Exercises: Microbial Growth Curves and the Microbiome

Name the four phases of a typical bacterial growth curve in a closed culture and describe in one sentence what happens to the population in each phase.

A bacterium divides by binary fission every 20 minutes. Starting with one cell, calculate the number of cells after 2 hours, assuming unrestricted growth.

Define the term microbiome. Name two parts of the human body that have their own microbiome and state one benefit that gut bacteria provide to the host.

A culture of yeast in a bioreactor at a brewery in Dublin was monitored:

1. a) Identify the lag, exponential, stationary and death phases from the data.

1. b) Calculate the number of generations in the exponential phase between 2 h and 6 h.

1. c) Explain why growth stops at 8 h and why numbers fall after 12 h.

A culture begins with $2\times10^3$ bacteria per mL. The generation time is 30 minutes and there is no lag phase.

1. a) Write an expression for the number of cells after $t$ hours.

1. b) Calculate the number of cells per mL after 5 hours.

1. c) State two reasons why the real culture would not reach this figure.

A person takes a course of broad-spectrum antibiotics for a chest infection and then develops diarrhoea.

1. a) Explain the link between the antibiotics and the gut microbiome.

1. b) Describe how probiotics or a diet rich in fibre might help restore the microbiome.

1. c) Explain why overuse of antibiotics may also encourage antibiotic-resistant bacteria.

A viable count of a bacterial culture gives the following results (log values are base 10).

1. a) Identify the phases shown.

1. b) Calculate the generation time during the exponential phase, given that one doubling raises the log value by 0.30.

1. c) Explain why a log scale is used to plot growth curves.

1. d) Describe two changes in the medium that cause the plateau.

Compare a closed batch culture with a continuous culture (chemostat) used in industrial production of an enzyme.

1. a) Describe how each system is operated.

1. b) Sketch in words the growth curve you would expect for each.

1. c) Evaluate which is more suitable for a product made during the stationary phase and which for a product made in the exponential phase.

Research suggests that the gut microbiome of infants differs depending on whether they are born vaginally or by caesarean section, and whether they are breastfed.

1. a) Suggest how birth method and feeding could influence the infant microbiome.

1. b) Explain the role of the gut microbiome in immune development and in vitamin K production.

1. c) A news article claims that a probiotic yoghurt "cures" allergies. Evaluate the claim and state what evidence would be needed.