When Antibiotics are injected, the bacteria that isn't resistant to it is killed off.
The remaining bacteria, (by total luck) have immunity from that antibiotic and since there is no competition in the environment, the population grows exponentially and quickly passing down its trait to its offspring.
This became a cycle that keeps happening every time steno is introduced to a new antibiotics. They develop resistant traits and pass it down with natural selection and that trait grows more common
Various other bacteria
Eventually, steno become totally resistant with various immunity traits which results antibiotics being totally ineffective and practically no way of treating it
All these resistant steno cells were infecting parts of Addie's lungs and so she when through a risky lung transplant surgery and thankfully she survived with no complications!