Describe in detail, using labelled diagrams, how you would purify a contaminated bacterial culture using the streak-plate procedure. (25 marks) Describe the differences between the structure of a Gram-positive and Gram-negative bacterial cell wall. (25 marks)

1. Describe in detail, using labelled diagrams, how you would purify a contaminated bacterial culture using the streak-plate procedure. (25 marks)

2. Describe the differences between the structure of a Gram-positive and Gram-negative bacterial cell wall. (25 marks)

3. Describe how environmental factors can affect bacterial growth. Use examples to illustrate your answer. (25 marks)

4. You are provided with data (Table 1) obtained during a study of the bacterial population of a broth culture.

An aliquot (1.0 ml) of broth culture was dispersed in 1000 ml of sterile distilled water and ten-fold dilutions of the resulting suspension were prepared (i.e. 10-4, 10-5, 10-6, 10-7 dilutions). Samples (100 µl) of each dilution were mixed with molten nutrient agar medium, poured into sterile petri dishes and allowed to set. The plates were incubated at 25oC for one week. After incubation, the number of bacterial colonies developing on the plates were counted. The following results were obtained:

Table 1. Dilution plate count of the bacterial broth culture.

Express the above results in terms of the number of colony-forming-units per ml of broth culture. (25 marks)