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THE CLA SSIFICATION OF
Bacterial Subspecies
Beyond the conventional genus–species naming system for
organisms, scientists often group members of a single
bacterial species into smaller divisions that reflect genetic
similarities or other shared features. These advanced
classifications provide progressively finer criteria for
distinguishing one microbe from another.
A subspecies is a genetically distinct population that is
often geographically isolated from other members of the
same species. The common food pathogen Salmonella
enterica, for example, has seven subspecies, including
S. enterica arizonae and S. enterica enterica. The latter is
the most common kind that is found in people and
warm-blooded animals with food poisoning.
Scientists can further divide closely related
species or subspecies by identifying distinguishing
characteristics, such as specific molecules or
genetic elements in the cells or their outer surfaces.
They refer to bacterial variants grouped this way as
a serovar (or serotype).
The relationship among bacterial serovars resembles
that which exists among different tomato varieties.
The Sweet 100 cherry tomato cultivar, whose fruit weighs
a mere 28 g / 1 oz, for example, differs markedly from the
Goliath beefsteak tomato variety, which can yield fruits
weighing 1.4 kg / 3 lb. Yet both types are readily identifiable
as tomatoes: Solanum lycopersicum.
Small genetic differences can likewise impart significant
variation among bacterial serovars, including the ability of
some to withstand multiple antibiotics. Sometimes those
differences are not even part of the microbe’s heritable
genome but are conferred when unrelated plasmid DNA is
transferred from one bacterium to freeload on another (see
Plasmids, next page).
Researchers have identified several thousand serovars of
S. enterica, nearly all of which belong to the enterica subspecies.
Common serovars associated with foodborne illness
Most bacteria that infect the human body are
aerobic for the simple reason that we have oxygen
in our blood and that oxygen keeps the growth of
anaerobic bacteria in check. A few kinds of
anaerobic bacteria, such as Clostridium perfringens,
can cause severe infections (tetanus and gas
gangrene), but normally only when they get into
deep wounds or dead, oxygen-starved tissue.
include Agona, Hadar, Heidelberg, and Typhimurium, which
as a body hint at Salmonella’s outsized role in human
disease.
All those subdivisions typically generate especially long
formal names, such as Salmonella enterica subspecies
enterica serovar Heidelberg, which is a variety that has shown
enhanced resistance to antibiotics in the United States.
Specialists commonly shorten the name to Salmonella
Heidelberg. Investigators have linked another serovar,
Salmonella Typhimurium, to a major outbreak in the United
States in 2008 and 2009, in which contaminated peanuts
and peanut-containing foods sickened hundreds.
Many other bacteria owe their ill repute to virulently
pathogenic serovars. A particularly potent example is E. coli
O157:H7. In this subgroup of what is a normally benign
species, a relatively small number of genetic changes have
occurred that enable it to cause severe illness, gastrointestinal
bleeding, and even death. Yet in the gut of a typical
person, 10 billion to 1 trillion E. coli of other serovars coexist
quite harmlessly with their host.
Vibrio cholerae has 139 serovars, of which only two are
pathogenic. Researchers have tied both to foodborne
illnesses that were associated with contaminated shellfish.
At an even more refined level of classification, specialists
sometimes refer to bacterial strains, which are usually isolated
from a particular source, such as an infected animal or
a human patient. No uniform naming convention exists for
strains, but scientists often give them numbers or other
designations based on the results of the tests they use to
distinguish among them. They labeled, for example, a multidrug-resistant
strain of S. enterica that belongs to the Typhimurium
serovar “definitive type 104.” Known as S. enterica
serotype Typhimurium DT104, the strain was first isolated in
1984 from patients in the U.K. Within several years, Salmonella
Typhimurium DT104 became common there, and in
the mid-1990s it appeared throughout the U.S.
Anaerobic foodborne pathogens, including
others within the Clostridium genus, have developed
infection strategies that rely on hosts eating
foods contaminated with their spores. Some
anaerobes, however, can do their worst damage
without ever inhabiting our bodies: foodborne
botulism is a relatively uncommon but potentially
deadly form of food poisoning in which Clostridium
botulinum releases a potent nerve toxin as it grows
in canned vegetables or other foods. Even heating
the food enough to kill the bacteria doesn’t destroy
the toxin they’ve already produced.
As cooks, we’re most interested in the three
main groups of bacteria that are associated with
food. The first group, sometimes called spoilage
bacteria, aren’t harmful on their own, but they
can produce rot and foul odors that make food
unappealing. Hard as it may be to believe, you
almost never get sick from consuming these types
of bacteria. Their presence does, however, often
signal contamination with other aerobic bacteria
that are pathogenic.
The second group includes both invasive
infectious bacteria, such as Salmonella and E. coli,
which can sicken humans by penetrating intestinal
or other body tissues, and noninvasive infectious
bacteria, such as Vibrio cholerae, which can
cause illness even without a full-blown invasion by
secreting toxins during their stay in our intestines.
Finally, we’ll examine food poisoning bacteria,
including Bacillus cereus and C. botulinum. In
addition to these three groups, other kinds of
bacteria can infect a wide range of body tissues
through the blood, respiratory system, and other
access routes. But, by definition, those infections
are not related to food.
THE BIOLOGY OF
Plasmids
A plasmid is not a living thing but rather a self-copying piece
of roving DNA, typically circular, that can reproduce only
inside a bacterium or some other organism. Plasmids differ
from viruses, which have fairly complicated protein structures
around their DNA or RNA; plasmids are just naked
DNA. When a plasmid infects a bacterium, it supplements
the normal genetic blueprint of the microbe, often bestowing
on the host bacterium dramatic
new capabilities, such as the power to
cause disease, live in a new environment,
or resist antibiotics.
Plasmids are passed on during the
normal replicative division of a bacterium,
which ensures that any plasmiddependent
traits persist in future
generations. In fact, bacterial strains
Spoilage Bacteria
Not all bacteria in food are dangerous; some are
merely annoying. Spoilage bacteria produce
liquids and gases that let us know that food has
become rotten. Vegetables and fruit may become
slimy or mushy, whereas meat usually starts to
stink. As disgusting as spoiled food can be, most
of the smell, color, and texture changes that
people associate with food gone bad are actually
medically harmless. With few exceptions, you
rarely get sick from spoilage bacteria. Food in
which spoilage bacteria have been very active,
however, is likely to be contaminated with other
bacteria that are pathogenic and could make you
very sick.
Unfortunately, this situation can fool people
into thinking the reverse is truethat if no sign of
spoilage is present, then the food must be safe.
This assumption is emphatically not true and is
a great example of how misinformation can kill
you. People can get very sick or even die from food
that shows no signs of spoilage. Furthermore, as we
noted, spoilage is not always so safe (see Spoiled
Fish and Cheese, page 139).
Interestingly, although most other chemicals
released by spoilage bacteria are not toxic to us,
they can often harm other bacteria. The toxins
either poison or repel species that might otherwise
are sometimes defined by the plasmids they incorporate.
Cell division is not the only way plasmid DNA transfers
from one microorganism to another, however. Those who
monitor foodborne illnesses must stay aware of one aspect
of plasmids’ ability to pass from one bacterium to another:
a process known as conjugation, which can occur during
cell-to-cell contact. Amazingly, the donor and recipient of the
plasmid transfer can belong to different
species, creating the possibility, for
instance, that a plasmid from S. enterica
could spread to E. coli and vice versa.
The details are beyond the scope of this
book, but it’s worth noting that some
deadly bacterial strains acquire their
pathological power from the promiscuous
proclivities of simple plasmids.
132 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 133