samples and counting bacteria by DEFT.
The antibody-direct epifluorescent filter
technique (Ab-DEFT) has been incorpo-
rated in the enumeration ofL. monocyto-
genesin ready-to-eat packaged salads and
other fresh vegetables, and in the detection
ofE. coliO157:H7 in ground beef, apple
juice, and milk. In addition to membrane
filtration of food to collect and concentrate
microbial cells on the membrane surface,
fluorescent antibody staining of the filter
surface and epifluorescence microscopy
are involved. The fluorescent antibody is
added to the filter, placed on a slide, and
examined under a microscope. By this
method,L. monocytogenescan be quantified
in agreement between the three methods
(Anon., 1997) and has demonstrated the
potential of Ab-DEFT as a rapid alternative
for the quantitation of Listeriain food.
However, nonspecific reactivity of the fluo-
rescent antibodies to indigenous microbial
populations has resulted in false-positive
reactions using Ab-DEFT.
Remote Inspection Biological Sensor
Biosensors provide an instantaneous
indication of the presence of specific
pathogens in a food sample without need
for enrichment and can detect generic E.
coliandSalmonella. They may provide con-
tinual feedback of pathogen loads in fluids
within a plant. The remote inspection bio-
logical sensor (RIBS) uses a laser spectro-
graphic technique. A laser beam is directed
onto the surface of a carcass. Based on the
characteristics of the reflected light, this
equipment can make a specific identifica-
tion of pathogenic bacteria and give a gen-
eral indication of the number of organisms
present (Anon., 1998). It has a sensitivity of
up to 5 CFUs per square centimeter and is
able to effectively discriminate target organ-
isms from background (Wyvill and Got-
tfried, 2004).Microcalorimetry
Heat produced as a result of a biological
reaction, such as the catabolic processes
occurring in growing microorganisms cul-
tured from contaminated samples, can be
measured by a sensitive calorimeter called a
microcalorimeter. This biophysical technique
has been applied to enumerate microorgan-
isms in food. The procedure correlates a
thermogram (a heat-generation pattern dur-
ing microbial growth) with the number of
microbial cells. After a reference thermo-
gram has been established, others obtained
from contaminated samples can be com-
pared to the reference.Radiometry and Infrared Spectrophotometry
The time required for the detection of cer-
tain levels of radioactivity by this biophysical
technique is inversely related to the number of
microorganisms in the sample. This method
can be employed for sterility testing of asepti-
cally packaged products. Results are available
in 4 to 5 days, compared with 10 days with
conventional methods. The enumeration of
microorganisms in food samples can be
accomplished in less than 24 hours.Hydrophobic Grid Membrane Filter System
This culturing method is used to detect
and enumerate E. coliin foods. An ISO-
GRID hydrophobic grid membrane filter
(HGMF) system is available to detect and
enumerate E. coli. The sample is filtered
through the membrane without use of an
enrichment step, and a complex medium
(SD-39) is used to detect the target organ-
ism. The test is completed in 48 hours,
including biochemical and serological con-
firmation of presumptive colonies.The Relationship of Microorganisms to Sanitation 61