CP 13 Flashcards
EVALUATION
● Contamination of the sample could lead to inaccurate readings
● Careful not to dig into the agar gel as this would affect the growth of the microbes
● Incubation temperature will affect the growth of microbes. Too hot = death, too cool = growth too slow
● Difficulty in identifying the colonies
● Not streaking the plate properly means that you would not get a gradual dilution of the colonies on the plate and could then mean bacteria will be growing in merging colonies making it difficult to isolate. This investigation can be adapted to test for antibiotic resistance. Remember, the same bacterial species and concentration must be used
Controlled variable
● Nutrients in the agar jelly
● Time left for incubation
● Aseptic technique
Procedure
disinfect surfaces / use benchcoat (1)
work near Bunsen flame / work in a sterile cabinet (1)
flame top of tube (1)
heat wire loop (1)
open lid of Petri dish to a small extent (1)
allow wire loop to cool (1)
• flame a wire loop / use of sterile swab (cotton bud) (1)
• first streaks of sample in straight /zig-zag lines on
surface of sterile agar plate (1)
• at least 3 streaks in total (1)
• second and third streak must pass through lines of
previous streak (1)
• incubate and take off individual colonies with wire loop
• only incubate at 20 degrees as above 30 °C close to human body temperature (1)
• so risk of incubating human pathogens which could infect students (1)
Describe how streak plating can be used to isolate individual bacterial colonies from a mix ed liquid culture. You may use a diagram in your answer. ( 5 )
- flame a wire loop / use of sterile swab (cotton bud)
- first streaks of sample in straight /zig-zag lines on surface of sterile agar plate
- at least 3 streaks in total
- second and third streak must pass through lines of previous streak
- incubate and take off individual colonies with wire loop
How can different colonies on an agar plate be
distinguished from each other?
Different colonies will have different sizes, colour and texture.
How can different colonies on an agar plate be
distinguished from each other?
Different colonies will have different
sizes, colour and texture.
Why should the lid of the petri dish not
be fully taped around?
To allow oxygen to enter, preventing the conditions from becoming anoxic
Why should the lid of the petri dish not
be fully taped around?
To allow oxygen to enter, preventing the
conditions from becoming anoxic
Why was it important that each set of streaks crossed over?
Sets of streaks must cross over so that clumps of bacteria in the first streaks can gradually be
spread out into subsequent streaks.
Why should incubation be carried out below 30 °C?
Temperatures above 30 °C are closer to human body temperature, so temperatures in this range would cause a risk of incubating human pathogens which could infect students.
Sterile (aseptic) technique is important in microbiology.
Explain why this is the case even in this activity, where the bacterial species have been selected because they are harmless to humans
The use of aseptic technique is vital in microbiology to ensure that there is no contamination of cultures by microorganisms from the environment and that people and the environment are not contaminated by the microorganisms being handled.
Even cultures thought to be low risk should be treated with caution as bacteria may mutate to form pathogenic strains, our knowledge of the hazards may be incomplete or the culture may have become contaminated.
Besides colour, what other features of colonies can be used to distinguish bacterial species?
Ideas might include overall shape, shape of the colony margin, whether colonies are shiny or dull, and height of colony (flat or raised).
