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The effects of microbes have been recognized for many centuries.
Summary of Section 1.1
Microbes could not be identified until microscopes were invented.
Summary of Section 1.1
The use of microbes to refute the idea of spontaneous generation highlighted
the need for well designed apparatus and good experimental practice.
Summary of Section 1.1
Microbes are of enormous importance to us and to our environment.
Summary of Section 1.2
Microbes include prions, viruses, bacteria, most protoctists and fungi.
Summary of Section 1.2
Some microbial species are extremely useful laboratory organisms.
Summary of Section 1.2
Viruses are obligate intracellular parasites.
Summary of Section 1.3
Viral genetic material shows great diversity as it can be DNA or RNA, singleor
double-stranded, and linear or circular.
Summary of Section 1.3
Viruses replicate by infecting a host cell, then hijacking the cell’s molecular
machinery to make more viral genomes and capsids. Released virions can go
on to infect new host cells, spreading the infection.
Summary of Section 1.3
The three phases of viral infection are initiation, when viruses gain entry to
the host cell, replication, when their nucleic acid is replicated and capsid
proteins are made, and release, when the new virions are assembled, become
mature and then leave the host cell.
Summary of Section 1.3
Virus structure is based on helical or icosahedral symmetry, or can be more
complex. The virions can be naked or covered with an envelope derived from
the host’s membrane.
Summary of Section 1.3
Viroids are naked, infectious RNA molecules.
Summary of Section 1.3
Virusoids are like large viroids, but require a proper virus for replication and
transport.
Summary of Section 1.3
Prions are infective agents with no nucleic acid.
Summary of Section 1.4
Mutant prion protein, PrPsc, produced from the mutant PrP gene, is an
abnormally shaped molecule that can change the conformation of normal,
PrPc, molecules into its own higher-order structure.
Summary of Section 1.4
Diseases caused by prions are well known in some domesticated animals, but
a new variant that gives rise to an acute, fatal disease in humans is causing
concern.
Summary of Section 1.4
Bacteria have a high degree of structural uniformity, but enormous metabolic
diversity.
Summary of Section 1.5
Bacterial taxonomy is complicated by the absence of conventional species.
Summary of Section 1.5
The outer surfaces of bacteria divide them into two major types by their
reaction to the Gram stain. Gram-negative bacteria have an outer membrane
lying outside the cell wall. Both types contain peptidoglycan, but Grampositives
have much more of this polymer in their walls than do Gramnegatives.
Summary of Section 1.5
Many bacteria secrete extracellular materials such as slime which may be
important for their pathogenicity.
Summary of Section 1.5
Many bacteria are motile; the most common mechanism is by a rotating
flagellar motor.
Summary of Section 1.5
Bacteria grow by binary fission and can achieve quite spectacular growth
rates.
Summary of Section 1.5
The growth of bacterial cultures can be described by a growth curve with lag,
exponential, stationary and death phases.
Summary of Section 1.5
Bacterial growth can be described mathematically using the parameters of
doubling time, td, and specific growth rate, μ.
Summary of Section 1.5
Multiple origins of replication at high growth rates allow complete copies of
the chromosome to be partitioned to all daughter cells.
Summary of Section 1.5
Some bacteria, mainly Gram-positives, can form endospores which are
extremely resistant to adverse environmental conditions.
Summary of Section 1.5
The known Archaea have been found predominantly in extreme
environments, e.g. at high temperatures and extremes of pH.
Summary of Section 1.5
The largest group of Archaea is the methanogens, which produce methane
and are therefore believed to contribute significantly to global warming.
Summary of Section 1.5
Bacteria are morphologically not very diverse, although reaction to the Gram
stain distinguishes one phylum from all the others.
Summary of Section 1.5
Bacteria are classified largely by their metabolic activities.
Summary of Section 1.5
The percentage GC content of bacterial DNA has been widely used as a
taxonomic criterion.
Summary of Section 1.5
The actinomycetes are among the most diverse of bacteria, and are renowned
for their antibiotic production.
Summary of Section 1.5
Fungi are osmotrophic, heterotrophic, predominantly haploid eukaryotes. The
hypha is the structural unit.
Summary of Section 1.6
The fungi show a great range of modes of nutrition, although the majority are
saprotrophs, which secrete digestive enzymes.
Summary of Section 1.6
Asexual reproduction is by means of mitotically produced modified hyphal
structures that produce mitospores.
Summary of Section 1.6
Sexual reproduction in fungi is complex, sometimes (in the Basidiomycota)
with many possible mating types. The diversity of structures and mechanisms
is the basis for their taxonomy.
Summary of Section 1.6
Sexual processes in Zygomycota and Ascomycota are triggered by
pheromones.
Summary of Section 1.6
Some Basidiomycota show an extended dikaryotic phase in which there are
two synchronously dividing nuclei per compartment.
Summary of Section 1.6
The sequence of sexual phases and the various properties shown by fungi
during them enables four characteristic life cycles to be recognized.
Summary of Section 1.6
Fungi are distributed by the dispersal of both their mitotic and meiotic spores
by air currents, animals or water. The spores may be actively discharged.
Summary of Section 1.6
Material detected in meteorites from Mars found on Earth may derive from
biological activity. This suggestion is highly controversial on a number of
grounds.
Summary of Section 1.7
More evidence to illuminate this question may be forthcoming from the
Beagle 2 expedition.
Summary of Section 1.7
Anaerobic bacteria may respire using terminal electron acceptors other than
oxygen.
Summary of Section 2.2
Electron acceptors with the highest redox potential are used preferentially, so
that bacteria using different terminal acceptors are zoned in anaerobic
sediments.
Summary of Section 2.2
Methanogens are anaerobic archaeons and use the end-products of anaerobic
decomposition as both carbon and energy sources, producing methane as
their end-product.
Summary of Section 2.2
The two most common pathways that generate energy and methane are:
(i) reduction of CO2 using H2 gas; (ii) breakdown of acetate to CO2 and CH4.
Summary of Section 2.2
Anaerobic respirers are important in mineral cycling, methanogens produce a
significant greenhouse gas, and both play key roles in the decomposition of
organic matter.
Summary of Section 2.2
Microbes can obtain energy not only from organic molecules (heterotrophy)
but also from light (phototrophy) and simple inorganic molecules
(chemotrophy), often switching between two, or sometimes all three, of these
types of energy metabolism.
Summary of Section 2.3
In both phototrophs and chemotrophs, energy released during electron
transport is conserved as an electrochemical gradient (proton pumping) which
is used to synthesize ATP.
Summary of Section 2.3
Among microbial phototrophs, oxygenic cyanobacteria have the same light
reactions as plants, with water as the electron donor; all other photosynthetic
bacteria are anoxygenic and anaerobic, using cyclic electron transport to
generate ATP and either non-cyclic electron transport (in green sulfur
bacteria) or reverse (ATP-powered) electron transport (in purple sulfur
bacteria) to generate reducing power (NADP.2H). Reduced sulfur compounds
such as H2S are used as electron donors in these last two processes.
Summary of Section 2.3
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