Exercise 3

How does varying the mutation rate affect the proportion of cells capable of resisting phage infection?

Answer – the simulator begins with one cell, with each of the three genes in the off state. This phasotype will have none of the capsular modifications responsible for phage resistance. More of the possible phasotypes (when considering these 3 genes) lead to a resistant phenotype than a sensitive phenotype. A higher mutation rate would speed up phase variable switching, leading to more phage resistant cells than a slower mutation rate.  

How does varying the mutation rate affect the genetic diversity of the final population?

Answer - increasing the mutation rate increases diversification of the population. This is beneficial in response to rapidly changing environments, but there are other factors involve as discussed in the subsequent questions.


What could an advantage of phase varying these genes, given that phage infection is undesirable?

Answer – The bacterial capsule is often highly immunogenic, and a target for cells of the immune system. While phage infection is undesirable, the modifications conferring phage sensitivity may also target the cells to the immune system. Creating a diverse population with many cells resistant to phage, but also resistant to immune attack makes phase varying these genes advantageous.

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