Indicators of Bacterial Pathogens ◾ 425
11. There should preferably be a direct relationship between the level of an indicator present
and the probability of the presence of an enteric pathogen in a food. This will help set up
regulatory standards or specifications for an indicator limit for the acceptance or rejection of
a food for consumption. For this criterion, it is very important to recognize whether the high
numbers of an indicator in a food have resulted from a high level of initial contamination
(and a greater chance for the presence of a pathogen) or from their growth in the food from
a very low initial contamination (in which case a pathogen may not be present even when
the indicator is present in high numbers).
It is apparent that no single bacterial group or species will be able to meet all the criteria of
an ideal indicator. Several bacterial groups or species satisfy many of these criteria. The characteristics, advantages, and disadvantages of some of the important and accepted indicator bacterial
groups and species (of enteric pathogens) are described here. 1–3
Coliform Group
Coliforms
In the coliform group, coliforms, fecal coliforms, and Esc. coli are discussed. They are not separate,
as both fecal coliforms (mostly Esc. coli) and Esc. coli belong to coliforms. Depending on the situation, food, water, and environmental samples are examined for one or more of the three.
Organisms and Sources
The term “coliform” does not have taxonomic value; rather, it represents a group of species from
several genera, namely, Escherichia, Enterobacter, Klebsiella, Citrobacter, and probably Aeromonas
and Serratia. The main reason for grouping them together is their many common characteristics.
They are all gram-negative, non-spore-forming rods; many are motile, are facultative anaerobes
resistant to many surface-active agents, and ferment lactose to produce acid and gas within 48
hours at 32°C or 35°C. Some species can grow at higher temperatures (44.5°C), whereas others
can grow at 4°C–5°C. All are able to grow in foods except in those that are at pH ≤ 4.0 (a few that
are acid resistant can grow or survive) and A W ≤ 0.92. All are sensitive to low-heat treatments and
are killed by pasteurization.
They can be present in the feces of humans and warm-blooded animals and birds. Some can
be present in the environment and contaminate food. Thus, some Klebsiella spp. and Enterobacter
spp. are found in soil, where they can multiply and reach high population levels. Some are found
in water and plants. 1,4–6
Occurrence and Significance in Food
Coliforms are expected to be present in many raw foods and food ingredients of animal and plant
origin. In some plant foods, they are present in very high numbers because of contamination from
soil. Because they can grow in foods, some even at refrigerated temperatures, a low initial number
can reach a high level during storage. The occurrence of some coliforms of nonfecal origin and
their ability to grow in many foods reduce the specificity of coliforms as an indicator of fecal contamination in raw foods. In contrast, in heat-processed (pasteurized) products, their presence is
11. There should preferably be a direct relationship between the level of an indicator present
and the probability of the presence of an enteric pathogen in a food. This will help set up
regulatory standards or specifications for an indicator limit for the acceptance or rejection of
a food for consumption. For this criterion, it is very important to recognize whether the high
numbers of an indicator in a food have resulted from a high level of initial contamination
(and a greater chance for the presence of a pathogen) or from their growth in the food from
a very low initial contamination (in which case a pathogen may not be present even when
the indicator is present in high numbers).
It is apparent that no single bacterial group or species will be able to meet all the criteria of
an ideal indicator. Several bacterial groups or species satisfy many of these criteria. The characteristics, advantages, and disadvantages of some of the important and accepted indicator bacterial
groups and species (of enteric pathogens) are described here. 1–3
Coliform Group
Coliforms
In the coliform group, coliforms, fecal coliforms, and Esc. coli are discussed. They are not separate,
as both fecal coliforms (mostly Esc. coli) and Esc. coli belong to coliforms. Depending on the situation, food, water, and environmental samples are examined for one or more of the three.
Organisms and Sources
The term “coliform” does not have taxonomic value; rather, it represents a group of species from
several genera, namely, Escherichia, Enterobacter, Klebsiella, Citrobacter, and probably Aeromonas
and Serratia. The main reason for grouping them together is their many common characteristics.
They are all gram-negative, non-spore-forming rods; many are motile, are facultative anaerobes
resistant to many surface-active agents, and ferment lactose to produce acid and gas within 48
hours at 32°C or 35°C. Some species can grow at higher temperatures (44.5°C), whereas others
can grow at 4°C–5°C. All are able to grow in foods except in those that are at pH ≤ 4.0 (a few that
are acid resistant can grow or survive) and A W ≤ 0.92. All are sensitive to low-heat treatments and
are killed by pasteurization.
They can be present in the feces of humans and warm-blooded animals and birds. Some can
be present in the environment and contaminate food. Thus, some Klebsiella spp. and Enterobacter
spp. are found in soil, where they can multiply and reach high population levels. Some are found
in water and plants. 1,4–6
Occurrence and Significance in Food
Coliforms are expected to be present in many raw foods and food ingredients of animal and plant
origin. In some plant foods, they are present in very high numbers because of contamination from
soil. Because they can grow in foods, some even at refrigerated temperatures, a low initial number
can reach a high level during storage. The occurrence of some coliforms of nonfecal origin and
their ability to grow in many foods reduce the specificity of coliforms as an indicator of fecal contamination in raw foods. In contrast, in heat-processed (pasteurized) products, their presence is