22 ◾ Fundamental Food Microbiology
Gram-Negative Aerobes
Acetobacter. Ellipsoid to rod-shaped (0.6 × 4 μm); occur singly or in short chains; motile or
nonmotile; aerobes; oxidize ethanol to acetic acid; mesophiles. Cause souring of alcoholic
beverages and fruit juices and used to produce vinegar (acetic acid). Can also spoil some
fruits (rot). Widely distributed in plants and in places where alcohol fermentation occurs.
Important species: Acetobacter aceti.
Acinetobacter. Rods (1 × 2 μm); occur in pairs or small chains; show twitching motility because
of the presence of polar fimbriae; strictly aerobic and grow between 20°C and 35°C. Found
in soil, water, and sewage. Important species: Acinetobacter calcoaceticus.
Alcaligenes. Rods or coccobacilli (0.5 × 1 μm); motile; present in water, soil, or fecal material;
mesophiles. Cause spoilage of protein-rich foods. Species: Alcaligenes faecalis.
Alteromonas. Most currently assigned Alteromonas species are of marine origin and might
be present in foods of marine origin. Need 100 mM NaCl for optimum growth (unlike
Pseudomonas). Because Alteromonas putrefacience (species reclassified as Shewanella
putrefaciens) has many characters similar to those of Pseudomonas, it was previously
designated as Pseudomonas putrefacience. Strains important in fish and meat spoilage.
Psychrotrophs.
Brucella. Coccobacilli (0.5 × 1.0 μm); mostly single; nonmotile. Different species cause
disease in animals, including cattle, pigs, and sheep. They are also human pathogens
and have been implicated in foodborne brucellosis. Brucella abortus causes abortion in
cows.
Campylobacter. Two species, Campylobacter jejuni and Cam. coli, are foodborne pathogens.
Small (0.2 × 1 μm) microaerophilic, helical, motile cells found in the intestinal tract of
humans, animals, and birds. Mesophiles.
Flavobacterium. Rods with parallel sides (0.5 × 3 μm); nonmotile; colonies colored; some
species psychrotrophic. Cause spoilage of milk, meat, and other protein foods. Species:
Flavobacterium aquatile. Gluconobacter. Many characteristics of this group are similar to
those of Acetobacter. Gluconobacter oxydans causes spoilage of pineapples, apples, and pears
(rot).
Moraxella. Very short rods, frequently approaching coccoid shape (1 × 1.5 μm); occur singly,
in pairs or short chains; may be capsulated; twitching motility may be present in some cells;
optimum growth at 30°C–35°C. Found in the mucous membrane of animals and humans.
Important species: Moraxella lacunata.
Pseudomonas. Straight or curved (0.5 × 5 μm); aerobes; motile rods; psychrotrophs (grow at
low temperatures). Found widely in the environment. This genus includes large numbers of
species. Some important species in foods are Pseudomonas fluorescens, Pse. aeruginosa, and
Pse. putida. Important spoilage bacteria, can metabolize a wide variety of carbohydrates,
proteins, and lipids in foods.
Psychrobacter. The genus was created in 1986 and contains one species: Psychrobacter immobilis. Coccobacilli (1 × 1.5 μm) and nonmotile. Can grow at 5°C or below, show optimum growth at 20°C, and are unable to grow at 35°C. Found in fish, meat, and poultry
products.
Xanthomonas. Most characteristics of this group are similar to those for Pseudomonas. Plant
pathogens, can thus cause spoilage of fruits and vegetables. Xanthomonas campestris strains
used to produce xanthan gum, which is used as a food stabilizer.
Gram-Negative Aerobes
Acetobacter. Ellipsoid to rod-shaped (0.6 × 4 μm); occur singly or in short chains; motile or
nonmotile; aerobes; oxidize ethanol to acetic acid; mesophiles. Cause souring of alcoholic
beverages and fruit juices and used to produce vinegar (acetic acid). Can also spoil some
fruits (rot). Widely distributed in plants and in places where alcohol fermentation occurs.
Important species: Acetobacter aceti.
Acinetobacter. Rods (1 × 2 μm); occur in pairs or small chains; show twitching motility because
of the presence of polar fimbriae; strictly aerobic and grow between 20°C and 35°C. Found
in soil, water, and sewage. Important species: Acinetobacter calcoaceticus.
Alcaligenes. Rods or coccobacilli (0.5 × 1 μm); motile; present in water, soil, or fecal material;
mesophiles. Cause spoilage of protein-rich foods. Species: Alcaligenes faecalis.
Alteromonas. Most currently assigned Alteromonas species are of marine origin and might
be present in foods of marine origin. Need 100 mM NaCl for optimum growth (unlike
Pseudomonas). Because Alteromonas putrefacience (species reclassified as Shewanella
putrefaciens) has many characters similar to those of Pseudomonas, it was previously
designated as Pseudomonas putrefacience. Strains important in fish and meat spoilage.
Psychrotrophs.
Brucella. Coccobacilli (0.5 × 1.0 μm); mostly single; nonmotile. Different species cause
disease in animals, including cattle, pigs, and sheep. They are also human pathogens
and have been implicated in foodborne brucellosis. Brucella abortus causes abortion in
cows.
Campylobacter. Two species, Campylobacter jejuni and Cam. coli, are foodborne pathogens.
Small (0.2 × 1 μm) microaerophilic, helical, motile cells found in the intestinal tract of
humans, animals, and birds. Mesophiles.
Flavobacterium. Rods with parallel sides (0.5 × 3 μm); nonmotile; colonies colored; some
species psychrotrophic. Cause spoilage of milk, meat, and other protein foods. Species:
Flavobacterium aquatile. Gluconobacter. Many characteristics of this group are similar to
those of Acetobacter. Gluconobacter oxydans causes spoilage of pineapples, apples, and pears
(rot).
Moraxella. Very short rods, frequently approaching coccoid shape (1 × 1.5 μm); occur singly,
in pairs or short chains; may be capsulated; twitching motility may be present in some cells;
optimum growth at 30°C–35°C. Found in the mucous membrane of animals and humans.
Important species: Moraxella lacunata.
Pseudomonas. Straight or curved (0.5 × 5 μm); aerobes; motile rods; psychrotrophs (grow at
low temperatures). Found widely in the environment. This genus includes large numbers of
species. Some important species in foods are Pseudomonas fluorescens, Pse. aeruginosa, and
Pse. putida. Important spoilage bacteria, can metabolize a wide variety of carbohydrates,
proteins, and lipids in foods.
Psychrobacter. The genus was created in 1986 and contains one species: Psychrobacter immobilis. Coccobacilli (1 × 1.5 μm) and nonmotile. Can grow at 5°C or below, show optimum growth at 20°C, and are unable to grow at 35°C. Found in fish, meat, and poultry
products.
Xanthomonas. Most characteristics of this group are similar to those for Pseudomonas. Plant
pathogens, can thus cause spoilage of fruits and vegetables. Xanthomonas campestris strains
used to produce xanthan gum, which is used as a food stabilizer.