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3.4 - Microbiology - Coggle Diagram
3.4 - Microbiology
Classifying bacteria by shape
- Bacteria can be distinguished from each other by their size, shape and staining characteristics as well as by their metabloic antigenic and gentic features.
3 main shapes of bacteria Spherical in shape
- Cocci (plural)
- Coccus (singular)
Rod Shaped
- Bacilli (plural)
- Bacillus (singular)
Spiral or corkscrew shaped
Classifying bacteria by Gram staining
- Bacteria can be classified as a Gram Positive (purple) or Gram Negative (red) according to how they react to the Gram stain.
Method
- Smear glass slide with the bacterial sample and use heat to fix
- Strain with crystal violent which bindsto peptidoglycan
- Treat with mordant Lugol's iodine to fix the stain to the peptidoglycan
- Decolourise with alcohol to remove unbound crystal violent stain.
- Counter stain with safranin.
Gram positive
- Thick peptidoglycan (murein)
- cell wall that retains crystal violent stain and so apprears violent or purple
Gram Negative
- Lipopolysaccharide layer that is washed away with the alcohol along with any crystal violent stain.
Gram negative, cells then stain pink or red wuth safranin.
- The lipoolysaccharide layer
provides protection by lysozyme and penicillin antibiotics. They are therefore more difficult to control inmedical settings
Counting microbes
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Method
- In order to obtain reliable results when counting microbes, diluting the original sample of bacteria to give between 20-200 colonies.
- Count colonies on the plate and multiply the dilution factor to find the number of bacterua in 1cm3 of the original sample. This is a viable count as only living cells can create a colony.
Growing microbes
- Microbes can be grown in Petri dishes or in large containers. They are usually grown in agar which is a jelly like sybstance, to which the required resources can be added.
Nutrients
- carbon source - Bacteria requires a source of carbon for growth, generally a organic compund such as glucose
Nitrogen source - Nitrogen needs to be provided for synthesis of nitrogenous compunds, it can be provided as organic or inorganic compunds.
Temperature
- Optimum temperature of the bacteria you wish to grow should be taken into account, growing bacteria at 37.C (body temperature) will grow bacteria able to live in the human body (human pathogens)
pH
- Optimum pH of the bacteria should be considered. Acid or Alkali can be used to change pH
Oxygen
- Obligate aerobe - Can only reproduce or metabolise in an oxygen -rish environment
- Facultative anaerobe - Thrive in an oxygen-rich environment but can alos respire anarobically
- Obligate Anaerobe - Unable to reproduce or metabbolise when there is oxygen present.
Aseptic techniques
Must be used
- To prevent contamination of the environment by the microbes being handled
- To prevent contamination of the cultures by microbes from the environment
- patri dishes and the nutrient agar should be sterilised before the agar is poured.
- An inoculating lupe is used to transfer bacteria should be sterilised before and after use by heating it to red heat on a Bunsen flame.
- Only lift patri dish lid slighty as this prevent microorgansims from the air contaminating it.
- After inoculation, the lid of the parti dish should be secured in the place by strips of adhesive tape labelled and dated.
Agar plates shoul dbe incubated at 25.C in school and laboratories for 24-48 hours, encouraging growth of the cultures without growing pathogens
- Sterilise plates and equiptment after use.
Key terms
- Aseptic techniques - Lab practise that maintains sterility and prevents contamination of the equiptment and the environment.
- Autoclave - Method of steriliation. Autoclave is sealed container where glass and metal equiptment is heated (at 121.C) in steam under pressure (for 15 mins after the required pressure is reached)
- Colony - Cluster of cells, or clone, whcih aries from a single bacterium or fungal spore by asexual reproduction.
- Gram stain - A method of staining the cell walls of bacteria as an aid to their identification
- Haemocytometer - Specialised microscope slide used to count the number of colonies. The result is a total cell count becuase it is not possible to distinguish between living and dead cells.