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MICROBES AS GER MS
Most threats to food safety can be linked to
microorganisms: living creatures, typically
consisting of a single cell, that can be seen only
with the aid of a microscope. These microbes have
colonized our planet in astonishing abundance.
No one knows how many kinds there are, but
many biologists believe the tally of species may be
well into the millionsand that’s just bacteria!
The vast majority of bacteria and other microscopic
life forms are perfectly harmless or even
beneficial. That’s lucky for us because the human
body accommodates a teeming menagerie of
microbes inside and out, including tiny mites,
fungi, viruses, and bacteria. Some researchers
estimate that as many as 500 bacterial species may
live on your skin alone, and research suggests that
the belly button, inner forearms, and buttocks are
also hot spots of microbial biodiversity. The adult
human gut is host to roughly 100 trillion microbes
spanning some 400 bacterial species. That’s pretty
THE TEC HNOLOGY OF
Bacterial Staining as a Form of ID
Among the many methods scientists have used to classify
bacteria, chemical staining has been a standby for more
than a century. The most common method is the Gram stain,
a technique that uses a dye developed by the Danish
physician Hans Christian Gram in 1884. Researchers label
bacteria that stain purple when doused with the dye “Grampositive”
(below left), whereas those that instead take up a
pink or red counterstain are “Gram-negative” (below right).
impressive if you consider that your entire body
contains just 10 trillion or so human cells. And we
should welcome these microbial guests because
our lives would be difficult or impossible without
them. Many of these tiny organisms play crucial
roles in digestion, for example.
But what about the bad bugs? Scientists refer
generically to microorganisms that cause disease
as pathogens or pathogenic; the public calls
them germs. Within the broad range of these
microorganisms, several major categories of
foodborne pathogens are the main concern of
cooks (see A Bestiary of Foodborne Pathogens,
page 108). Each class of microbes presents unique
risks and hazards. Cooking and storage methods
that thwart one kind of pathogen, for instance,
may be useless against another. To maintain good
culinary hygiene in the kitchen, a working
understanding of the differences and similarities
among microbial contaminants is crucial.
The difference in stain absorption depends on the makeup
of the bacteria’s protective outer wall. Only some species
have a relatively thick, chain-mail-like layer of sugars and
peptides that absorbs Gram’s dye, which is called crystal
violet. Although he did not know exactly how the stain
worked, Gram found he could diagnose diseases by using
crystal violet to differentiate among bacteria causing similar
symptoms—a useful practice that continues to this day.
A Window into the World of Single-Celled Life
The microscope, the device that opened the microbial
universe to human observation, boasts a truly international
history. One thousand years ago, pioneers in Europe
and the Middle East laid the foundations for the technology
by producing simple magnifying lenses that are convex:
thick in the middle and thinner toward the edge.
The first true microscope dates to late 16th- century Holland,
when a lens grinder there placed two lenses inside
a tube and saw that the apparatus could greatly magnify
objects in its view. The Tuscan astronomer G alileo, who
is more commonly associated with telescopes, studied
the initial crude device, then crafted a better version that
used a compound lens.
In 1665, British physicist Robert Hooke published the
first microscope-aided scientific study, Micrographia, an
illustrated book that detailed previously unseen marvels
such as the porous microstructure of cork that confers
its buoyancy. Shortly thereafter, Dutch fabric merchant
Antony van Leeuwenhoek began constructing simple
but refined single-lens microscopes. He used his instruments
to describe insect parts, blood cells, sperm, parasitic
worms, protists, and what he called “animalcules”—
tiny organisms in dental plaque that were the first
recorded observations of living bacteria.
Frequent innovations in the centuries since have produced
microscopes that today enable researchers to
magnify matter at the subatomic level.
We used this microscope,
and several others like it, in
creating the pictures for this book.
Objective lenses magnify the image. A good microscope may
come equipped with five or six lenses of various magnifications.
Eyepiece lenses focus
the magnified light into
the observer’s eyes.
Digital camera allows users to
quickly document objects they
see through the eyepiece.
A mechanical stage holds the glass slide with the specimen.
Turning the associated knob allows fine adjustments to the
portion of the slide that’s in view.
A condenser concentrates light and shines
it through specimens to reveal their details.
Specialized versions show specimens on a
dark background.
An illuminator beams light
through the specimen
from below.
106 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 107