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Although spoilage and pathogenic bacteria
often contaminate food simultaneously,
you can never assume that the absence of
spoilage bacteria means the absence of
pathogens.
compete for limited food resources. This strategy
is, in fact, the basis of fermentation, the process
by which food-dwelling microbes break sugars
down into acids or alcohols.
A great example of a bacterium that uses this
tactic is Lactobacillus bulgaricus, a species that
emits large quantities of lactic acid, preventing the
growth of most other bacteria. L. bulgaricus
thrives in the acid, which gives the food it inhabits
a distinctive odor and flavor. In certain contexts
this spoilage is, however, desirable.
For example, this is how yogurt is made.
Makers inoculate milk with a particular strain of
L. bulgaricus (or other related Lactobacillus species
or the lactic acid-producing bacteria Streptococcus
thermophilus) and incubate it for a time at a suitable
temperature. As a side effect of the bacterial
growth, the milk thickens into yogurt.
The preparation of fermented foods invariably
involves cultivating bacteria, yeast, or fungi that
secrete chemicals that are poisonous to other
microorganisms. Food processors use related
Lactobacillus species such as L. plantarum to
produce fermented foods including sauerkraut,
pickles, and Korean kimchi. San Francisco-style
sourdough bread derives its characteristic tangy
flavor from L. sanfranciscensis.
Nevertheless, not all Lactobacillus species are
beneficial. Specialists consider some to be
spoilage bacteria, particularly when they grow on
meat.
A Toxic Invasion
The secretions of some other foodborne bacteria
are not nearly as benign as those of their spoilagecausing
cousins. Although some invasive infectious
bacteria can cause disease without emitting
a toxin, most pathogenic ones release an associated
bacterial toxin. Intriguing evidence suggests
that a bacterium can communicate with its kin by
emitting chemical signals, which allow a group of
microbes to gang up and coordinate their invasion.
This process, called quorum sensing,
enables the bacteria to build up their numbers
before starting toxin production. Some researchers
suspect this is why the onset of certain infections
is so sudden.
Bacteria often secrete toxins specifically to
harm us. It’s nothing personal; it’s just part of their
life cycle. A common strategy among gastrointestinal
bacteria is to release toxins that bring on
diarrhea, in which a gram of fecal matter can
contain millions of copies of the bacterium. The
fact that diarrhea is hard to control and often
messy boosts the probability that the bacteria will
contaminate food or water and spread to other
people, thereby continuing their life cycle. Over
millions of years, bacteria have evolved this
mechanism for dispersal. Unfortunately for us,
their drive to survive means we may face discomfort,
illness, and even death.
Common infectious foodborne bacteria include
Campylobacter jejuni, E. coli, Listeria monocytogenes,
Yersinia enterocolitica, and several species of
Salmonella, Shigella, and Vibrio. Some of these
pathogens, such as Listeria, prey upon susceptible
people with undeveloped or compromised immune
systems who eat contaminated food.
Every year in the United States, about 2,500
people fall seriously ill from a Listeria infection,
also known as listeriosis. Of those sickened, about
one in five ultimately dies; this is among the
highest mortalities for any foodborne infection.
But the risk is far from uniform. Pregnant
women, in whom one-third of all such infections
occur, are 20 times more likely to get listeriosis
than other healthy adults. Listeriosis places each
such patient and her unborn baby or newborn at
grave risk. Those living with AIDS are even more
vulnerable; according to the CDC, they are nearly
300 times more likely to contract listeriosis than
people with normal immune systems are.
Other pathogens, including V. cholerae, are
better known as agents of waterborne disease
cholera in particular. A few species of Vibrio live in
saline environments, such as salt-marsh mud.
Nevertheless, food also can transmit a Vibrio
infection; researchers have cited raw or undercooked
seafood from the Gulf of Mexico, Latin
America, and Asia as culprits, for example.
Despite their differences, the detailed properties
of these infectious pathogens are less important to
a cook than the big picture: each microbe can
contaminate food and infect those who eat it, and
nearly all can be transmitted by the fecal-oral route.
THE ORIGIN OF
Microbial Taints and Off-Flavors
Everyone knows the archetypal odor of sour milk, perhaps
the most readily recognizable sign of bacterial food spoilage.
More research has gone into investigating the sources
of tainted and off-tasting milk than those of perhaps any
other food, and the list of ways in which it can be fouled is
extensive. Pseudomonas bacteria, for example, produce an
enzyme that can leave milk tasting fruity. Other bacteria
give milk malty, acidic, rancid, or musty off-flavors.
Many other foods go bad through similar bacterial action.
A fermentation process gone wrong because of the presence
of uninvited Bacillus microbes, for instance, can create bittertasting
cheese. The bacterial secretions indole and skatole
produce not only bad breath but also the reek of rotting
potatoes. Streptococcus can produce a cheesy off-flavor in
Poison Left Behind
Many bacteria produce harmful toxins. Infectious
bacteria secrete those toxins inside your body,
where the chemicals cause various forms of
cellular damage that can make you ill. Some
bacteria, however, synthesize toxins well before
you eat them. Even though the bacteria typically
cannot survive in the human body and do not
produce an infection, their toxins can still wreak
havoc inside you.
Toxin production is typical of anaerobic pathogens
such as Clostridium, but aerobic bacteria also
can release poisons. One such pathogen, Staphylococcus
aureus, is ubiquitous in natureit even
grows on your skin. Although people know Staphylococcus
more commonly as the source of staph
infections and toxic shock syndrome, the bacterium
can also taint food with a toxin that it secretes. The
illness that results from this and other foodborne
toxins is known as food poisoning.
Because the toxins are already present, food
poisoning is characterized by an abrupt onset of
symptoms. Commonly, someone leaves out
susceptible food at an improper storage temperature
(often room temperature) long enough for
bacteria to multiply and produce toxins. If the
microbes remain undisturbed for extended
periods of time, dangerous levels of bacteria and
their toxins can accumulate even in refrigerated
food, although this happens less often.
In some instances, when a bacterial toxin is
susceptible to heat, or heat labile, it breaks down
readily at elevated temperatures. If you heat or
reheat contaminated food to a high enough
temperature, it will destroy some toxins, leaving
the food safe to eat. Other bacterial toxins tolerate
heat very well, however, which thwarts this
strategy unless you heat the tainted food to
excessive temperatures.
Bacteria have yet other ways to evade our
attempts to kill them. Anaerobic bacteria have
The contamination of food alone
is often insufficient to create
a bacterial infection. An infection
can be abetted by poor hygiene,
improper food storage, or
inadequate cooking safeguards.
canned hams. Everybody likes the earthy smell after a good
rain, but the actinobacteria that release geosmin, which
contributes to that pleasant aroma, add the same chemical to
produce the more disagreeable smells of tainted fish, bread,
flour, navy beans, and clams.
Fishmongers’ and butchers’ shops provide clear olfactory
confirmation that fish and meats are especially prone to
invasion by microbes that secrete odoriferous chemicals.
Vibrio, Achromobacter, and Pseudomonas bacteria can all
generate off-putting “fishy” flavors or smells. The growth of
spoilage bacteria in meat, fish, and cheese can yield a pungent
bouquet from the chemicals putrescine, cadaverine,
histamine, and tyramine. This odor is a telltale indicator that
hygienic practices may be absent in a kitchen.
134 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 135