248 Modern Food Microbiology
measured by use of a radioactivity counter. For organisms that utilize glucose,
14 C-glucose is usually
employed. For those that cannot utilize this compound, others such as
14 C-formate or
14 C-glutamate
are used. The overall procedure consists of using capped 15-ml serum vials to which are added anywhere from 12 to 36 ml of medium containing the labeled metabolite. The vials are made either aerobic
or anaerobic by sparging with appropriate gases and are then inoculated. Following incubation, the
headspace is tested periodically for the presence of
14 CO 2 . The time required to detect the labeled CO 2
is inversely related to the number of organisms in a product. The Bactec is a commercially available
detection system.
The use of radiometry to detect the presence of microorganisms was first suggested by Levin
et al.
124 It is confined largely to clinical microbiology, but some applications have been made to
foods and water. The experimental detection of S. aureus, Salmonella Typhimurium, and spores of
putrefactive anaerobe 3679 and Clostridium botulinum in beef loaf was studied by Previte.
165 The
inocula employed ranged from about 10
4 to 10
6 /ml of medium, and the detection time ranged from
2 hours for S.Typhimurium to 5–6 hours for C. botulinum spores. For these studies, 0.0139 µCi of
14 C-glucose per milliliter of tryptic soy broth was employed. In another study, Lampi et al.
119 found
that one cell per milliliter of S.Typhimurium or S. aureus could be detected by a radiometric method
in 9 hours. For 10
4 cells, 3–4 hours were required. With respect to spores, a level of 90 of putrefactive
anaerobe (PA) 3679 was detected in 11 hours, whereas 10
4 were detectable within 7 hours. These
and other investigators have shown that spores required 3–4 hours longer for detection than vegetable
cells. From the findings of Lampi et al., the radiometric detection procedure could be employed as a
screening procedure for foods containing high numbers of organisms, for such foods produced results
by this method within 5–6 hours, whereas those with lower numbers required longer times.
The detection of nonfermenters of glucose by this method is possible when metabolites such as
labeled formate and/or glutamate are used. It has been shown that a large number of foodborne
organisms can be detected by this method in 1–6 hours. The radiometric detection of 1–10 coliforms
in water within 6 hours was achieved by Bachrach and Bachrach
9 by employing
14 C-lactose with
incubation at 37
◦ C in a liquid medium. It is conceivable that a differentiation can be made between
fecal E. coli and total coliforms by employing 45.5
◦ C incubation along with 37
◦ C incubation.
Radiometry has been used to detect organisms in frozen orange juice concentrate.
78 The investigators
used
14 C-glucose, four yeasts, and four lactic acid bacteria, and at an organism concentration of 10
4
cells, detection was achieved in 6–10 hours. Of 600 juice samples examined, 44 with counts of 10
4 /ml
were detected in 12 hours and 41 of these in 8 hours. No false negatives occurred, and only two false
positives were noted. The method was used for cooked foods to determine if counts were <10
5 cfu/ml,
and the results were compared to APC. Of 404 samples consisting of seven types of foods, around
75% were correctly classified as acceptable or unacceptable within 6 hours.
174 No more than five were
incorrectly classified. The study employed
14 C-glucose, glutamic acid, and sodium formate.
Fluorogenic and Chromogenic Substrates
Some of the fluorogenic and chromogenic substrates employed in culture media in food microbiology are
1. 4-methylumbelliferyl-β-d-glucuronide (MUG),
2. 4-methylumbelliferyl-β-d-galactoside (MUGal),
3. 4-methylumbelliferyl phosphate (MUP),
4. o-nitrophenyl-β-d-galactopyranoside (ONPG),
measured by use of a radioactivity counter. For organisms that utilize glucose,
14 C-glucose is usually
employed. For those that cannot utilize this compound, others such as
14 C-formate or
14 C-glutamate
are used. The overall procedure consists of using capped 15-ml serum vials to which are added anywhere from 12 to 36 ml of medium containing the labeled metabolite. The vials are made either aerobic
or anaerobic by sparging with appropriate gases and are then inoculated. Following incubation, the
headspace is tested periodically for the presence of
14 CO 2 . The time required to detect the labeled CO 2
is inversely related to the number of organisms in a product. The Bactec is a commercially available
detection system.
The use of radiometry to detect the presence of microorganisms was first suggested by Levin
et al.
124 It is confined largely to clinical microbiology, but some applications have been made to
foods and water. The experimental detection of S. aureus, Salmonella Typhimurium, and spores of
putrefactive anaerobe 3679 and Clostridium botulinum in beef loaf was studied by Previte.
165 The
inocula employed ranged from about 10
4 to 10
6 /ml of medium, and the detection time ranged from
2 hours for S.Typhimurium to 5–6 hours for C. botulinum spores. For these studies, 0.0139 µCi of
14 C-glucose per milliliter of tryptic soy broth was employed. In another study, Lampi et al.
119 found
that one cell per milliliter of S.Typhimurium or S. aureus could be detected by a radiometric method
in 9 hours. For 10
4 cells, 3–4 hours were required. With respect to spores, a level of 90 of putrefactive
anaerobe (PA) 3679 was detected in 11 hours, whereas 10
4 were detectable within 7 hours. These
and other investigators have shown that spores required 3–4 hours longer for detection than vegetable
cells. From the findings of Lampi et al., the radiometric detection procedure could be employed as a
screening procedure for foods containing high numbers of organisms, for such foods produced results
by this method within 5–6 hours, whereas those with lower numbers required longer times.
The detection of nonfermenters of glucose by this method is possible when metabolites such as
labeled formate and/or glutamate are used. It has been shown that a large number of foodborne
organisms can be detected by this method in 1–6 hours. The radiometric detection of 1–10 coliforms
in water within 6 hours was achieved by Bachrach and Bachrach
9 by employing
14 C-lactose with
incubation at 37
◦ C in a liquid medium. It is conceivable that a differentiation can be made between
fecal E. coli and total coliforms by employing 45.5
◦ C incubation along with 37
◦ C incubation.
Radiometry has been used to detect organisms in frozen orange juice concentrate.
78 The investigators
used
14 C-glucose, four yeasts, and four lactic acid bacteria, and at an organism concentration of 10
4
cells, detection was achieved in 6–10 hours. Of 600 juice samples examined, 44 with counts of 10
4 /ml
were detected in 12 hours and 41 of these in 8 hours. No false negatives occurred, and only two false
positives were noted. The method was used for cooked foods to determine if counts were <10
5 cfu/ml,
and the results were compared to APC. Of 404 samples consisting of seven types of foods, around
75% were correctly classified as acceptable or unacceptable within 6 hours.
174 No more than five were
incorrectly classified. The study employed
14 C-glucose, glutamic acid, and sodium formate.
Fluorogenic and Chromogenic Substrates
Some of the fluorogenic and chromogenic substrates employed in culture media in food microbiology are
1. 4-methylumbelliferyl-β-d-glucuronide (MUG),
2. 4-methylumbelliferyl-β-d-galactoside (MUGal),
3. 4-methylumbelliferyl phosphate (MUP),
4. o-nitrophenyl-β-d-galactopyranoside (ONPG),