However, the Stenotrophomonas began to develop resistance to the antibiotics due to natural selection. In bacterial populations, some bacteria are more resistant to others due to variations of traits. In Addie's body, some Stenotrophomonas have more pores in their cell membrane than others.
Bacteria with fewer pores are more resistant than bacteria with more pores. This is because antibiotics have to find an opening in the cell membrane to kill bacteria, so the more pores, the higher the chance of the bacteria getting through the cell membrane. This results in the bacteria with more pores being killed off, leaving the bacteria with less pores.
As the antibiotics kill off more and more of the less resistant bacteria, the more resistant bacteria reproduce, passing their resistant traits on to offspring. The percentage of resistant bacteria continues increasing, while the percentage of less resistant bacteria continues decreasing.
= Bacteria is pan-resistant
= Less-resistant Stenotrophomonas
= More-resistant Stenotrophomonas
= Zoom in
= ECMO machine
= Addie's lungs
This gives more room for the resistant bacteria to grow, and eventually, the entire bacterial population becomes antibiotic resistant. Addie is continuously given antiobiotics, to the point where all the bacteria became pan-resistant (resistant to all types of antibiotics,) and the ZOI's created by the antibiotics were ineffective.
In the end, Addie's condition became so severe that she was forced to get a lung transplant.