The question I would ask is Why would a healthy young woman be attacked (de novo?) by a not uncommon bug?    Surely the main cause must be a systemic disease (diabetes?), with infection a secondary feature, taking advantage of some weakness in the body's tissues or defences, in which case the source of the PA is irrelevant.

Thank you for your input, Anthony. In the meantime, I managed to find out that the patient had a traumatic operation for cholesteatoma. I presume she contracted Pseudomonas in the hospital at that point. Afterwards, she had several episodes of otorrhea with pruritus and ear pain, for which she received different antibiotics, without an antibiogram. She then had to have a couple more ear surgeries because she developed a brain abscess and mastoid bone necrosis. We're seeing her now after all this, and we'll keep her on 3 weeks of i.v. ceftazidime. She doesn't have diabetes and she's not otherwise immunosuppressed. 

I think the root cause is a bone weakness, allowing CSF to seep into the mastoid, creating a pool of fluid as a culture medium and a back track for infection from the ear to the brain, forming an abscess.   It is very unusual to have a brain abscess from bilateral ear disease -- see my RG question on the topic.  It is also unusual to have bilateral cholesteatoma, so hopefully this case will be written up for publication.

Thank you for the interesting theory, Anthony. The cholesteatoma was unilateral (right ear), and the abscess was on the side of the right ear, the one originally operated on. The left ear was affected only recently, and the extent of damage to this ear is far less serious (no brain abscess, no bone necrosis). The problem is the infection keeps coming back and we don't know if it's because in the past the antibiotic treatments were not given for an adequate amount of time (the patient is very compliant with doctors' recommendations). She received a month of antibiotherapy in our care before her most recent operation (she has surgery on both ears), which was changed by her surgeon and stopped a week after the operation. Otorrhea, otodynia, and pruritus returned and ear cultures became positive for P. aeruginosa. Even 4 days into our i.v. ceftazidime treatment, the cultures were still positive. However, 10 days into the treatment, the cultures are negative. We don't know if we should continue the i.v. treatment for longer than 3 weeks. The bug is resistant to Cipro, so per os options are limited. Our goal is to eliminate the bug from any niche it may be hiding, considering the long (about 7 years) history of repeated infections (granted we don't know if they were all P. aeruginosa).

I still think the problem is not with the PA, nor where it is hiding out, nor the antibiotics, but with the weakened defences in the bone or ear.   Hippocrates would probably argue that once the causal CSF fistula on the right was presumably blocked off at surgery, a centrifugal brain process diverted to the opposite left ear to decompress there and restart the cycle.  If the patient is obese, intracranial hypertension may be present.

A detailed and lengthy discussion of a case with similarities to yours was published by Stankovic et al as Case 2-2013 in NEJM 2013;368:267.   Unfortunately, this was all based on the assumption that the multiple cranial complications were the result of ear disease, whereas the following facts point to a primary cerebral cause:

Thank you, Anthony. Yes, I had found that paper a while back. We'll see how our patient will do after we stop her treatment. I may write the case report, but I'm still gathering up data from her previous hospital discharges. Unfortunately, I don't think I'll have an accurate account of her treatments, signs and symptoms in-between hospitalizations.