224 Modern Food Microbiology
for solidification, which occurs in about 30 minutes. It is attractive for enumerating psychrotrophic
organisms because there is no exposure to hot molten agar, which can lower numbers of psychrotrophs
since some are extremely heat sensitive. On the other hand, colonies on Redigel tend to be rather small
in size. In a comparison of this method with Petrifilm, ISO-GRID, and the spiral plater using seven
different foods, all were statistically comparable.
21
SimPlate
r
is a culture method that is based on the activity of several enzymes common to many
foodborne organisms. The growth medium contains substrates that are hydrolyzed by enzymes to
release MUG (see Chapter 11), and this fluorescent compound is visible under long-wave ultraviolet
light. The special plates have holes or wells, and they come in two sizes—84 or 198 incubation
wells. The technique is in essence an MPN method. Unlike conventional plating methods, it does
not allow for the characterization of colony features. In a comparative study employing seafoods, no
significant differences were found among aerobic plate counts by Petrifilm, Redigel, ISO-GRID, and
SimPlate.
26 In a study employing 751 food samples, SimPlate was found to be a suitable alternative
to the conventional plate method, Petrifilm, and Redigel.
16 However, some foods (raw liver, wheat
flour, and nuts) gave false-positive results. A comparison of SimPlate to the standard plate count by
six laboratories on the enumeration of heterotrophic bacteria in water found that the two methods
produced comparable results.
61
MOST PROBABLE NUMBERS
In this method, dilutions of food samples are prepared as for the SPC. Three serial aliquots or
dilutions are then planted into 9 or 15 tubes of appropriate medium for the three- or five-tube method,
respectively. Numbers of organisms in the original sample are determined by use of standard MPN
tables. The method is statistical in nature, and MPN results are generally higher than SPC results.
This method was introduced by McCrady in 1915. It is not a precise method of analysis; the 95%
confidence intervals for a three-tube test range from 21 to 395. When the three-tube test is used, 20 of
the 62 possible test combinations account for 99% of all results, whereas with the five-tube test, 49 of
the possible 214 combinations account for 99% of all results.
131 In a collaborative study on coliform
densities in foods, a three-tube MPN value of 10 was found to be as high as 34, whereas in another phase
of the study, the upper limit could be as high as 60.
121 Although Woodward
131 concluded that many
MPN values are improbable, this method of analysis has gained popularity. Among the advantages it
offers are the following:
1. It is relatively simple.
2. Results from one laboratory are more likely than SPC results to agree with those from another
laboratory.
3. Specific groups of organisms can be determined by use of appropriate selective and differential
media.
4. It is the method of choice for determining fecal coliform densities.
Among the drawbacks to its use are the large volume of glassware required (especially for the
five-tube method), the lack of opportunity to observe the colonial morphology of the organisms, and
its lack of precision.
TEMPO
r
is an MPN-based method that employs an enumerating card with a specific medium that
allows rapid fluorescent detection of target organisms, and results may be obtained within 24 h. It
obviates the need for serial dilutions.
for solidification, which occurs in about 30 minutes. It is attractive for enumerating psychrotrophic
organisms because there is no exposure to hot molten agar, which can lower numbers of psychrotrophs
since some are extremely heat sensitive. On the other hand, colonies on Redigel tend to be rather small
in size. In a comparison of this method with Petrifilm, ISO-GRID, and the spiral plater using seven
different foods, all were statistically comparable.
21
SimPlate
r
is a culture method that is based on the activity of several enzymes common to many
foodborne organisms. The growth medium contains substrates that are hydrolyzed by enzymes to
release MUG (see Chapter 11), and this fluorescent compound is visible under long-wave ultraviolet
light. The special plates have holes or wells, and they come in two sizes—84 or 198 incubation
wells. The technique is in essence an MPN method. Unlike conventional plating methods, it does
not allow for the characterization of colony features. In a comparative study employing seafoods, no
significant differences were found among aerobic plate counts by Petrifilm, Redigel, ISO-GRID, and
SimPlate.
26 In a study employing 751 food samples, SimPlate was found to be a suitable alternative
to the conventional plate method, Petrifilm, and Redigel.
16 However, some foods (raw liver, wheat
flour, and nuts) gave false-positive results. A comparison of SimPlate to the standard plate count by
six laboratories on the enumeration of heterotrophic bacteria in water found that the two methods
produced comparable results.
61
MOST PROBABLE NUMBERS
In this method, dilutions of food samples are prepared as for the SPC. Three serial aliquots or
dilutions are then planted into 9 or 15 tubes of appropriate medium for the three- or five-tube method,
respectively. Numbers of organisms in the original sample are determined by use of standard MPN
tables. The method is statistical in nature, and MPN results are generally higher than SPC results.
This method was introduced by McCrady in 1915. It is not a precise method of analysis; the 95%
confidence intervals for a three-tube test range from 21 to 395. When the three-tube test is used, 20 of
the 62 possible test combinations account for 99% of all results, whereas with the five-tube test, 49 of
the possible 214 combinations account for 99% of all results.
131 In a collaborative study on coliform
densities in foods, a three-tube MPN value of 10 was found to be as high as 34, whereas in another phase
of the study, the upper limit could be as high as 60.
121 Although Woodward
131 concluded that many
MPN values are improbable, this method of analysis has gained popularity. Among the advantages it
offers are the following:
1. It is relatively simple.
2. Results from one laboratory are more likely than SPC results to agree with those from another
laboratory.
3. Specific groups of organisms can be determined by use of appropriate selective and differential
media.
4. It is the method of choice for determining fecal coliform densities.
Among the drawbacks to its use are the large volume of glassware required (especially for the
five-tube method), the lack of opportunity to observe the colonial morphology of the organisms, and
its lack of precision.
TEMPO
r
is an MPN-based method that employs an enumerating card with a specific medium that
allows rapid fluorescent detection of target organisms, and results may be obtained within 24 h. It
obviates the need for serial dilutions.