Microbiology

• Microorganisms decay dead organisms, releasing and recycling nutrients.
• Bacteria reproduce asexually, by binary fission and can do so very rapidly.
• What are the shapes of bacteria?

  Coccus - spherical

  Bacillus - rod shaped

  Spirillum - spiral
• Bacteria tend to group. they may be single, in pairs or in chains/clusters.
• What does the metabolic feature autotrophic mean?

  They synthesise all their cell constituents using carbon dioxide as the carbon source.
• What does the metabolic feature phototrophic mean?

  Photosynthesis with chlorophyll as the electron donor.
• What is an antigen?

  A molecule that causes the immune system to produce antibodies against it. These may be individual molecules or those on the surface of the bacterial cells.
• Gram stain:

  1)Application of crystal violet

  2)Application of grams iodine

  3)Alcohol wash

  4)application of safranin
• Why is gram staining used?

  To distinguish between the two types of bacteria. Gram positive and gram negative.
• Gram positive bacteria have a thick peptidoglycan cell wall but not a lipopolysccharide membrane. They therefore retain the crystal violet stain when washed with alcohol and appear purple under a microscope.
• Gram negative bacteria have a thin peptidoglycan cell wall but have a lipopolysaccharide membrane which is soluble in alcohol. When washed with alcohol this membrane dissolves and therefore loses the crystal violet stain. Instead they take up the safranin stain and appear red under a microscope.
• As bacterial metabolism is regulated by the enzymes, the range of 25-45 C is suitable for most bacteria. The optimum for mammalian pathogens Is around 37C.
• In the lab nutrients are supplied in nutrient media, which may be provided as a nutrient agar or nutrient liquid broth. The carbon source is usually glucose, and the nitrogen needed for amino acids and nucleic acid synthesis is provided as nitrate ions.
• Most bacteria favour slightly alkaline conditions, whereas most fungi favour in numeral to slightly acidic conditions.
• Obligate aerobes only survive and metabolise in the presence of oxygen.
• Obligate anaerobes can only survive and metabolise in the absence of oxygen.
• Facultative anaerobes prefer to metabolise in the presence of oxygen but can survive and metabolise in the absence of it.
• The lag phase is when there is little to no cell division and there is intense metabolic activity such as enzyme synthesis.
• The log phase is when there is exponential increase in numbers and there are no limiting factors. Cell division>death rate.
• The stationary phase is where growth stops due to limiting factors that prevent further growth of the population. Carrying capacity has been reached.
• The death phase is when limiting factors cause the population size to decrease. Death rate>cell division. This may be as a result of toxic waste or lack of nutrients.
• What are the types of sterilisation?

  Autoclave

  Gamma radiation
• How to sterilise equipment in an autoclave? 


  Place equipment in a sealed autoclave bag.

  Heat to 121 C

  Under high pressure for 15 mins.
• How to sterilise plastic equipment using gamma irradiation?

  Plastic equipment is often sterilised before use by gamma radiation.

  Once plastic equipment has been used, it can be disposed of using a biohazard waste bin or put in an autoclave.
• Total cell count - living and dead cells in a bacterial sample.

  Total viable count - Living cells in a known volume of liquid medium.
• A haemocytometer can be used to calculate the number of microbes in a sample in a sample. It is a microscope slide with regular chamber at the centre which is engraved with a grid. A coverslip is supported over the chamber and the depth of the chamber is known. The number of cells in a specific volume of medium can be counted.
• What inaccuracies may occur if the sample is under diluted?

  Colonies might merge and clumps. This would mean that counting may be inaccurate resulting in an under-estimate of cell numbers.
• What inaccuracies might occur if the culture is over-diluted?

  There will be too few colonies own each plate to count to be statistically correct.
• A colourimeter can be used to measure the turbidity of a culture as cell numbers increase. Measurements of the bacteria population are derived by finding the absorbance of the suspension and then reading from a standard graph of light absorbance plotted against the number of bacterial cells. The result is a total cell count because the colourimeter cannot distinguish between living and dead cells.