Conventional, Molecular, and Biosensor-Based Detection Methods ◾ 539
media, such as plate count agar (PCA), tryptic soy agar, nutrient agar, brain-heart infusion agar,
and others. Different media can give different results. However, PCA is recommended for colonyforming unit (CFU) determination of several groups. The temperature and time of plate incubation required for the colonies to develop differs for the microbial groups being enumerated. For
SPC, it is 32°C for 48 hours; for APC, it is 35°C for 48 hours; and for psychrotrophic counts, it
is 7°C for 10 days or 10°C for seven days. The same procedure with specific modifications can
be used to determine thermophilic, thermoduric, and anaerobic groups present in a food sample.
The specific group(s) to be tested depends on their relative importance in a food. For a vacuumpackaged refrigerated food, the most important groups will be psychrotrophic, anaerobic, and
facultative anaerobic groups (see Chapter 21).
CFU in Nonselective Differential Chromogenic Media
A nonselective medium is supplemented with an agent capable of differentiating the colonies
produced by specific group(s) of microorganisms that differ in metabolic or physiological characteristics from one another in the population by producing chromogenic pigments. pH indicators
or oxidation-reduction indicators are often used in the medium. Thus, the colonies of cells capable
of metabolizing lactose to lactic acid are differentiated from those that do not ferment lactose by
growing them in an agar medium supplemented with lactose as a carbon source and a pH indicator, such as bromocresol purple. The lactose-fermenting colonies will be yellow, and the others that
cannot ferment lactose, but grow, will be white against a purple background. Differential methods are also used in specific media for the enumeration of proteolytic, lipolytic, and pectinolytic
microbial groups in a food.
CFU in Selective Agar Media
A medium can be supplemented with one or more selective or inhibitory agents, such as antibiotics, salts, and so forth, and used by pour or surface plating of serially diluted samples. In the presence of such agents, only the microorganisms resistant to them can grow. Incubation conditions
to stimulate colony formation differ with the organisms being studied. Enumeration of aciduric
bacteria in a medium at pH 5.0, yeasts and molds in a medium at pH 3.5, and Clostridium perfringens in the presence of cycloserine are examples of selective enumeration of specific groups present
in a food. Halophilic and osmophilic microorganisms can also be enumerated similarly by specific
selective procedures.
CFU in Selective-Differential Chromogenic Agar Media
In this method, a medium is supplemented with one or more selective agents to allow selective
growth of specific resistant microbial group(s) while inhibiting growth of other sensitive associative microorganisms. In addition to selective agent(s), a medium is also supplemented with agent(s)
that enable each type among the selective microbial groups to produce colonies with pigments
(chromogenic) that differ in characteristics from one another. Violet red bile agar for coliforms,
KF-azide agar for Enterococcus spp., V-J agar or Baird-Parker agar for Sta. aureus, and media
recommended for the enumeration of some pathogens (e.g., Yersinia enterocolitica, Campylobacter
jejuni, Salmonella enterica, Lis. monocytogenes, Clo. perfringens, Aeromonas hydrophila) are selective
as well as differential agar media. Because of the presence of one or more selective compounds,
they allow selective growth and colony formation of several closely related species. The differential
media, such as plate count agar (PCA), tryptic soy agar, nutrient agar, brain-heart infusion agar,
and others. Different media can give different results. However, PCA is recommended for colonyforming unit (CFU) determination of several groups. The temperature and time of plate incubation required for the colonies to develop differs for the microbial groups being enumerated. For
SPC, it is 32°C for 48 hours; for APC, it is 35°C for 48 hours; and for psychrotrophic counts, it
is 7°C for 10 days or 10°C for seven days. The same procedure with specific modifications can
be used to determine thermophilic, thermoduric, and anaerobic groups present in a food sample.
The specific group(s) to be tested depends on their relative importance in a food. For a vacuumpackaged refrigerated food, the most important groups will be psychrotrophic, anaerobic, and
facultative anaerobic groups (see Chapter 21).
CFU in Nonselective Differential Chromogenic Media
A nonselective medium is supplemented with an agent capable of differentiating the colonies
produced by specific group(s) of microorganisms that differ in metabolic or physiological characteristics from one another in the population by producing chromogenic pigments. pH indicators
or oxidation-reduction indicators are often used in the medium. Thus, the colonies of cells capable
of metabolizing lactose to lactic acid are differentiated from those that do not ferment lactose by
growing them in an agar medium supplemented with lactose as a carbon source and a pH indicator, such as bromocresol purple. The lactose-fermenting colonies will be yellow, and the others that
cannot ferment lactose, but grow, will be white against a purple background. Differential methods are also used in specific media for the enumeration of proteolytic, lipolytic, and pectinolytic
microbial groups in a food.
CFU in Selective Agar Media
A medium can be supplemented with one or more selective or inhibitory agents, such as antibiotics, salts, and so forth, and used by pour or surface plating of serially diluted samples. In the presence of such agents, only the microorganisms resistant to them can grow. Incubation conditions
to stimulate colony formation differ with the organisms being studied. Enumeration of aciduric
bacteria in a medium at pH 5.0, yeasts and molds in a medium at pH 3.5, and Clostridium perfringens in the presence of cycloserine are examples of selective enumeration of specific groups present
in a food. Halophilic and osmophilic microorganisms can also be enumerated similarly by specific
selective procedures.
CFU in Selective-Differential Chromogenic Agar Media
In this method, a medium is supplemented with one or more selective agents to allow selective
growth of specific resistant microbial group(s) while inhibiting growth of other sensitive associative microorganisms. In addition to selective agent(s), a medium is also supplemented with agent(s)
that enable each type among the selective microbial groups to produce colonies with pigments
(chromogenic) that differ in characteristics from one another. Violet red bile agar for coliforms,
KF-azide agar for Enterococcus spp., V-J agar or Baird-Parker agar for Sta. aureus, and media
recommended for the enumeration of some pathogens (e.g., Yersinia enterocolitica, Campylobacter
jejuni, Salmonella enterica, Lis. monocytogenes, Clo. perfringens, Aeromonas hydrophila) are selective
as well as differential agar media. Because of the presence of one or more selective compounds,
they allow selective growth and colony formation of several closely related species. The differential