Bacteria
The Kingdom Monera consists of all prokaryotes, that is, unicellular organisms that lack
nuclear membranes. This taxonomic kingdom consists of two distinct groups:
Eubacteria and Archaebacteria.
Morphologically, archaebacteria and eubacteria differ in some key aspects. While most
members of both groups have cell walls, their cell membranes are chemically different, as
are their overall chemical makeup.
Most archaebacteria live in extremely hostile environments, such as extremely saline
waters or hot sulfur springs.
Some eubacteria also live in these harsh environments, but others inhabit locations
ranging from surface soils to the intestinal tracks of termites.
Though extremely small (most bacteria are significantly smaller than eukaryotic cells),
bacteria fill several important roles in the natural world. We are most familiar with
bacteria as the cause of diseases from strep throat to bubonic plague.
However, comparatively few bacteria cause diseases. Most are beneficial to other
organisms. Some bacteria are photoautotroph, producing food from inorganic material
and light. In some cases, particular photoautotrophs have the ecologically important
ability to fix nitrogen from the atmosphere, making it available to the roots of plants.
Other bacteria are saprophytes, breaking down dead organic material. Still others live
symbiotically in the digestive tracks of other organisms and aid in the digestion of diverse
food materials.
The structure and reproductive cycles of the Monerans are relatively simple compared to
those of the eukaryotes. They lack distinct nuclei and complex organelles. They have
small prokaryotic chromosomes and plasmids rather than the complex chromosomes
found in eukaryotes. Most Monerans reproduce by binary fission.
Eubacteria
Eubacteria, also known as the true bacteria, have a bad reputation. They are seen as
disease causing agents. Every day new products come out advertising their ability to
destroy these microscopic but dangerous creatures. In reality, only a small percentage of
these unicellular organisms cause disease. The rest fulfill many important roles in the
natural world. Eubacteria can be photoautotrophs, saprophytes, or symbionts.
Diversity of Eubacteria
The Eubacteria are an ancient and diverse group. Different species have evolved to fit in
every type of environment and lifestyle. They are often classified by their oxygen
requirements and by the type of nutrition in which they engage.
Nutrition
A great many of the most familiar eubacteria are heterotrophs, meaning they must take
food in from outside sources. Of the heterotrophs, the majority are saprophytes, which
consume dead material, or parasites, which live on or within another organism at the
host's expense.
In addition to the heterotrophs, there are many kinds of autotrophic bacteria, able to
produce their own food. These autotrophs may be photosynthetic or chemosynthetic and
may or may not use oxygen in their synthetic pathways. Cyanobacteria are the largest
group of photosynthetic eubacteria. The cells of these bacteria are often much larger than
other bacteria, which in the past led this group to be classified as algae rather than
bacteria. In fact, cyanobacteria are still sometimes referred to as blue-green algae. These
eubacteria possess pigment molecules, including chlorophyll a, the same type of
chlorophyll found in higher plants. Unlike plants, in cyanobacteria the pigments are not
contained within membrane-bound chloroplasts.
Oxygen Requirements
Respiration of eubacteria may be aerobic (using air) or anaerobic. (without air). The
anaerobes undergo a form of respiration called fermentation. Among anaerobes, some
can live in the presence or absence of oxygen. These are called facultative anaerobes.
Some are indifferent to the presence of oxygen, but others have two respiratory pathways,
one that uses oxygen and one that does not. The other group of anaerobes, the obligate
anaerobes, are actually poisoned in the presence of oxygen.
Gram Staining
In addition to respiratory and nutritional habits, one other important feature used to
classify bacteria is Gram staining. Gram's stain will highlight peptidoglycan if it
appears in a cell wall. Not all groups of eubacteria have peptidoglycan, so all eubacteria
may be classified as either Gram-positive (able to bind Gram's stain) or Gram-negative
(unable to bind Gram's stain).
Structure
As we just saw, eubacteria are extremely diverse and specialized to their environments.
Surprisingly, the structure of most eubacterial cells is relatively simple.
