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Unit 15: Microbiological Techniques. Assignment 2: P5
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Amina AliUnit 15: Microbiological Techniques
Assignment 2: Classification of micro-organisms
Task 1
P5: Describe the main groups of micro-organisms by their principal taxonomic characteristics
Taxonomic classification is a hierarchical system used to categorise micro-organisms. Taxonomic
rank is the relative level of a group of organisms. The order of classification from the most specific to
the broadest is species, genus, family, order, subclass, class, superclass, subphylum, phylum,
kingdom and domain.
Bacteria
Bacterial cells are unicellular organisms. They are prokaryotic cells because they lack a nucleus and
membrane-bound organelles. They occur in five main shapes: bacillus (rod shape), coccus (spherical
shape), spirilla (spiral shape), vibrio (curved shape) and spirochete (corkscrew). Most bacteria have a
cell wall made of peptidoglycan, the cell divides by binary fission and they sometimes have flagella
for motility. The difference in their cell wall structure is a major feature used in classifying these
organisms.
Bacteria can be classified as either Gram-positive or Gram-negative, according to the way the cell
wall structure stains. Can be further characterised based on their response to gaseous oxygen into
the following groups: aerobic (living in the presence of oxygen), anaerobic (living without oxygen),
and facultative anaerobes (can live in both environments).
According to the way bacteria obtain energy, they are classified as heterotrophs or autotrophs.
Autotrophs make their own food by using the energy from the sunlight or chemical reactions, in
which case they are called chemoautotrophs. Heterotrophs obtain their energy by consuming other
organisms. Bacteria that use decaying life forms as a source of energy are called saprophytes.
Archaea
Archaea or Archaebacteria differ from true bacteria because their cell wall lack peptidoglycans. They
are prokaryotic cells which have avidity to extreme conditions. All Archaean’s can be divided into the
groups based on their habits: methanogens (methane-producing organisms), halophiles (archaea
that live in salty environments), thermophiles (archaea that live at extremely hot temperatures) and
psychrophiles (cold-temperature archaea).
Archaea use different energy sources such as hydrogen gas, carbon dioxide and sulphur. Some use
sunlight to make energy but in the same way plants do. These archaea absorb sunlight using their
membrane pigment, bacteriorhodopsin, this reacts with light thus forming the energy molecule
adenosine triphosphate (ATP).
Fungi
Fungi (mushroom, mould and yeast) are eukaryotic cells with a membrane-bound nucleus. Fungi can
be unicellular or dimorphic but most are multicellular. Fungal cell walls are rigid and contain complex
polysaccharides called chitin (adds structural strength) and glucans. Fungi thrive in moist and slightly
acidic environment and can grow with or without light or oxygen. Fungi reproduce asexually,
sexually or both through fragmentation, budding or producing spores.
Fungi are heterotrophic as they use nutrients by either absorbing organic material from their
environment (decomposers), through symbiotic relationships with plants (symbionts) or harmful
relationships with a host (parasites). They form structures called hyphae (branching filaments) which
help absorb nutrients from the environment.
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