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Summary Unit 3.4. - Microbiology
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Notes on bacterial properties, aseptic technique, serial dilution and antibiotics
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3.4 MicrobiologyBacterial structure:
Always present Sometimes present
Cell membrane Pili
Cytoplasm Murein cell wall
Mesosome Glycocalyx
Nucleoid Flagellum
Ribosomes
Plasmids
Food granules
Also known as prokaryotes
Come in both heterotrophic and autotrophic forms
They are very important in decaying dead organisms and play an important role in the
nutrient cycle
Disease causing bacteria are pathogens
They reproduce asexually by binary fission (splitting in two) by mitosis
Bacteria can be identified by: Shape, size, staining characteristics plus metabolic, antigenic
and genetic features
Rod shaped bacteria = Bacillus e.g. E.coli
Spherical bacteria = Coccus e.g. staphylococcus
Spiral shaped bacteria = spirillum e.g. Spirillum
Bacteria are grouped in pairs, chains or clusters
Smallest bacteria are archaea and the largest is the sulphur bacterium
Bacteria walls contain peptidoglycan/murein which is molecules of polysaccharides and
polypeptides cross linked and the cross linking provides strength so the wall prevents against
osmotic lysis
Bacteria are either gram positive or gram negative – after staining, gram positive bacteria
are purple and gram-negative bacteria are pink
The difference is that gram positive have a basic cell wall structure whereas gram negative
has an additional lipopolysaccharide layer (fatty layer)
The lack of polysaccharide in gram positive bacteria is what allows it to bind to the stain (it
retains the crystal violet)
Gram positive bacteria are more susceptible to antibiotics and lysozyme (found in human
tears)
Gram positive bacteria example is Staphylococcus aureus
The Gram-negative bacteria have the lipopolysaccharide layer protecting the cell so they are
resistant to lysozyme and antibiotics like penicillin and they don’t retain the crystal violet
stain
Gram negative bacteria example is Salmonella
Gram staining steps:
1. Add crystal violet dye which binds to peptidoglycan so all bacteria stain purple
2. Add iodine which binds crystal violet to peptidoglycan more strongly fixing the stain
in gram positive bacteria
3. Add ethanol which removes unbound crystal violet and lipopolysaccharide so gram
negative bacteria become colourless and gram-positive stay purple
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