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Place an unwashed hand on a petri dish
full of growth medium, and this is what
sprouts forth: a menagerie of microbial
life, not all of it friendly. For more on how
to wash your hands and kitchen equipment
properly, see Hygiene, page 196.
pathogens to theoretically infect a continent or
perhaps even the entire worldshould some
supervillain figure out a way to distribute them.
The point is simply to illustrate what an
incredibly tiny quantity of pathogen it takes to
cause a tremendous amount of disease. That
extreme ratio is one of the primary reasons for
the ubiquity of foodborne illness. Looked at
another way, if a tour boat flushes one liter of
feces into a very large lake and it becomes diluted
in 100 cubic kilometers of water, a liter of water
from the lake could still contain enough pathogens
to infect an average person. Indeed, after an
outbreak of Escherichia coli sickened 21 children
in 1991, investigators determined they had
become infected after swimming in a lake near
Portland, Oregon, that was contaminated with
feces from other bathers.
Pathogenic bacteria generally have infectious
doses that are higher, so the ratio is not as extreme.
If a strain of E. coli has an infectious dose of 100
million organisms, for example, you might think
you would have to eat a gram of feces to get to
sicka very unlikely scenario.
But bacteria often multiply on the food after
contamination. Under favorable conditions,
a single E. coli bacterium can produce millions of
progeny in just a day. So even a tiny amount of
fecal contamination that puts a small number of
the wrong bacteria on food can cause a problem.
This might lead you to conclude that ingesting
fecal matter is a seriousand often deadly
public health problem. And indeed, the CDC
study estimates that 9.6 million annual cases of
foodborne disease are linked to fecal contamination.
But fecal matter leads to such illnesses only
when it harbors pathogens. Fortunately, most
people and domestic animals do not routinely
excrete pathogen-laden feces. Among humans in
particular, most pathogenic organisms in feces
emerge during the course of a foodborne illness
or during a limited window of a few days to
a week afterward.
Unfortunately, this discussion implies a rather
uncomfortable fact: we all regularly consume fecescontaminated
food. For a variety of practical
reasons, we can’t always follow those two simple
rules about not consuming feces or body fluids.
Most of the time, we get away with it. But given
the minuscule quantities of organisms needed to
contaminate food, how do we reduce the risk?
The next chapter, Food Safety, describes various
approaches to achieving that goal, all of which
mainly boil down to minimizing the opportunity
for pathogens to get into your kitchenand
preventing those that do get in from establishing
a foothold.
Don’t Eat That….
The technical term for the transmission of contaminated
feces from one person to another is the
fecal-oral route. Contamination by the human
fecal-oral route normally occurs in a very straightforward
way: via poor hygiene. Namely, after
using the toilet, people who handle food either do
not wash their hands or do so improperly.
Cross-contamination of one food source by
another or by contaminated water also spreads
fecal matter. Human fecal contamination can
even occur in the ocean via filter-feeding clams
and oystersmore on that shortly.
Aside from exposure to human feces, foodborne
illness spreads chiefly through four other
types of contamination: animal feces, soil-based
and free-floating microbes, human spittle, and
animal flesh. Animal fecal contamination of food
occurs primarily on the farm or in the slaughterhouse.
Washing baths are particularly prone to
contamination by animal feces because even
a small fleck of feces in a washing tank that
cleans multiple carcasses can contaminate all of
them. Animal feces also can contaminate fruit
and vegetable crops, either in the field or through
cross-contamination at various points along their
path from initial production to the dinner plate.
Environmental contamination involves
generally ubiquitous microbes. Clostridium
botulinum, for example, is widespread in soil,
whereas many Vibrio species thrive in seawater.
Staphylococcus aureus and related species normally
live quietly on human skin, in the nose,
and elsewhere in the environment, but they can
do considerable damage if they are allowed to
grow on food and produce toxins that cause food
poisoning. Staphylococcus species alone can
secrete up to seven different kinds of poisons.
Human oral contamination mainly occurs from
spittle. Group A streptococcus, the bacterial
strains that are to blame for strep throat, are
common malefactors spread this way. Most
restaurants use “sneeze guards” at salad bars to cut
down on oral transmission of strep infections by
blocking the fine mists of spittle that people eject
during sneezes or coughs.
Finally, flesh contamination, although not as
common, is the primary source of infection by
some parasitic worms and by a form of salmonella
that infects hen ovaries and subsequently contaminates
their eggs.
Even if you know the foodborne pathogen that
is causing an infection, however, tracing that
infection back to its original source of contamination
can be tedious and ultimately futile. Consider
the bacterial pathogen Yersinia enterocolitica.
According to the U.S. Food and Drug Administration,
the microbe can be transmitted through
meat, oysters, fish, and raw milk, among other
foods. But the species is common in soil and water
samples, as well as in animals such as beavers, pigs,
and squirrels.
Poor sanitation and sterilization by food
handlers could contribute to contamination. So
could infected workers who spread the disease
through poor hygiene. So for that one foodborne
pathogen, an illness could arise from environmental,
human fecal, or animal fecal contamination.
Likewise, campylobacter infections are normally
associated with fecal contamination. But
raw milk can become contaminated by a cow with
an infected udder as well. Many other foodborne
pathogens can also exploit multiple avenues to
reach the kitchenone reason why they can be so
difficult to avoid ingesting.
Even so, a review of the records of foodborne
outbreaks in which the source has been identified
suggests that an overwhelming majority are linked
to fecal contamination. And that means that most
food contamination occurs through an external
sourceit is basically dirt (or worse) on the
outside that never reaches the interior of the food.
There are some important exceptions to this
rule of thumb: oysters and clams, for example,
are filter feeders and can internalize feces from
contaminated water. Salmonella can contaminate
intact eggs. Nevertheless, the fact that most
microbial contamination arrives from a source
beyond the food itself has multiple implications
for food safety and kitchen practices, which are
the subject of the next chapter.
Common Misconceptions
About Microbes
As we began looking closer at research on the
main kinds of foodborne pathogens, we were
frankly somewhat surprised to learn just how large
a fraction of foodborne illness is caused by contamination
by human or animal fecal matter. Like
many people with culinary training, we had
assumed that the problem was intrinsic to the food
supply. Before a pig becomes pork, for example,
the worm Trichinella spiralis that causes the
disease trichinellosis (also called trichinosis) can
infect the animal. Salmonella lurks in eggs and
chickens as a matter of course. We had naively
assumed that all food pathogens are somehow just
present in the food or its environment.
But for the vast majority of foodborne illnesses,
that just isn’t the case. Consider trichinella, which
burrows into the muscle of contaminated pigs.
Our mothers taught us to always cook pork
well-doneor else. That dire warning is repeated
in many cookbooks, web sites, and even culinary
schools. Fear of trichinellosis has inspired countless
overcooked pork roasts.
In reality, however, the U.S. pork industry has
succeeded in essentially purging trichinella from
pig farmsand just in case any slips through,
the industry routinely freezes the meat, which
Fighting viral threats (top) may have the
biggest impact on foodborne illnesses in
developed countries, whereas reducing
contamination by bacteria (middle) and
protists (bottom) could lead to the largest
decrease in deaths.
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MICROBIOLOGY FOR COOKS 117