18 ◾ Fundamental Food Microbiology
Viruses
Viruses are regarded as noncellular entities or particles. Bacterial viruses (bacteriophages)
important in food microbiology are widely distributed in nature. 5 They are composed of nucleic
acids (DNA or RNA) and several proteins. The proteins forming the head (surrounding the
nucleic acid) and tail are called capsids. Some also carry another layer called an envelope made
of a lipid bilayer, and these viruses are called “enveloped viruses.” Some viruses carry appendages or surface molecules for attachment to host cells. A bacteriophage attaches itself to the
surface of a host bacterial cell and inoculates its nucleic acid into the host cell. Subsequently,
many phages form inside a host cell and are released outside following lysis of the cell. This is
discussed in Chapter 14.
Several pathogenic viruses have been identified as causing foodborne diseases in humans. The
two most important viruses implicated in foodborne outbreaks are hepatitis A and norovirus.
Both are single-stranded RNA viruses. Hepatitis A is a small, naked, polyhedral enteric virus
approximately 30 nm in diameter. The RNA strand is enclosed in a capsid. In addition, other
emerging viruses responsible for foodborne diseases include Hepatitis E virus (HEV) and Nipah
virus. Viral diseases are discussed in Chapters 28 and 29.
important Microorganisms in Food
Important Mold Genera
Molds are important in food because they can grow even in conditions in which many bacteria
cannot grow, such as low pH, low water activity (A W ), and high osmotic pressure. Many types
of molds are found in foods. 6,7 They are important spoilage microorganisms. Many strains also
produce mycotoxins and have been implicated in foodborne intoxication (see Chapter 28). Some
mycotoxins are carcinogenic or mutagenic and cause organ-specific pathology, such as hepatotoxic
(liver toxicity) or nephrotoxic (kidney toxicity). Many are used in food bioprocessing. Finally,
many are used to produce food additives and enzymes. Some of the most common genera of molds
found in food are listed here (also see Figure 9.1).
1. Aspergillus. It is widely distributed and contains many species important in food. Members
have septate hyphae and produce black-colored asexual spores on conidia. Many are xerophilic (able to grow in low A W ) and can grow in grains, causing spoilage. They are also
involved in spoilage of foods such as jams, cured ham, nuts, and fruits and vegetables (rot).
Some species or strains produce mycotoxins (e.g., Aspergillus flavus produces aflatoxin).
Many species or strains are also used in food and food-additive processing. Asp. oryzae is
used to hydrolyze starch by α-amylase in the production of sake. Asp. niger is used to process
citric acid from sucrose and to produce enzymes such as β-galactosidase.
2. Alternaria. Members are septate and form dark-colored spores or conidia. They cause rot in
tomatoes and rancid flavor in dairy products. Some species or strains produce mycotoxins
(Alternaria toxin). Species: Alternaria citri.
3. Fusarium. Many types are associated with rot in citrus fruits, potatoes, and grains. They
form cottony growth and produce septate, sickle-shaped conidia. They produce mycotoxins:
fumonosins, zearalenone, tricothecenes, and dexoynivalenol (DON). Species: Fusarium verticillioides, Fus. graminearum, Fus. proliferatum, and so forth.
Viruses
Viruses are regarded as noncellular entities or particles. Bacterial viruses (bacteriophages)
important in food microbiology are widely distributed in nature. 5 They are composed of nucleic
acids (DNA or RNA) and several proteins. The proteins forming the head (surrounding the
nucleic acid) and tail are called capsids. Some also carry another layer called an envelope made
of a lipid bilayer, and these viruses are called “enveloped viruses.” Some viruses carry appendages or surface molecules for attachment to host cells. A bacteriophage attaches itself to the
surface of a host bacterial cell and inoculates its nucleic acid into the host cell. Subsequently,
many phages form inside a host cell and are released outside following lysis of the cell. This is
discussed in Chapter 14.
Several pathogenic viruses have been identified as causing foodborne diseases in humans. The
two most important viruses implicated in foodborne outbreaks are hepatitis A and norovirus.
Both are single-stranded RNA viruses. Hepatitis A is a small, naked, polyhedral enteric virus
approximately 30 nm in diameter. The RNA strand is enclosed in a capsid. In addition, other
emerging viruses responsible for foodborne diseases include Hepatitis E virus (HEV) and Nipah
virus. Viral diseases are discussed in Chapters 28 and 29.
important Microorganisms in Food
Important Mold Genera
Molds are important in food because they can grow even in conditions in which many bacteria
cannot grow, such as low pH, low water activity (A W ), and high osmotic pressure. Many types
of molds are found in foods. 6,7 They are important spoilage microorganisms. Many strains also
produce mycotoxins and have been implicated in foodborne intoxication (see Chapter 28). Some
mycotoxins are carcinogenic or mutagenic and cause organ-specific pathology, such as hepatotoxic
(liver toxicity) or nephrotoxic (kidney toxicity). Many are used in food bioprocessing. Finally,
many are used to produce food additives and enzymes. Some of the most common genera of molds
found in food are listed here (also see Figure 9.1).
1. Aspergillus. It is widely distributed and contains many species important in food. Members
have septate hyphae and produce black-colored asexual spores on conidia. Many are xerophilic (able to grow in low A W ) and can grow in grains, causing spoilage. They are also
involved in spoilage of foods such as jams, cured ham, nuts, and fruits and vegetables (rot).
Some species or strains produce mycotoxins (e.g., Aspergillus flavus produces aflatoxin).
Many species or strains are also used in food and food-additive processing. Asp. oryzae is
used to hydrolyze starch by α-amylase in the production of sake. Asp. niger is used to process
citric acid from sucrose and to produce enzymes such as β-galactosidase.
2. Alternaria. Members are septate and form dark-colored spores or conidia. They cause rot in
tomatoes and rancid flavor in dairy products. Some species or strains produce mycotoxins
(Alternaria toxin). Species: Alternaria citri.
3. Fusarium. Many types are associated with rot in citrus fruits, potatoes, and grains. They
form cottony growth and produce septate, sickle-shaped conidia. They produce mycotoxins:
fumonosins, zearalenone, tricothecenes, and dexoynivalenol (DON). Species: Fusarium verticillioides, Fus. graminearum, Fus. proliferatum, and so forth.