Figure 1.2: Structure of Eubacteria
Eubacteria have circular DNA molecules called plasmids. Plasmids are not housed in a
centralized nucleus because eubacteria, as prokaryotes, lack a nuclear membrane. Instead,
plasmids are usually found in relatively clear areas in the cytoplasm called nucleoids.
The rest of the cytoplasm is filled with ribosomes, the cell's protein synthesis machinery.
While eubacteria lack the organized organelles found in eukaryotic cells, many eubacteria
have specialized internal membranes. For example, cyanobacteria have membranes that
contain chlorophyll and other chemicals required to carry out photosynthesis.
Motility
Many eubacteria are motile. In most cases, rotating structures called flagella enable them
to move. The term flagella is also used to refer to similar motility structures in protists
and other eukaryotic cells, but the two are not the same and should not be confused.
Prokaryotic flagella are composed of protein subunits called flagellin, while eukaryotic
flagella are made of arrays of microtubules made of tubulin.
Shape
Eubacteria are often classified by their shape. They fall into three main shape categories.
Spherical eubacteria are called Cocci; rod-shaped eubacteria are known as Bacilli; spiral
or helically-shaped eubacteria are Spirilla.
Figure1.4: Common shapes of eubacteria
Reproduction
Eubacteria reproduce by binary fission. In this process, the genetic material is replicated,
and the two copies move to separate nucleoid regions. Next, the plasma membrane
pinches inward, producing two equal daughter cells. While these daughter cells are
completely independent of each other, in some species they remain together, forming
colonies and filaments. Binary fission can take place very rapidly, on the order of about
one split every 20 minutes, accounting for the amazing reproduction ability of
eubacteria.
Archaebacteria
The name "archaebacteria," with its prefix meaning "ancient," suggests that this is an
extremely old group. The fact that most of these Monerans live in extremely hostile
environments similar to those found on primitive Earth leads many to believe that
archaebacteria may have been the earliest forms of life on the planet.
Diversity of Archaebacteria
While some archaebacteria are heterotrophic, the vast majority are chemoautotrophs,
meaning they produce their own food from chemicals found in their environments. Based
on the method by which they do this and the type of environment in which they are
found, archaebacteria can be classified into four groups: methanogens, halophiles,
sulfur reducers, and thermoacidophiles.
Methanogens
Methanogens are anaerobic, feeding on decaying plant and other organic material,
producing water and methane gas. They can be found in bogs and marshes, deep in the
oceans, and in the gastrointestinal tracks of cellulose- fermenting herbivores where they
aid in the digestion of cellulose.
Halophiles
Halophiles are phototrophs (producing their energy from light) that use a purple version
of chlorophyll called bacteriorhodosin. They live in extremely salty conditions such as
those found in the Great Salt Lake and the Dead Sea. Such environments present two
challenges. First, the difference in salt concentration inside and outside the cell is
tremendous, creating huge osmotic pressure. While other organisms would rapidly lose
all of their water and die, halophiles have adapted to survive within such a difference in
water gradient. Second, the salty environments are very alkaline, some having a pH of up
to 11.5. Beyond simply surviving within these inhospitable environments, halophiles
have incorporated the conditions into their unique photosynthetic pathway. Most
halophiles are aerobes.
Sulfur Reducers
Like methanogens, sulfur reducers live near volcanic vents and pools. As their name
suggests, they use the abundant inorganic sulfur found near these vents, along with
hydrogen, as food. They also have very high heat tolerances, living in temperatures up to
85 degrees Celsius.
Thermoacidophiles
Thermacidophiles also live off of sulfur, but they do so by oxidizing it, combining the
sulfur with oxygen molecules rather than hydrogen. Like the methanogens and sulfur
reducers, these archaebacteria live near volcanic vents and pools and thus are adapted to
high temperatures (65 to 80 degrees Celsius). Unlike the other two classes, though,
thermoacidophiles also prefer extremely acidic conditions, living in environments with a
pH as low as 1.0. Almost all thermoacidophiles are obligate anaerobes.