Download Chapter 20 Viruses, Bacteria, and Archaea

Title
Chapter 20
Viruses,
Bacteria, and
Archaea
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Table 20.1
Viruses
•
•
•
•
•
The viruses are a biological enigma. They have a DNA or
RNA genome, but they can reproduce only by using the
metabolic machinery of a host cell.
Viruses are noncellular, and therefore cannot be assigned a
two-part binomial name, as are organisms.
In 1884, Pasteur suspected something smaller than bacteria
caused rabies; he chose a Latin term for “poison.”
In 1892, Russian biologist Dimitri Ivanowsky, working with
the tobacco mosaic virus, confirmed Pasteur’s hypothesis that
an infectious agent smaller than a bacterium existed.
With the invention of the electron microscope, these
infectious agents could be seen for the first time.
Viral Structure
• All viruses have at least two parts:
–
–
An outer capsid is composed of protein subunits.
An inner core contains either DNA (deoxyribonucleic
acid) or RNA (ribonucleic acid), but not both.
• The viral capsid may be surrounded by an outer
membranous envelope; if not, the virus is said to be
naked.
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Tobacco mosaic virus
Parasitic Nature
• Viruses infect all sorts of cells, from bacteria to human
cells, but they are host specific.
• Virus Evolution
– Some believe that viruses originated from the very cells
that they infect.
– For example, viral nucleic acids originated from the host
cell genome.
– Therefore, viruses evolved after cells came into existence;
new viruses are probably evolving now.
– Others suggest that viruses arose before the three
domains.
Reproduction of Bacteriophages
• There are two types of bacteriophage life cycles,
termed the lytic cycle and the lysogenic cycle.
• In the lytic cycle, viral reproduction occurs, and the
host cell undergoes lysis, a breaking open of the cell to
release viral particles.
• In the lysogenic cycle, viral reproduction does not
immediately occur, but reproduction may take place
sometime in the future.
Fig. 20.3
The Bacteria
• Characteristics of Bacterial Cells
– Bacteria are the more common type of prokaryote.
– Bacterial cell walls are protected by peptidoglycan, a
complex of polysaccharides linked by amino acids.
– The Gram stain procedure (developed in the late 1880s
by Hans Christian Gram) differentiates bacteria.
•Gram-positive bacteria stain purple, whereas
Gram-negative bacteria stain pink.
•This difference is dependent on the thick or thin
(respectively) peptidoglycan cell wall.
– Bacteria and archaea have three basic shapes.
•A spirillum is spiral-shaped.
•A bacillus is an elongated or rod-shaped bacteria.
•Coccus bacteria are spherical.
•Cocci and bacilli tend to form clusters and chains of a
length typical of the particular species.
• Bacterial Metabolism
– Bacteria differ in their need for, and tolerance of, oxygen
(O2).
•Obligate anaerobes are unable to grow in the presence of O2;
this includes anaerobic bacteria that cause botulism, gas
gangrene, and tetanus.
•Facultative anaerobes are able to grow in either the presence or
absence of gaseous O2.
•Aerobic organisms (including animals and most prokaryotes)
require a constant supply of O2 to carry out cellular respiration.
Major Groups of Bacteria
• Proteobacteria
– Alpha Proteobacteria
Includes many symbionts of plants and animals, and some
pathogens. Rhizobium species live symbiotically in root
nodules of legumes (e.g., beans) and convert atmospheric
nitrogen to a form usable by plants (nitrogen fixation).
– Purple sulfur bacteria are photoautotrophs that do not
produce oxygen.
• Cyanobacteria
– Gram negative; photosynthetic. Inhabit ponds, lakes,
swimming pools, moist soil, dead logs, tree bark. Contain
chlorophyll a and use a photosynthetic process similar to
that of plants and algae. Many species also fix nitrogen.
– They photosynthesize in the same manner as plants and
are believed to be responsible for first introducing oxygen
into the primitive atmosphere.
– Cyanobacteria are common in fresh and marine waters, in
soil, and on moist surfaces, but they are also found in
harsh habitats, such as hot springs.
– Lichens are a symbiotic relationship where the
cyanobacteria provide organic nutrients to the fungus and
the fungus protects and supplies inorganic nutrients.
These "jelly balls" are colonies of the cyanobacteria species Nostoc pruniforme
("Mare's eggs"), living in freshwater ponds. (Determination microscopically
confirmed by Prof. Dr. L. Kies, Hamburg).
Anabaena
Oscillatoria
Lichen
• Gram-positive Bacteria
– Actinomycetes
Several species of the genus Streptomyces produce
antibiotics such as streptomycin, erythromycin,
chloramphenicol, and the tetracyclines. Some
actinomycetes cause serious lung disease or generalized
infections in humans and other animals.
– Lactic acid bacteria ferment sugar, producing lactic acids
as the main end product. Inhabit decomposing plant
material, milk, and other dairy products; responsible for
the characteristic taste of yogurt, pickles, sauerkraut, and
green olives.
– Clostridia are anaerobic. One species causes tetanus;
another causes gas gangrene. Clostridium botulinum can
cause botulism, and often fatal type of food poisoning.
Microscopic image of the bacterial spore formation of Bacillus
subtilis (ATCC 6633) Spore staining, magnification:1,000.
(green) spores, (red) vegetatives.
– The mycoplasmas are a group of bacteria that lack cell
walls. They inhabit soil and sewage; some are parasitic on
plants or animals. Some inhabit human mucous
membranes but do not generally cause diseases; one
species causes a mild type of bacterial pneumonia in
humans.
• Chlamydias
– Chlamydias lack peptidoglycan in their cell walls. Sexually
transmitted chlamydias are the major cause of pelvic
inflammatory disease in women.
• Spirochetes
– Spirochetes are spiral-shaped bacteria. Some species are
free-living, whereas others form symbiotic associations; a
few are parasitic. The spirochete of greatest medical
importance is Treponema pallidum, which causes syphilis.
Table 20.2
The Archaea
Types of Archaea
• Three main types of archaea are distinguished based
on their unique habitats: methanogens, halophiles, and
thermoacidophiles.
These Archaea species live in extreme heat near deep sea vents.
Extremophiles that survive in extremely cold environments.
Three groups of Archaea
• Crenarchaeota
– Some extremely thermophilic (thrive at 70° C, or some
thrive at temperatures greater than 100° C); others
psychrophilic (can live at temperatures below 15° C).
• Euryarchaeota
– Includes methanogens, halophiles (not all extreme), and
acidophiles. Some extreme thermophiles included in this
phylum.
• Nanoarchaeota