Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1264 Chapter 23: Pathogens and Infection
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uniform temperature and constantly renews itself. It is not surprising that many
microorganisms have evolved the ability to survive and reproduce in this desirable
niche. In this section, we discuss some of the common features that microorganisms
must have in order to colonize the human body or cause disease, and we
explore the wide variety of organisms that are known to cause disease.
The Human Microbiota Is a Complex Ecological System That Is
Important for Our Development and Health
The human body contains about 10 13 human cells, as well as a microbiota consisting
of approximately 10 14 bacterial, fungal, and protozoan cells, which represent
thousands of microbial species—the so-called normal flora. The combined
genomes of the various species of the human microbiota, called the microbiome,
contain more than 5 × 10 6 genes—more
MBoC6 m24.01/23.01
than 100 times greater than the number
of genes in the human genome itself. A consequence of this genomic diversity
is that the microbiota expands the range of biochemical and metabolic activities
available to the humans.
The microbiota is usually confined to the skin, mouth, digestive tract, and
vagina. With the exception of microbes colonizing the skin, it consists primarily
of anaerobic bacteria, with distinct communities of species inhabiting each body
part. These communities vary considerably between individual humans, even
between close relatives or identical twins. Although the microbiota of an individual
is generally consistent over time, it is influenced by a variety of factors, including
age, diet, health status, and antibiotic use.
There are various ecological relationships that these microbes have with their
host. In mutualism, both the microbe and host benefit. The anaerobic bacteria
that inhabit our intestines, for example, gain shelter and a nutrient supply but also
contribute to the digestion of our food, produce important nutrients for us, and
are essential for the normal development of our gastrointestinal tract and innate
and adaptive immune systems. In commensalism, the microbe benefits but offers
no benefit and causes no harm: for example, we are infected with many viruses
that have no noticeable effect on our health. In parasitism, the microbe benefits
to the detriment of the host, as is often the case for pathogens.
Many infectious diseases are caused by a single pathogen. There is increasing
evidence, however, that an imbalance in the community of microbes that constitute
the microbiota can contribute to some diseases, including autoimmune and
allergic diseases, obesity, inflammatory bowel disease, and diabetes. Remarkably,
in such cases of microbiota imbalance (referred to as dysbiosis), the transfer of the
microbiota from a healthy individual to someone suffering from the disease can
be beneficial and sometimes curative, as in the case of Clostridium difficile colitis
caused by overgrowth of the bacterium.
Figure 23–1 Parasitism at many levels.
(A) Most animals harbor parasites, an
example being the blacklegged tick or deer
tick (Ixodes scapularis), shown here on a
human finger. Although ticks of this species
thrive on white-tailed deer and other wild
mammals, they can also live on humans.
(B) Ticks themselves harbor their own
parasites including the bacterium Borrelia
burgdorferi, stained here with a vital dye
that labels living bacteria green and dead
bacteria red. These spiral-shaped bacteria
live in deer ticks and can be transmitted to
humans during a tick’s blood meal. Borrelia
burgdorferi causes Lyme disease, which
is characterized by a bull’s-eye-shaped
skin rash and fever; if the infection is left
untreated, various complications can
result, including arthritis and neurological
abnormalities. The idea that parasites have
their own parasites was noted by Jonathan
Swift in 1733:
“So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite ‘em;
And so proceed ad infinitum.”
(A, from Acorn, White-Footed Mice and
Tick Cycle Augment Risks of Lyme Disease
in 2012. March 14, 2012. Reprinted with
permission of Anita Sil; B, courtesy of
M. Embers.)
Pathogens Interact with Their Hosts in Different Ways
If it is normal for us to live with a community of microbes, why are some of
them capable of causing us illness or death? Although the ability of a particular