Dude, go get an incision and drainage done. If your acne has grown to a size of a 50cent coin and its actively draining pus, antibiotics itself would not work any miracles. Best is to open up the cyst, drain of the pus and have it washed.

Hey there, from your xrays, it does look like there's some erosion on the ischial bones.
If you're fully wheelchair bound and have been coming in and out of pressure ulcers, then you really are MORE LIKELY to get an extension of the infection from the skin/fat/muscle into the bones.
To diagnose osteomyelitis, there really is no need to take a sample of your discharge to make a diagnosis.
Diagnosis is made mostly on xrays.
The only reason why we would take a sample of your discharge is to see what organisms will grow and what antibiotics will respond to it.

If you're having a lot of discharge, and you're already having fever with nausea. you really need to get it treated. Your bones doesnt look badly eroded now. Dont let it get worse. Get it treated asap.
After you're done with the treatment and and good skin coverage over your buttock area, you have to be really strict with your 2 hourly turning.

I would assume you've lost sensation from around your umbilicus all the way down to your toes. Therefore, you wouldn't know when your pain receptors are firing from all the damage caused by constant compression of your skin against your bones when you sit for prolonged hours. Even if there is no pain and you feel awesome and ready to take the world by storm, 2 hourly turning is really really important.
I've a granny who left the ward with grade 1 bed sores and came back 2 weeks later with grade 4 bed sore. that's just two weeks. turns out her daughter didnt follow the 2 hourly turning instruction.

hope this bit will help.

dude, tham~
how many xrays have you got in the past 12 months?