Page 7 - The Science of Infectious Diseases

Clostridium botulinum produces a toxin so potent that just 14
ounces (400 g) would be enough to kill everyone on the planet.
Microbes can also trigger a response by the immune system
that is itself dangerous. The immune system deploys antibodies,
white blood cells, and other defenses to rid the body of the invad-
ing microbes. It is the body’s own immune system that causes
many of the symptoms people experience when infected by a
microbe, including fever, headache, body ache, boils, and rashes.
Usually, the immune system defeats the invaders and returns the
body to health, but sometimes the immune reaction is so severe
that it damages healthy tissues and even causes death.

Microscopic Diversity
Bacteria come in several shapes, including rod shaped (bacilli, or
bacillus for one cell), round (cocci, or coccus for one), and spiral
(spirilla, or spirillum for one). The shape of the bacteria is often
refl ected in its name. For example, the rodlike bacteria that curdle
milk into buttermilk are called Lactobacillus bulgaricus. The round
bacteria that cause skin boils, cellulitis, and life-threatening toxic
shock syndrome are called Staphylococcus aureus. Some bacte-
ria have tails, and others are covered with fi brous strands known
as fl agella if they are long and pili if they are short. These exten-
sions can move like whips, propelling the bacterial cell through
its environment and allowing it to attach to objects or other cells.
Bacteria are incredibly small. Scientists measure them in units
called microns, abbreviated as μm. One micron equals one-
millionth of a meter, or one-thousandth of a millimeter. Round
bacteria average 0.5 to 2.0 µm in diameter. Rod-shaped bacteria
average 0.25 to 1.5 µm in diameter and 1 to 10 µm in length. For
example, the disease-causing, rod-shaped Escherichia coli bac-
terium, known as E. coli, averages 1.1 to 1.5 µm in diameter by
2.0 to 6.0 µm in length. Spiral bacteria average 0.4 to 1.7 μm in
diameter and up to 60 μm in length. The smallest objects visible
to the human eye are about 100 μm long. Therefore, most bac-
teria can only be seen with a microscope, but the Epulopiscium

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