past, 6,000 cases of ready-to-eat pudding
were recalled because of mold contamination
(FDA, 1996a). During 1996, two manufac-
turers of fruit juice issued recalls on products
contaminated with mold (FDA, 1996b).
Yeasts
Yeasts are generally unicellular. They dif-
fer from bacteria in their larger cell sizes and
morphology, and because they produce buds
during the process of reproduction by fis-
sion. The generation time of yeasts is slower
than that of bacteria, with a typical time of
2 to 3 hours in foods, leading from an origi-
nal contamination of one yeast/g of food to
spoilage in approximately 40 to 60 hours.
Like molds, yeasts can be spread through the
air or by other means and can alight on the
surface of foodstuffs. Yeast colonies are gen-
erally moist or slimy in appearance and
creamy white. Yeasts prefer an Awof 0.90 to
0.94, but can grow below 0.90. In fact, some
osmiophilic yeasts can grow at an Awas low
as 0.60. These microorganisms grow best in
the intermediate acid range, a pH of from
4.0 to 4.5. Yeasts are more likely to grow on
foods with lower pH and on those that are
vacuum packaged. Food that is highly con-
taminated with yeasts will frequently have a
slightly fruity odor.
Bacteria
Bacteria are unicellular microorganisms
(prokaryotic cells) that are approximately
1 μm in diameter, with morphology variation
from short and elongated rods (bacilli) to
spherical or ovoid forms. Cocci (meaning
“berry”) are spherically shaped bacteria.
Individual bacteria closely combine in vari-
ous forms, according to genera. Some sphere-
shaped bacteria occur in clusters similar to
a bunch of grapes (e.g., staphylococci). Other
bacteria (rod-shaped or sphere-shaped) are
linked together to form chains (e.g., strepto-
cocci). Also, certain genera of sphere-shaped
bacteria are formed together in pairs (diploid
formation), such as pneumococci. Microor-
ganisms, such as Sarciniaspp., form as a
group of four (tetrad formation). Other
genera appear as an individual bacterium.
Some bacteria possess flagella and are
motile.
Bacteria produce pigments ranging from
variations of yellow to dark shades, such as
brown or black. Certain bacteria have pig-
mentation of intermediate colors—red,
pink, orange, blue, green, or purple. These
bacteria cause food discoloration, especially
among foods with unstable color pigments,
such as meat. Some bacteria also cause dis-
coloration by slime formation.
Some species of bacteria produce spores,
which may be resistant to heat, chemicals,
and other environmental conditions. Some
of these spore-forming bacteria are ther-
mophilic microorganisms that produce a
toxin that can cause foodborne illness.Viruses
Viruses are infective microorganisms with
dimensions that range from 20 to 300 nm, or
about 1/100 to 1/10 the size of a bacterium.
Most viruses can be seen only with an elec-
tron microscope. A virus particle consists of
a single molecule of DNA or RNA, sur-
rounded by a coat made from protein.
Viruses cannot reproduce outside of another
organism and are obligate parasites of all liv-
ing organisms, such as bacteria, fungi, algae,
protozoa, higher plants, and invertebrate
and vertebrate animals. When a protein cell
becomes attached to the surface of the
appropriate host cell, either the host cell
engulfs the virus particle or the nucleic acid
is injected from the virus particle into the
host cell, as with bacteriophages active
against bacteria.
In animals, some infected host cells die,
but others survive infection with the virus
and resume their normal function. It is notThe Relationship of Microorganisms to Sanitation 27