Bacteria
“Bacteria” is a plural word. The singular for this word is “bacterium” (bacter = rod, staff).
Bacteria are prokaryotes, which means that they have no true nucleus. They do have one
chromosome of double-stranded DNA in a ring. Most bacteria lack or have very few internal
membranes, which means that they don’t have some kinds of organelles (like mitochondria or
chloroplasts). Most bacteria are benign (benign = good, friendly, kind) or beneficial, and only a
few are “bad guys” or pathogens.
There are some bacteria relatives that can do photosynthesis--they don’t have chloroplasts, but
their chlorophyll and other needed chemicals are built into their cell membranes. These
organisms are called Cyanobacteria (cyano = blue, dark blue) or blue-green algae, although
they’re not really algae (real algae are in Kingdom Protista).
Shapes
Most bacteria are one of three shapes (although there are a few other possibilities):
coccus (sing.), cocci (pl.): are spherical
(coccus = a berry)
bacillus (sing.), bacilli (pl.): are rod-shaped
(bacillum = a little stick)
spirillum (sing.), spirilla (pl.): are spiral
(spiro = spiral, coil).
While many bacteria live singly, others are found in aggregates or clusters. These aggregates are
named based on the arrangement of the bacterial cells of which they are composed. Using cocci
as an example:
diplococcus: are in sets of two (diplo = double, two; pneumo = lungs)
streptococcus: are in chains (strepto = bent, twisted, pliable)
staphylococcus: are in clusters (staphylo = a bunch of grapes).
Pathenogenic Bacteria
One person who worked with bacteria was Dr. Robert Koch, a German physician. He is famous
for several discoveries related to bacteria:
1. Colony Isolation - He noted bacteria growing on a spoiled potato and realized that each
colony he saw grew from one bacterium that had landed on the potato. He realized he
could remove a bit of one of the colonies and transfer it to a sterile medium to start a
pure culture of that species of bacterium. This is called single colony isolation.
2. Use of Agar - Koch developed the use of agar (a polysaccharide isolated from seaweed) to
solidify nutrient media upon which to raise/grow bacteria. Because agar is a complex
polysaccharide, most bacteria cannot digest it, and it remains solid at body temperature,
enabling researchers to incubate bacteria, encouraging their growth.
3. He was the first person to actually connect certain disease(s) to specific bacteria. He
established four criteria, called “Koch’s Postulates” which if met, prove a specific
pathogen causes a specific disease (in animals):
Koch’s Postulates:
1. the same pathogen must be found in all diseased individuals (those showing the same
symptoms)
2. the pathogen must be isolated from the diseased subjects and grown in pure culture on
some nutrient medium,
3. the same disease must be induced in experimental animals by transferring bacteria from
the pure culture into their bodies, and
4. after the disease develops, the same pathogen must be isolated from the experimental
animals.
If all four of these steps can be demonstrated, then it can be said that the pathogen in
question causes that disease. Koch specifically proved that anthrax and tuberculosis were
caused by specific species of bacteria. Other people have demonstrated this relationship for a
number of other bacterial diseases. Interestingly, this has never been done for the bacterium
that causes syphilis. While we know what it is and how to treat it, no one has been able to grow
it in culture.
Roles of Bacteria in the Environment
Bacteria are vital to maintaining the living world. Some are producers that capture energy by
photosynthesis. Others are decomposers that break the nutrients in dead matter and the
atmosphere. Still other bacteria have human uses.
Decomposers - Every living thing depends directly or indirectly on a supply of raw materials. If
these materials were lost when an organism died, life could not continue. Before long, plants
would drain the soil of minerals and die, and animals that depend on plants for food would
starve. As decomposers, bacteria help the ecosystem recycle nutrients, therefore maintaining
equilibrium in the environment. When a tree dies, armies of bacteria attack and digest the dead
tissue, breaking it down into simpler materials, which are released into the soil.
Bacteria also help with critical steps in sewage treatment. Sewage contains human waste,
discarded food, and chemical waste. Bacteria break down complex compounds in the sewage
into simpler ones. This process produces purified water, nitrogen and carbon dioxide gases, and
leftover products that can be used as fertilizers.
