Indicators of Bacterial Pathogens ◾ 427
other coliforms by the indole production from tryptone, methyl red reduction resulting from acid
production (red coloration), Voges–Proskauer reaction (production of acetyl-methyl carbinol from
glucose), and citrate utilization as a C-source (IMViC) reaction patterns. Esc. coli Type I and Type
II give IMViC reaction patterns, respectively, of + + – – and – + – –. The – + – – reaction pattern
of Esc. coli Type II could also be a result of slow or low production of indole from tryptone (or
peptone). The IMViC tests are conducted with an isolate obtained after testing a food sample for
the coliform group or fecal coliform group. However, there is a concern now about the adequacy
of these reaction patterns to identify Esc. coli types.
Initially, Esc. coli types were used as indicators of fecal contamination and the possible presence
of enteric pathogens (in food) with the considerations that they are nonpathogenic and occur normally in the GI tract of humans, animals, and birds in high numbers. However, it is now known
that some variants and strains of Esc. coli are pathogenic (e.g., Esc. coli O157:H7). None of the
methods mentioned previously are able to differentiate pathogenic from nonpathogenic Esc. coli
strains; this can be achieved only through specific tests designed to identify different pathogenic
Esc. coli strains. This discussion emphasizes the value of Esc. coli as an indicator. 1 The significance
and importance of pathogenic Esc. coli have been discussed previously (Chapters 26 and 27).
Occurrence and Significance in Food
Esc. coli is present in the lower intestinal tract of humans and warm-blooded animals and birds. Its
presence in raw foods is considered an indication of direct or indirect fecal contamination. Direct
fecal contamination occurs during the processing of raw foods of animal origin and because of
poor personal hygiene of food handlers. Indirect contamination can occur through sewage and
polluted water. In heat-processed (pasteurized) foods, its presence is viewed with great concern.
Its value as an indicator of fecal contamination and the possible presence of enteric pathogens is
much greater than that of coliform and fecal coliform groups. However, the time to complete the
tests (IMViC) is relatively long (ca. five days). Some direct-plating methods have been developed
that give an indication of Esc. coli in a shorter time. There are several other inadequacies of Esc. coli
as an indicator. Esc. coli strains may die at a faster rate in dried, frozen, and low-pH products than
some enteric pathogens, and some enteric pathogens can grow at low temperatures (0°C–2°C) at
which Esc. coli strains can die. In addition, Esc. coli strains can be injured by sublethal stresses in
higher degrees than some enteric pathogens and may not be effectively detected by the recommended selective media unless a prior resuscitation (repair) step is included.
Enterobacteriaceae Group
The methods recommended for detecting coliforms, fecal coliforms, and Esc. coli are based on
the ability of these bacterial species to ferment lactose to produce gas and acid. In contrast, some
enteric pathogens do not ferment lactose, such as most Salmonella serovars. Thus, instead of only
enumerating coliforms or fecal coliforms in a food, enumeration of all the genera and species in
the Enterobacteriaceae 1 family is advocated. Because this family includes not only coliforms but
also many genera and species that are enteric pathogens, enumeration of the whole group can be
a better indicator of the level of sanitation, possible fecal contamination, and possible presence
of enteric pathogens (Table 30.1). In European countries, this concept has been used to a certain degree. The method includes the enumeration of organisms from colony-forming units in a
selective-differential agar medium containing glucose instead of lactose. 1
other coliforms by the indole production from tryptone, methyl red reduction resulting from acid
production (red coloration), Voges–Proskauer reaction (production of acetyl-methyl carbinol from
glucose), and citrate utilization as a C-source (IMViC) reaction patterns. Esc. coli Type I and Type
II give IMViC reaction patterns, respectively, of + + – – and – + – –. The – + – – reaction pattern
of Esc. coli Type II could also be a result of slow or low production of indole from tryptone (or
peptone). The IMViC tests are conducted with an isolate obtained after testing a food sample for
the coliform group or fecal coliform group. However, there is a concern now about the adequacy
of these reaction patterns to identify Esc. coli types.
Initially, Esc. coli types were used as indicators of fecal contamination and the possible presence
of enteric pathogens (in food) with the considerations that they are nonpathogenic and occur normally in the GI tract of humans, animals, and birds in high numbers. However, it is now known
that some variants and strains of Esc. coli are pathogenic (e.g., Esc. coli O157:H7). None of the
methods mentioned previously are able to differentiate pathogenic from nonpathogenic Esc. coli
strains; this can be achieved only through specific tests designed to identify different pathogenic
Esc. coli strains. This discussion emphasizes the value of Esc. coli as an indicator. 1 The significance
and importance of pathogenic Esc. coli have been discussed previously (Chapters 26 and 27).
Occurrence and Significance in Food
Esc. coli is present in the lower intestinal tract of humans and warm-blooded animals and birds. Its
presence in raw foods is considered an indication of direct or indirect fecal contamination. Direct
fecal contamination occurs during the processing of raw foods of animal origin and because of
poor personal hygiene of food handlers. Indirect contamination can occur through sewage and
polluted water. In heat-processed (pasteurized) foods, its presence is viewed with great concern.
Its value as an indicator of fecal contamination and the possible presence of enteric pathogens is
much greater than that of coliform and fecal coliform groups. However, the time to complete the
tests (IMViC) is relatively long (ca. five days). Some direct-plating methods have been developed
that give an indication of Esc. coli in a shorter time. There are several other inadequacies of Esc. coli
as an indicator. Esc. coli strains may die at a faster rate in dried, frozen, and low-pH products than
some enteric pathogens, and some enteric pathogens can grow at low temperatures (0°C–2°C) at
which Esc. coli strains can die. In addition, Esc. coli strains can be injured by sublethal stresses in
higher degrees than some enteric pathogens and may not be effectively detected by the recommended selective media unless a prior resuscitation (repair) step is included.
Enterobacteriaceae Group
The methods recommended for detecting coliforms, fecal coliforms, and Esc. coli are based on
the ability of these bacterial species to ferment lactose to produce gas and acid. In contrast, some
enteric pathogens do not ferment lactose, such as most Salmonella serovars. Thus, instead of only
enumerating coliforms or fecal coliforms in a food, enumeration of all the genera and species in
the Enterobacteriaceae 1 family is advocated. Because this family includes not only coliforms but
also many genera and species that are enteric pathogens, enumeration of the whole group can be
a better indicator of the level of sanitation, possible fecal contamination, and possible presence
of enteric pathogens (Table 30.1). In European countries, this concept has been used to a certain degree. The method includes the enumeration of organisms from colony-forming units in a
selective-differential agar medium containing glucose instead of lactose. 1