for the detection of
Listeriain meat and poultry products, the Federal
Drug Administration (FDA) protocol for detecting
the organism in dairy, fruit, vegetable, and seafood
products, and the Netherlands Government Food
Inspection Service (NGFIS) method, used for all