Nitrogen Fixers - Plants and animals depend on bacteria for nitrogen. You may recall that plants
need nitrogen to make amino acids, the building blocks of proteins. Nitrogen gas (N 2) makes up
approximately 80 percent of Earth's atmosphere. However, plants cannot use nitrogen gas
directly. Nitrogen must first be changed chemically to ammonia (NH 3) or other nitrogen
compounds. Expensive synthetic fertilizers contain these nitrogen compounds, but certain
bacteria in the soil produce them naturally. The process of converting nitrogen gas into a form
plants can use is known as nitrogen fixation. Nitrogen fixation allows nitrogen atoms to
continually cycle through the biosphere.
Human Uses of Bacteria - Many of the remarkable properties of bacteria provide us with
products we depend on every day. For example, bacteria are used in the production of a wide
variety of foods and beverages. Bacteria can also be used in industry. One type of bacteria can
digest petroleum, making it very helpful in cleaning up small oil spills. Some bacteria remove
waste products and poisons from water. Others can even help to mine minerals from the ground.
Still others are used to synthesize drugs and chemicals through the techniques of genetic
engineering.
Our intestines are inhabited by large numbers of bacteria, including E. coli. The term coli was
derived from the fact that these bacteria were discovered in the human colon, or large
intestine. In the intestines, the bacteria are provided with a warm and safe home, plenty of
food, and free transportation. These bacteria also make a number of vitamins that the body
cannot produce by itself. So both we, and the bacteria benefit from this symbiotic relationship.
Biologists continue to discover new uses for bacteria. For example, biotechnology companies
have begun to realize that bacteria adapted to extreme environments may be a rich source of
heat-stable enzymes. These enzymes can be used in medicine, food production, and industrial
chemistry.
Response
1. What are good bacteria called? Bad bacteria? Where have you heard these terms before?
2. What factors can be used to identify prokaryotes?
3. Explain how Cyanobacteria can perform photosynthesis without membrane bound
organelles (i.e. Chloroplasts).
4. Label each image of bacteria below based on their shape (coccus, bacillus, or spirillum)
5. Give at least two examples of how bacteria maintain equilibrium in the environment.
6. Identify the parts of a prokaryote:
7. What are some ways that human use bacteria? Which was most surprising to you? Why?
ENRICHMENT: Acid-Fast Bacteria
Almost all bacteria can be classified as Gram-
positive or Gram-negative by a process called
Gram staining. Very few bacteria are Gram-
positive. Most bacteria, yeasts, and fungi are
Gram-negative. How bacteria respond to the
Gram-staining procedure can provide
information about the nutritive requirements,
cell wall composition, and other traits of the
bacteria. When treated with special dyes,
Gram-positive bacteria appear deep violet in
color. Gram-negative bacteria appear pink. The
Gram-staining technique can help doctors
identify bacteria and choose the correct
antibiotics to treat bacterial infections.
Some bacteria, however, are completely resistant to Gram staining. These bacteria are known as
acid-fast bacteria, and are identified by using the acid-fast stain. In this procedure, a sample of
unknown bacteria is dyed and then washed with acidified alcohol. This will remove most of the
dye. Those bacteria that “hold fast” to the dye will be strongly stained and readily identifiable
as acid-fast bacteria. Because acid-fast bacteria are a major cause of disease, it is particularly
important to be able to identify them.
The acid-fast bacteria form a homogeneous group composed of the genera Mycobacterium and
Nocardia. Mycobacteria are usually rod shaped, and are found in soil, water, and animals. Many
species are saprophytic (feed on dead organic matter); others cause diseases such as diphtheria,
tuberculosis, and leprosy.
Acid-fast bacteria are characterized by their high lipid (fat) content. Lipids and waxes make up
as much as 40 percent of the dry weight of acid-fast bacteria. These lipids and waxes are the
key to testing for acid-fast bacteria. The lipids and waxes absorb dye so it can’t be removed
with acidified alcohol. This is how acid-fast bacteria “hold fast” to dye and remain stained while
bacteria are washed clean.
1. How are acid-fast bacteria different from other bacteria?
2. Why is it important to wash the bacteria sample after it has been dyed?