3.4 microbiology SWA
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1
What are the three bacterial shapes?
Cocci (spherical), bacilli (rod), and spirilla (spiral).
2
What colour are Gram-positive bacteria after staining?
Purple.
3
What colour are Gram-negative bacteria after staining?
Red/pink.
4
Why do Gram-negative bacteria stain red?
They have a thin peptidoglycan layer and an outer lipopolysaccharide layer that is washed away by alcohol.
5
List the four Gram stain reagents in order.
Crystal violet, Lugol’s iodine, alcohol/ethanol, safranin.
6
What is an obligate aerobe?
An organism that requires oxygen for metabolism.
7
What is a facultative anaerobe?
An organism that grows better with oxygen but can survive without it.
8
Define aseptic technique.
Procedures used to prevent contamination of cultures and the environment.
9
At what temperature should school cultures be incubated?
25°C.
10
Why is 37°C avoided in school labs?
To prevent the growth of human pathogens.
11
Define a colony.
A visible mass of cells produced from a single original bacterium.
12
What is the difference between total and viable counts?
Total counts include living and dead cells; viable counts only include living cells.
13
What is the purpose of serial dilution?
To reduce the concentration of bacteria so individual colonies can be counted.
14
Name the machine used to sterilise equipment with high pressure.
Autoclave.
15
What is the function of a haemocytometer?
A specialised microscope slide used to perform a total cell count.
16
What is the lag phase?
A period of no growth where bacteria adapt and synthesise enzymes.
17
What happens during the stationary phase?
The birth rate equals the death rate due to limiting factors.
18
Why is the Petri dish lid only partially taped?
To allow oxygen in and prevent the growth of anaerobic pathogens.
19
What is the log phase?
A period of exponential growth where the population doubles at a constant rate.
20
How should a loop be sterilised?
By heating in a Bunsen burner flame until it glows red.
21
What are the conditions in an autoclave?
121°C for 15 minutes at high pressure.
22
Define a pathogen.
A microorganism that causes disease.
23
Why do Gram-positive bacteria retain crystal violet?
They have a thick peptidoglycan cell wall that traps the stain–iodine complex.
24
Name a nitrogen source for bacterial growth media.
Nitrate ions or amino acids.
25
What is the purpose of a blank in colorimetry?
To set the absorbance to zero using only the growth medium.
26
How do you calculate the number of bacteria in the original sample?
Number of colonies multiplied by the dilution factor.
27
What are mesophiles?
Bacteria that grow best at moderate temperatures, such as 37°C.
28
What is the role of an inoculating loop?
To transfer bacteria from a culture to a fresh medium.
29
Why is the Petri dish incubated upside down?
To prevent condensation dripping onto the agar surface.
30
What is turbidimetry?
Measuring the cloudiness of a culture to estimate bacterial population size.
31
Describe the Gram staining procedure.
Crystal violet, iodine, alcohol, then safranin.
32
What is the function of crystal violet?
Primary stain that colours cells purple.
33
What is the function of Lugol’s iodine?
Mordant that fixes the stain.
34
What is the function of alcohol?
Decolourises Gram-negative bacteria.
35
What is the function of safranin?
Counterstains Gram-negative bacteria pink.
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