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