252 Modern Food Microbiology
detects the presence of the homologous antibody; in the direct method, it detects the presence of
the antigen. The use of the indirect method eliminates the need to prepare FA for each organism of
interest. The FA technique obviates the necessity of pure culture isolations of salmonellae if H antisera
are employed. A commonly employed conjugate is polyvalent salmonellae OH globulin labeled with
fluorescein isothiocyanate with somatic groups A to Z represented. Because of the cross-reactivity
of salmonellae antisera with other closely related organisms (e.g., Arizona, Citrobacter, E. coli),
false-positive results are to be expected when naturally contaminated foods are examined. The early
history and development of the FA technique for clinical microbiology has been reviewed by Cherry
and Moody
31 and for food applications by Ayres
7 and Goepfert and Insalata.
69 The popularity of this
method for foodborne pathogens has waned with the advent of molecular and other detection methods.
Enrichment Serology
The use of enrichment serology (ES) is a more rapid method for recovering salmonellae from foods
than the conventional culture method (CCM). Originally developed by Sperber and Deibel,
193 it is
carried out in four steps: pre-enrichment in a nonselective medium for 18 hours; selective enrichment
in selenite–cystine and/or tetrathionate broth for 24 hours; elective enrichment in M broth for either
6–8 hours or 24 hours; and agglutination with polyvalent H antisera at 50
◦ C for 1 hour. Results can
be obtained in 50 hours (depending on elective enrichment time used) compared to 96–120 hours by
CCM. A modified ES method has been proposed involving a 6-hour pre-enrichment, thus making it
possible to obtain results in 32 hours.
200
Overall, the ES method provides results in 32–50 hours compared to 92–120 hours for CCM, results
are comparable to both CCM and FA, and no specialized equipment or training is needed. Possible
disadvantages to its use are the need for a minimum of about 10
7 cells per milliliter and its lack of
response to nonmotile salmonellae. The latter can be overcome by use of a slide agglutination test
from the elective enrichment broth employing polyvalent O antiserum.
193
The Oxoid Salmonella rapid test (OSRT) is a variation of ES. It consists of a culture vessel containing two tubes, each of which contains dehydrated enrichment media in the lower compartments and
dehydrated selective media in the upper compartments. The media are hydrated with sterile distilled
water, and a special salmonellae elective medium is added to the culture vessel along with a novobiocin
disk, followed by 1 ml of pre-enrichment culture of sample. Following incubation at 41
◦ C for 24 hours,
media in the upper compartment (selective media) of each tube are examined for color change, indicating the presence of salmonellae. Positive tubes are further tested with the Oxoid Salmonella latex test
(2 minutes). Final confirmation of salmonellae is made by use of traditional biochemical and serologic
tests.
Salmonella 1–2 Test
This method is similar to ES and OSRT. ES relies on antibody reaction with flagellated salmonellae
strains. Unlike ES, the 1–2 Test employs the use of a semisolid phase. The test is conducted in a specially
designed plastic device that has two chambers, one for selective broth and the other for a nonselective
motility medium. In addition to selective ingredients, the latter contains the amino acid l-serine,
which is elective for salmonellae. Following inoculation of the selective medium chamber, the device
is incubated, during which time motile salmonellae move into the nonselective medium chamber.
The latter contains flagellar antibodies, and when the motile organisms enter the antibody area, an
detects the presence of the homologous antibody; in the direct method, it detects the presence of
the antigen. The use of the indirect method eliminates the need to prepare FA for each organism of
interest. The FA technique obviates the necessity of pure culture isolations of salmonellae if H antisera
are employed. A commonly employed conjugate is polyvalent salmonellae OH globulin labeled with
fluorescein isothiocyanate with somatic groups A to Z represented. Because of the cross-reactivity
of salmonellae antisera with other closely related organisms (e.g., Arizona, Citrobacter, E. coli),
false-positive results are to be expected when naturally contaminated foods are examined. The early
history and development of the FA technique for clinical microbiology has been reviewed by Cherry
and Moody
31 and for food applications by Ayres
7 and Goepfert and Insalata.
69 The popularity of this
method for foodborne pathogens has waned with the advent of molecular and other detection methods.
Enrichment Serology
The use of enrichment serology (ES) is a more rapid method for recovering salmonellae from foods
than the conventional culture method (CCM). Originally developed by Sperber and Deibel,
193 it is
carried out in four steps: pre-enrichment in a nonselective medium for 18 hours; selective enrichment
in selenite–cystine and/or tetrathionate broth for 24 hours; elective enrichment in M broth for either
6–8 hours or 24 hours; and agglutination with polyvalent H antisera at 50
◦ C for 1 hour. Results can
be obtained in 50 hours (depending on elective enrichment time used) compared to 96–120 hours by
CCM. A modified ES method has been proposed involving a 6-hour pre-enrichment, thus making it
possible to obtain results in 32 hours.
200
Overall, the ES method provides results in 32–50 hours compared to 92–120 hours for CCM, results
are comparable to both CCM and FA, and no specialized equipment or training is needed. Possible
disadvantages to its use are the need for a minimum of about 10
7 cells per milliliter and its lack of
response to nonmotile salmonellae. The latter can be overcome by use of a slide agglutination test
from the elective enrichment broth employing polyvalent O antiserum.
193
The Oxoid Salmonella rapid test (OSRT) is a variation of ES. It consists of a culture vessel containing two tubes, each of which contains dehydrated enrichment media in the lower compartments and
dehydrated selective media in the upper compartments. The media are hydrated with sterile distilled
water, and a special salmonellae elective medium is added to the culture vessel along with a novobiocin
disk, followed by 1 ml of pre-enrichment culture of sample. Following incubation at 41
◦ C for 24 hours,
media in the upper compartment (selective media) of each tube are examined for color change, indicating the presence of salmonellae. Positive tubes are further tested with the Oxoid Salmonella latex test
(2 minutes). Final confirmation of salmonellae is made by use of traditional biochemical and serologic
tests.
Salmonella 1–2 Test
This method is similar to ES and OSRT. ES relies on antibody reaction with flagellated salmonellae
strains. Unlike ES, the 1–2 Test employs the use of a semisolid phase. The test is conducted in a specially
designed plastic device that has two chambers, one for selective broth and the other for a nonselective
motility medium. In addition to selective ingredients, the latter contains the amino acid l-serine,
which is elective for salmonellae. Following inoculation of the selective medium chamber, the device
is incubated, during which time motile salmonellae move into the nonselective medium chamber.
The latter contains flagellar antibodies, and when the motile organisms enter the antibody area, an