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Microbes don't like to share their food, so many of them emit chemicals like acids or alcohol that they
can tolerate but that their competitors can't. Human chefs use this trait to their advantage to prepare
fermented foods, to which the emitted chemicals provide flavor and texture. Yogurt, kimchi,
sauerkraut, and sourdough bread are all examples.
responded to a fundamental challenge to their
survivalthe fact that there is a lot of oxygen in
the worldby evolving the ability to form bacterial
spores. The microbe grows a cocoon-like
protective covering that encases the dormant
bacterium, shielding it from oxygen, dehydration,
and other potentially lethal environmental
conditions. By forming spores, bacteria can hide
out for months or years until conditions improve.
This is such an effective strategy that some
aerobic bacteria also produce spores to cope with
unfavorable environs. Many bacteria that live in
seasonal ponds can produce spores when the
ponds dry up, for example. The tough spore
coverings can even protect some Bacillus bacteria
from exposure to the extreme cold, hard vacuum,
and harsh radiation of outer space.
Understandably, bacterial spores are problematic
in the kitchen because they are much harder
to kill or inactivate than normal bacterial cells are.
Unfortunately, throughout history human cooks
have unintentionally provided invaluable assistance
to anaerobic bacteria. Because most spoilage
bacteria are aerobic, people have invented many
schemes for preserving food for long periods of
time by limiting its contact with oxygen. A layer of
fat seals oxygen out of traditional French duck
confit, for example, just as it prohibits oxygen from
reaching pemmican prepared by North American
Indians. The same preservation method is part of
traditional sausage makingparticularly the
preparation of dry, preserved sausages. An airtight
seal is fundamental to canning food and, more
recently, the concept has been expanded to
include cooking sous vide (see chapter 9). But
these well-meaning techniques have a nasty side
effect. They improve the growing conditions for
anaerobic bacteria, which makes the food more
susceptible to contamination by bacterial spores.
And because spores can survive heat or other
measures that kill most bacteria, fully cooked food
can be full of viable spores. If spore-containing
food is eaten quickly after its preparation, the
still-dormant spores are unlikely to cause any
trouble. But if the spores are allowed time to revert
into active bacteria, they can quickly reproduce
and produce toxins.
Canning can prove to be a particular problem
because people store canned food at room temperature
for many months. If the canning is done
How Pathogenic E. Coli Can Get into Food
Most varieties of E. coli bacteria, which live in the gastrointestinal (GI) tracts
of humans and other warm-blooded animals and are shed through their
feces, are either beneficial or harmless. The same cannot be said for
pathogenic strains such as E. coli O157:H7, which has been blamed for
multiple foodborne outbreaks linked to contaminated milk, meat, and
1
Life stages
Bacterial cells divide and
reproduce on food or in
animal GI tracts.
2
Contamination
Spinach is contaminated by
E. coli-laden feces at the farm.
Contamination risk
Low High
Cross-contamination may occur
during washing or processing.
Contamination risk
Low High
produce such as spinach and alfalfa. Infections with pathogenic strains
cause painful cramping and bloody diarrhea. They are particularly dangerous
for young children and the elderly, in whom life-threatening anemia and
kidney damage can develop. Fortunately, good kitchen practices—including
safe storage and cooking— can minimize the risk of infection.
3a
3b
Unsafe storage
Spinach is left out in warm, wet
conditions before serving.
Contamination risk
Low High
Safe storage
Spinach is refrigerated.
Contamination risk
Low High
An E. coli bacterium uses its long flagella
to move around.
4a
4b
Unsafe cooking
Spinach is unwashed or
insufficiently washed before
serving. Adding mayonnaise or
salad dressing may compound
contamination by fueling bacterial
multiplication.
Contamination risk
Low High
Safe cooking
Spinach is thoroughly washed
or cooked.
Contamination risk
Low High
136 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 137