Characteristics of Predominant Microorganisms in Food ◾ 23
Gram-Negative Facultative Anaerobes
Aeromonas. Small rods (0.5 × 1.0 μm); occur singly or in pairs; motile; psychrotrophs. Found in
aquatic environments; generally a fish or marine pathogen. Aeromonas hydrophila has been
suspected as a potential foodborne pathogen.
Citrobacter. Straight rods (1 × 4 μm); single or in pairs; usually motile; mesophiles.
Found in the intestinal contents of humans, animals, and birds and in the environment. Included in the coliform group as an indicator of sanitation. Important species:
Citrobacter freundii.
Cronobacter. (Previously known as Enterobacter) rods (1 × 3 μm): oxidase-negative, catalasepositive, facultatively anaerobic, motile (peritrichous flagella); mesophiles. Found in the
intestinal contents of humans, animals, and birds and in the environment. Involved with
powdered infant formula. Important species: Cronobacter sakazakii.
Escherichia. Straight rods (1 × 3 μm); motile or nonmotile; mesophiles. Found in the intestinal
contents of humans, warm-blooded animals, and birds. Many strains are nonpathogenic,
but some strains are pathogenic to humans and animals and involved in foodborne diseases.
Used as an indicator of sanitation (theoretically nonpathogenic strains) in coliform and fecal
coliform groups. Important species: Escherichia coli.
Enterobacter. Straight rods (1 × 2 mm); motile; mesophiles. Found in the intestinal contents of
humans, animals, and birds and in the environment. Included in the coliform group as an
indicator of sanitation. Important species: Enterobacter aerogenes.
Edwardsiella. Small rods (1 × 2 μm); motile. Found in the intestines of cold-blooded animals
and in fresh water. Can be pathogenic to humans, but involvement in foodborne disease is
not shown.
Erwinia. Small rods (1 × 2 μm); occur in pairs or short chains; motile; facultative anaerobes;
optimum growth at 30°C. Many are plant pathogens and cause spoilage of plant products.
Species: Erwinia amylovora.
Hafnia. Small rods (1 × 2 μm); motile; mesophiles. Found in intestinal contents of humans,
animals, and birds and in the environment. Associated with food spoilage and may cause
food poisoning. Species: Hafnia alvei.
Klebsiella. Medium rods (1 × 4 μm); occur singly or in pairs; motile; capsulated; mesophiles.
Found in the intestinal contents of humans, animals, and birds; soil; water; and grains.
Included in the coliform group as an indicator of sanitation. Important species: Klebsiella
pneumoniae.
Morganella. Small rods (0.5 × 1 μm); motile; mesophiles. Found in the intestinal contents of
humans and animals. Can be pathogenic but has not been implicated in foodborne disease.
Species: Morganella morganii.
Plesiomonas. Small rods (0.5 × 1.0 μm); motile. Found in fish and aquatic animals. Plesiomonas
shigelloides has been suspected as a potential foodborne pathogen.
Proteus. Straight, small rods (0.5 × 1.5 μm); highly motile; form swarms on agar media; some
grow at low temperatures. Occur in the intestinal contents of humans and animals and the
environment. Many involved in food spoilage. Species: Proteus vulgaris.
Salmonella. Medium rods (1 × 3 μm); usually motile; mesophiles. There are more than 2500
serovars, and all are regarded as pathogens. Found in the intestinal contents of humans,
animals, birds, and insects. Major cause of foodborne diseases. Species: Salmonella enterica
ssp. enterica. (See Chapter 26 for the new naming system.)
Gram-Negative Facultative Anaerobes
Aeromonas. Small rods (0.5 × 1.0 μm); occur singly or in pairs; motile; psychrotrophs. Found in
aquatic environments; generally a fish or marine pathogen. Aeromonas hydrophila has been
suspected as a potential foodborne pathogen.
Citrobacter. Straight rods (1 × 4 μm); single or in pairs; usually motile; mesophiles.
Found in the intestinal contents of humans, animals, and birds and in the environment. Included in the coliform group as an indicator of sanitation. Important species:
Citrobacter freundii.
Cronobacter. (Previously known as Enterobacter) rods (1 × 3 μm): oxidase-negative, catalasepositive, facultatively anaerobic, motile (peritrichous flagella); mesophiles. Found in the
intestinal contents of humans, animals, and birds and in the environment. Involved with
powdered infant formula. Important species: Cronobacter sakazakii.
Escherichia. Straight rods (1 × 3 μm); motile or nonmotile; mesophiles. Found in the intestinal
contents of humans, warm-blooded animals, and birds. Many strains are nonpathogenic,
but some strains are pathogenic to humans and animals and involved in foodborne diseases.
Used as an indicator of sanitation (theoretically nonpathogenic strains) in coliform and fecal
coliform groups. Important species: Escherichia coli.
Enterobacter. Straight rods (1 × 2 mm); motile; mesophiles. Found in the intestinal contents of
humans, animals, and birds and in the environment. Included in the coliform group as an
indicator of sanitation. Important species: Enterobacter aerogenes.
Edwardsiella. Small rods (1 × 2 μm); motile. Found in the intestines of cold-blooded animals
and in fresh water. Can be pathogenic to humans, but involvement in foodborne disease is
not shown.
Erwinia. Small rods (1 × 2 μm); occur in pairs or short chains; motile; facultative anaerobes;
optimum growth at 30°C. Many are plant pathogens and cause spoilage of plant products.
Species: Erwinia amylovora.
Hafnia. Small rods (1 × 2 μm); motile; mesophiles. Found in intestinal contents of humans,
animals, and birds and in the environment. Associated with food spoilage and may cause
food poisoning. Species: Hafnia alvei.
Klebsiella. Medium rods (1 × 4 μm); occur singly or in pairs; motile; capsulated; mesophiles.
Found in the intestinal contents of humans, animals, and birds; soil; water; and grains.
Included in the coliform group as an indicator of sanitation. Important species: Klebsiella
pneumoniae.
Morganella. Small rods (0.5 × 1 μm); motile; mesophiles. Found in the intestinal contents of
humans and animals. Can be pathogenic but has not been implicated in foodborne disease.
Species: Morganella morganii.
Plesiomonas. Small rods (0.5 × 1.0 μm); motile. Found in fish and aquatic animals. Plesiomonas
shigelloides has been suspected as a potential foodborne pathogen.
Proteus. Straight, small rods (0.5 × 1.5 μm); highly motile; form swarms on agar media; some
grow at low temperatures. Occur in the intestinal contents of humans and animals and the
environment. Many involved in food spoilage. Species: Proteus vulgaris.
Salmonella. Medium rods (1 × 3 μm); usually motile; mesophiles. There are more than 2500
serovars, and all are regarded as pathogens. Found in the intestinal contents of humans,
animals, birds, and insects. Major cause of foodborne diseases. Species: Salmonella enterica
ssp. enterica. (See Chapter 26 for the new naming system.)