424 ◾ Fundamental Food Microbiology
toxins, specific methods are recommended for their detection and identification. The total aerobic
plate count (TAPC), also known as mesophilic aerobic bacteria (MAB) or standard plate count
(SPC), is not an indicator of the possible presence of pathogens. Instead, it is an indicator of the
microbiological quality of a food as well as a measure of the level of sanitation used in the handling
(processing and storage) of a food, such as cooked, ready-to-eat foods, pasteurized milk, and spices.
Criteria for ideal indicators
Two types of microbiological indicator assays are used: index organisms and indicator organisms. 1,2 An index organism is an organism, a group of organisms, or their metabolic byproducts
that is predictive of the presence of a pathogen. However, this system is not as effective as an indicator organism, which can indicate the presence of a pathogen. Several criteria were suggested for
selecting a bacterial group or species as an indicator of enteric foodborne pathogens. 1–3 Some are
listed with brief explanations:
1. The indicator should preferably contain a single species or a few species with some common
and identifiable biochemical and other characteristics in order to be able to identify them
from the many different types of microorganisms that might be present in a food.
2. The indicator should be of enteric origin, that is, it should share the same habitat as the enteric
pathogens and should be present when and where the pathogens are likely to be present.
3. The indicator should be nonpathogenic so that its handling in the laboratory does not require
safety precautions as required for pathogens.
4. The indicator should be present in the fecal matter in much higher numbers than the enteric
pathogens so that they can be easily detected (enumerated or isolated) even when a food is
contaminated with small amounts of fecal matter.
5. The indicator should be detected (enumerated or isolated) and identified within a short time,
easily, and economically, so that a product, following processing, can be distributed quickly,
and several samples from a batch can be tested.
6. The indicator should be detected by using one or more newly developed molecular biology
techniques for rapid identification.
7. The indicator should be detected (enumerated or isolated) even in the presence of large numbers of associated microorganisms, which can be achieved by using compounds that inhibit
growth of associated microorganisms but not of the indicator.
8. The indicator should have a growth and survival rate in a food the same as that of the enteric
pathogens. It should not grow more slowly or die off faster than the pathogens in a food. If it
dies off more rapidly than the pathogen, then, theoretically, a food can be free of the indicator during storage but can still have pathogens.
9. The indicator should not suffer sublethal injury more (in degree) than the pathogens do
when exposed to physical and chemical stresses. If the indicator is more susceptible to sublethal stresses, it will not be detected by the selective methods used in the enumeration, and
a food may show no or very low acceptable levels of the indicator even when the pathogens
are present at higher levels.
10. The indicator should preferably be present when the pathogens are present in a food; conversely, it should be absent when the enteric pathogens are absent. Unless such correlations
exist, the importance of an indicator to indicate the possible presence of a pathogen in a food
reduces greatly.
toxins, specific methods are recommended for their detection and identification. The total aerobic
plate count (TAPC), also known as mesophilic aerobic bacteria (MAB) or standard plate count
(SPC), is not an indicator of the possible presence of pathogens. Instead, it is an indicator of the
microbiological quality of a food as well as a measure of the level of sanitation used in the handling
(processing and storage) of a food, such as cooked, ready-to-eat foods, pasteurized milk, and spices.
Criteria for ideal indicators
Two types of microbiological indicator assays are used: index organisms and indicator organisms. 1,2 An index organism is an organism, a group of organisms, or their metabolic byproducts
that is predictive of the presence of a pathogen. However, this system is not as effective as an indicator organism, which can indicate the presence of a pathogen. Several criteria were suggested for
selecting a bacterial group or species as an indicator of enteric foodborne pathogens. 1–3 Some are
listed with brief explanations:
1. The indicator should preferably contain a single species or a few species with some common
and identifiable biochemical and other characteristics in order to be able to identify them
from the many different types of microorganisms that might be present in a food.
2. The indicator should be of enteric origin, that is, it should share the same habitat as the enteric
pathogens and should be present when and where the pathogens are likely to be present.
3. The indicator should be nonpathogenic so that its handling in the laboratory does not require
safety precautions as required for pathogens.
4. The indicator should be present in the fecal matter in much higher numbers than the enteric
pathogens so that they can be easily detected (enumerated or isolated) even when a food is
contaminated with small amounts of fecal matter.
5. The indicator should be detected (enumerated or isolated) and identified within a short time,
easily, and economically, so that a product, following processing, can be distributed quickly,
and several samples from a batch can be tested.
6. The indicator should be detected by using one or more newly developed molecular biology
techniques for rapid identification.
7. The indicator should be detected (enumerated or isolated) even in the presence of large numbers of associated microorganisms, which can be achieved by using compounds that inhibit
growth of associated microorganisms but not of the indicator.
8. The indicator should have a growth and survival rate in a food the same as that of the enteric
pathogens. It should not grow more slowly or die off faster than the pathogens in a food. If it
dies off more rapidly than the pathogen, then, theoretically, a food can be free of the indicator during storage but can still have pathogens.
9. The indicator should not suffer sublethal injury more (in degree) than the pathogens do
when exposed to physical and chemical stresses. If the indicator is more susceptible to sublethal stresses, it will not be detected by the selective methods used in the enumeration, and
a food may show no or very low acceptable levels of the indicator even when the pathogens
are present at higher levels.
10. The indicator should preferably be present when the pathogens are present in a food; conversely, it should be absent when the enteric pathogens are absent. Unless such correlations
exist, the importance of an indicator to indicate the possible presence of a pathogen in a food
reduces greatly.