Microbial techniques and culturingEdexcel A-Level Biology B: Mind map

Exam questions on Microbial techniques and culturing

1. A technician transfers a sample of a harmless strain of Escherichia coli from a nutrient broth culture to a nutrient agar plate using an inoculating loop, then incubates the plate at 25 °C.
   The technician worked close to a lit Bunsen burner and lifted the lid of the Petri dish only slightly. Explain why these precautions reduce contamination.2 marks
2. A microbiologist wants to isolate a salt-tolerant bacterium from a seawater sample that contains many species of bacteria.
   Describe how streak plating could be used to obtain isolated colonies of the bacterium from a mixed culture.2 marks
3. A broth culture of bacteria was serially diluted by transferring 1 cm³ into 9 cm³ of sterile diluent five times, giving a dilution of 10⁻⁵ in the final tube. A 0.1 cm³ sample of this final dilution was spread on nutrient agar and incubated, and 46 colonies were counted.
   Calculate the number of viable bacteria per cm³ in the original culture. Give your answer in standard form.3 marks