CHAPTER 27: PROKARYOTES
The bacterial classification is currently being reconsidered because of the many new discoveries of
the past decade (thanks to all that cool technology!), so various possible systems are used, such as
Kingdom Monera, Kingdom Prokaryota, Kingdoms Archaeprokaryota and Euprokaryota, Kingdoms
Archaemonera and Eumonera, Domain Prokaryota, Domains Archaeprokaryota and Euprokaryota.
In general, any ancient group of bacteria includes the thermophiles, halophiles and methanogens. The
modern classification groups include those that we are more familiar with, most of which have cell
walls (the exceptions are the eubacterial mycoplasmas, which lack cell walls). The first prokaryotes
probably formed from the interaction and aggregation of viscous protobionts. They existed as the
only life forms for +/- two billion years. Even today, they are the most numerous cellular life forms.
Over 4,000 species of bacteria have been identified. Projections for future estimates run close to
+4,000,000. Most are unicellular, but some are colonial.
Many microfossils of prokaryotes exist, and they date back as far as the earliest Precambrian time.
Numerous microfossils have been recovered from the Ediacaran hills of Australia and include
Clostridium sp. as well as Eobacterium isolatum (age = +3 billion years). Modern bacteria have a size
range from 1 to 5 µm. One noted exception is Epulopiscium fishelsoni, which measures over 600-µm
long and 80 µm wide. It is a symbiont in the intestine of the sturgeon fish, and its habitat is the Great
Barrier Reef in Australia.
Bacteria can play a positive or negative role in every known environment and natural cycle. Examples of
helpful bacteria include those involved in nitrogen fixation; preparation of antibiotics; yogurt
production; the production of industrial compounds, such as acetone and vinegar; use in the process of
bioremediation of contaminated soil and/or water (Flavobacterium); and the production of vitamin K in
the human colon. Other bacteria are considered harmful. Examples of harmful bacteria include those
that cause tetanus, botulism, gas gangrene, anthrax and whooping cough.
BACTERIAL BODY DESIGN
All of the Eubacteria except the mycoplasmas have cell walls. Most of the bacterial cell walls are
composed of a carbohydrate substance known as peptidoglycan, which is a combination of n-acetyl
glucosamine (NAG) and n-acetyl muramic acid (NAM). Anything that can disrupt the integrity of the
bacterial cell wall, such as antibiotics or the lysozyme enzyme of human tears, can destroy the
bacterial cell. There are two genetically determined patterns of bacterial cell wall design. These two
designs are the basis for most Eubacterial classification. Each cell wall design produces one of two
possible responses to the staining sequence developed by Christian Gram. One is Gram positive and the
other is Gram negative. There are three main body shapes for cell-wall producing bacteria. They are:
(1) bacillus, (2) coccus and (3) spiral.
BACTERIAL STRUCTURES
Bacteria usually only have a single DNA molecule, and it is usually free floating. Sometimes, however, it
forms special associations, such as forming a bacterial chromosome, a plasmid, or special respiratory
membranes that attach to the internal cell membrane surface. Other specialized body features may
include structures such as a periplasmic space, mesosome, capsule, endospore, bacterial ribosome,
chromatophore, pilus and flagellum.
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REPRODUCTION
Bacteria can reproduce asexually (by binary fission or budding) or sexually by transformation,
transduction or conjugation.
NUTRITIONAL MODES
Bacteria exhibit various modes of nutrition. Some are autotrophs (chemosynthetic or photosynthetic),
some are saprophytes and others are heterotrophs. Bacteria also exhibit various respiratory modes.
Some live aerobically, others anaerobically and others can switch back and forth (are facultative) as
needed.
TOXINS
Some bacterial produce toxins. There are two toxin categories: (1) endotoxins and (2) exotoxins.
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