Taxonomy, Role, and Significance of Microorganisms in Foods
21
more of the cited references should be consulted. Some of the phylogenetic features of these bacteria
are presented in the Appendix.
Acinetobacter (A • ci • ne’to • bac • ter; Gr. akinetos, unable to move). These Gram-negative rods
show some affinity to the family Neisseriaceae, and some that were formerly achromobacters and
moraxellae are placed here. Also, some former acinetobacters are now in the genus Psychrobacter.
They differ from the latter and the moraxellae in being oxidase negative. They are strict aerobes that
do not reduce nitrates. Although rod-shaped cells are formed in young cultures, old cultures contain
many coccoid-shaped cells. They are widely distributed in soil and water and may be found on many
foods, especially refrigerated fresh products. The mol% G + C content of DNA for the genus is 39–47.
(See Chapter 4 for a further discussion relative to meats.) It has been proposed, based on DNA–rRNA
hybridization data, that the genera Acinetobacter, Moraxella, and Psychrobacter be placed in a new
family (Moraxellaceae), but this proposal has not been approved.
Aeromonas (ae • ro • mo’nas; gas producing). These are typically aquatic Gram-negative rods formerly
in the family Vibrionaceae but now in the family Aeromonadaceae.
32 As the generic name suggests,
they produce copious quantities of gas from the fermented sugars. They are normal inhabitants of the
intestines of fish, and some are fish pathogens. The mol% G + C content of DNA is 57–65. (The
species that possesses pathogenic properties is discussed in Chapter 31.)
Alcaligenes (al • ca • li’ge • nes; alkali producers). Although Gram negative, these organisms sometimes stain Gram positive. They are rods that do not, as the generic name suggests, ferment sugars
but instead produce alkaline reactions, especially in litmus milk. Nonpigmented, they are widely
distributed in nature in decomposing matter of all types. Raw milk, poultry products, and fecal matter are common sources. The mol% G + C content of DNA is 58–70, suggesting that the genus is
heterogeneous.
Alteromonas (al • te • ro • mo’nas; another monad). These are marine and coastal water inhabitants that
are found in and on seafoods; all species require seawater salinity for growth. They are Gram-negative
motile rods that are strict aerobe.
17
Arcobacter (Ar’co • bac • ter; L. arcus, bow). This genus was created during revision of the genera
Campylobacter, Helicobacter, and Wolinella,
45 and the three species were once classified as Campylobacter. They are Gram-negative curved or S-shaped rods that are quite similar to the campylobacters
except they can grow at 15
◦ C and are aerotolerant. They are found in poultry, raw milk, shellfish, and
water; and in cattle and swine products.
51,52 These oxidase- and catalase-positive organisms cause
abortion and enteritis in some animals, and the latter in humans is associated with A. butzleri.
Bacillus (ba • cil’lus). These are Gram-positive spore-forming rods that are aerobes in contrast to
the clostridia, which are anaerobes. Although most are mesophiles, psychrotrophs and thermophiles
exist. The genus contains only two pathogens: B. anthracis (cause of anthrax) and B. cereus. Although
most strains of the latter are nonpathogens, some cause foodborne gastroenteritis (further discussed
in Chapter 24). This genus has been delimited by the transfer of a number of its former species to
eight new genera: Alicyclobacillus, Aneurinibacillus, Brevibacillus, Gracilibacillus, Paenibacillus,
Virgibacillus, and Salibacillus.
5 Also, the former group 5 Bacillus species are now in the genus
Geobacillus, and the former B. stearothermophilus is now G. stearothermophilus.
36
21
more of the cited references should be consulted. Some of the phylogenetic features of these bacteria
are presented in the Appendix.
Acinetobacter (A • ci • ne’to • bac • ter; Gr. akinetos, unable to move). These Gram-negative rods
show some affinity to the family Neisseriaceae, and some that were formerly achromobacters and
moraxellae are placed here. Also, some former acinetobacters are now in the genus Psychrobacter.
They differ from the latter and the moraxellae in being oxidase negative. They are strict aerobes that
do not reduce nitrates. Although rod-shaped cells are formed in young cultures, old cultures contain
many coccoid-shaped cells. They are widely distributed in soil and water and may be found on many
foods, especially refrigerated fresh products. The mol% G + C content of DNA for the genus is 39–47.
(See Chapter 4 for a further discussion relative to meats.) It has been proposed, based on DNA–rRNA
hybridization data, that the genera Acinetobacter, Moraxella, and Psychrobacter be placed in a new
family (Moraxellaceae), but this proposal has not been approved.
Aeromonas (ae • ro • mo’nas; gas producing). These are typically aquatic Gram-negative rods formerly
in the family Vibrionaceae but now in the family Aeromonadaceae.
32 As the generic name suggests,
they produce copious quantities of gas from the fermented sugars. They are normal inhabitants of the
intestines of fish, and some are fish pathogens. The mol% G + C content of DNA is 57–65. (The
species that possesses pathogenic properties is discussed in Chapter 31.)
Alcaligenes (al • ca • li’ge • nes; alkali producers). Although Gram negative, these organisms sometimes stain Gram positive. They are rods that do not, as the generic name suggests, ferment sugars
but instead produce alkaline reactions, especially in litmus milk. Nonpigmented, they are widely
distributed in nature in decomposing matter of all types. Raw milk, poultry products, and fecal matter are common sources. The mol% G + C content of DNA is 58–70, suggesting that the genus is
heterogeneous.
Alteromonas (al • te • ro • mo’nas; another monad). These are marine and coastal water inhabitants that
are found in and on seafoods; all species require seawater salinity for growth. They are Gram-negative
motile rods that are strict aerobe.
17
Arcobacter (Ar’co • bac • ter; L. arcus, bow). This genus was created during revision of the genera
Campylobacter, Helicobacter, and Wolinella,
45 and the three species were once classified as Campylobacter. They are Gram-negative curved or S-shaped rods that are quite similar to the campylobacters
except they can grow at 15
◦ C and are aerotolerant. They are found in poultry, raw milk, shellfish, and
water; and in cattle and swine products.
51,52 These oxidase- and catalase-positive organisms cause
abortion and enteritis in some animals, and the latter in humans is associated with A. butzleri.
Bacillus (ba • cil’lus). These are Gram-positive spore-forming rods that are aerobes in contrast to
the clostridia, which are anaerobes. Although most are mesophiles, psychrotrophs and thermophiles
exist. The genus contains only two pathogens: B. anthracis (cause of anthrax) and B. cereus. Although
most strains of the latter are nonpathogens, some cause foodborne gastroenteritis (further discussed
in Chapter 24). This genus has been delimited by the transfer of a number of its former species to
eight new genera: Alicyclobacillus, Aneurinibacillus, Brevibacillus, Gracilibacillus, Paenibacillus,
Virgibacillus, and Salibacillus.
5 Also, the former group 5 Bacillus species are now in the genus
Geobacillus, and the former B. stearothermophilus is now G. stearothermophilus.
36