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Viruses have developed many ways
to invade their targets. A virus can
fuse with a cell, poke a hole in its
protective membrane, or try some
other tactic to get its genetic
information inside, such as injecting
that information into the host
cell or tricking the cell into engulfing
its attacker. Whatever method
the virus uses, the successful
delivery of DNA or RNA moves the
cycle of infection into high gear.
VIRUSES
Bacteria are tiny living things. Viruses are quite
different, so much so that they blur the distinction
between what is alive and what is just a complex
chemical.
A virus consists of at least two main components:
a biological information moleculeeither
DNA or RNAon the inside and a viral protein
coat, or capsid, on the exterior. A few varieties of
virus include a third component called an envelope,
which surrounds the capsid.
Think of a virus as a nanometer-scale syringe or
hypodermic needle. The “syringe” is the viral
protein coat, which is a complicated structure that
usually has a geometric shape. Its function is to
infect a host cell by injecting or otherwise inserting
the DNA or RNA into that cell, where it
mingles with the DNA and RNA of the host.
The information molecules of the virus contain
the blueprints for building more identical viruses.
Once inside the cell, the viral DNA or RNA
hijacks the cell’s own molecular machinery for
building proteins and forces it to makes copies of
the virus, thus effectively converting the cell into
A computer-generated model of
rotavirus particles shows proteins
that protrude from the surface of the
virus particles and bind to target cells
to help the virus gain entry.
a virus factory. The virus may also cause its host
cell to make toxins.
None of this activity is good for the cell, which
usually dies, sometimes bursting in the process to
release lots of new copies of the virus. As the
human immune system cleans up the dead cells
and responds to the virus, it produces inflammation
and other symptoms. Many viruses can block
their hosts from mounting an effective defense,
and some actually trick the host’s immune system
into attacking healthy cells.
Dangerous but Not Exactly Alive
Viruses differ from bacteria in many fundamental
ways that matter to food safety. Unlike bacteria,
which can increase their numbers dramatically on
or in foodeven precooked foodviruses can
reproduce only within the cells of living hosts. So
viral contamination levels, at worst, remain
constant in prepared food or ingredients; the
contamination does not increase over time.
Even though viruses do not reproduce independently
the way that bacteria do, they do reproduce
in a parasitic way, so they are subject to natural
selection. They co-evolve with their host species
and, over time, become quite specialized. Although
most viruses infect just a single species,
some adapt and cross over to infect other species.
The rabies virus, for example, can infect most
mammals, including humans. Meanwhile, the
influenza virus can infect humans and a few other
animalsnotably pigs and birdsand the West
Nile virus can infect humans, birds, and horses,
among other animals.
Many viruses specialize in infecting human
cells, and those that do are either neutral or
pathological. Unlike bacteria, which sometimes
benefit humans, no natural human viruses are
known to be beneficial. Nearly all viruses that
cause foodborne illness are specialized to live in
humans and do not infect plants or other animals.
Perhaps the most important way in which
viruses differ from bacteria is how they die.
Because viruses aren’t alive in the same way that
bacteria are, you can’t kill them: instead, you must
inactivate viral pathogens. Refrigeration or
freezing do not inactivate viruses, but heat can do
so. The thermal inactivation curve for a virus is
very similar to the thermal death curve for bacteria
that we discussed in the previous section on
bacterial death. Like thermal death, thermal
inactivation is an exponential phenomenon that
depends on time and temperature.
Unfortunately, much less is known about how
heat inactivates viruses than about how heat kills
bacteria. Unlike many bacteria, most viruses are
hard to grow in a laboratory. The problem is
particularly acute for foodborne viruses that infect
human gut cells; those cells can themselves be
difficult and expensive to culture.
Notorious Noroviruses
The noroviruses aptly illustrate the conundrum
that many viral pathogens pose to science. Although
noroviruses are among the most common
foodborne pathogens, thought to collectively
cause more than nine million cases of foodborne
illness each year in the United Statesand to
sicken many millions more around the globe
few details have emerged about the mysterious
microbes.
Noroviruses have been infecting humanity
from time immemorial, yet they were unknown to
THE M ATHEM ATICS OF
Spreading an Infection Around
A little math demonstrates how easy it is for noroviruses to
infect people. One study by researchers in Hong Kong
suggests that 1 g / 0.04 oz of feces from an infected patient
can harbor 300 million particles of norovirus genotype II, the
strain that accounts for most outbreaks. If that small amount
of feces were to get dispersed in an Olympic-size swimming
pool (about 2.5 million l / 660,000 gal), the resulting dilution
would still leave one viral particle per 8 ml / 1½ tsp of water.
A vegetable rinsed in that water could be infectious.
Contamination can build up at the source of the food as
well. Oysters or clams routinely become contaminated from
the discharge of raw sewage coming from the boats that
harvest them. One study showed that 85% of boats operating
in a productive oyster area in the U.S. in 1993 lacked proper
sewage-holding facilities, meaning that they instead discharged
their sewage directly into the sea—despite laws
science until an outbreak of foodborne gastroenteritis,
or intestinal inflammation, in 1968, at a
school in Norwalk, Ohio. Following that episode,
related viruses were found in similar outbreaks
worldwide. Microbiologists originally lumped the
burgeoning group under the name Norwalk virus.
They subsequently became known as Norwalklike
viruses (NLVs) then, in 2002, were officially
classified under the genus Norovirus.
It took some 40 years after noroviruses were
discovered for researchers to successfully cultivate
the viral particles in a laboratorya feat not
accomplished until 2007. In the meantime,
investigators learned what they could from genetic
sequencing of noroviruses’ viral RNA, epidemiological
studies of infected humans, and research
on related viruses that infect cats and mice.
Noroviruses mainly sicken humans, and
contamination occurs chiefly via the fecal–oral
route. Investigators of outbreaks have implicated
foods, such as salad dressing, raspberries, sandwiches,
and cake frosting, served in a wide range
of places, from schools to cruise ships to some of
the world’s best restaurants (see Food Poisoning
at The Fat Duck, page 155). According to CDC
estimates, infected food handlers are responsible
for half of all norovirus outbreaks. The viruses
can also affect people who eat foods that were
Noroviruses are among the most common
foodborne pathogens, but they were only
recently discovered, and their mechanism
of action remains unclear.
forbidding the practice. Investigations of three separate
gastroenteritis outbreaks suggested that a single crew member
who is stricken with a norovirus can contaminate miles of
oyster beds through fecal discharge into the water.
That may seem incredible, but consider that a single adult
oyster can suck in and spit out as much as 230 l / 60 gal of
seawater a day as it feeds on microorganisms that it filters out
of the water. Norovirus-contaminated feces that discharge
into the ocean and are diluted to a concentration of one virus
per 100 ml / 3.4 oz of water (12 times more dilute than in the
swimming pool example above) could theoretically expose
an oyster to some 2,300 viral particles every day. Because
oysters grow over a period of months or years, they filter
a tremendous amount of seawater, meaning that the virus can
survive and accumulate within oysters (or clams), then infect
a person who eats the shellfish.
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MICROBIOLOGY FOR COOKS 153