Firstly, the fact that it has slough means that it's not likely its a stage II. Stage II's are usually the "pink, partial, painful" type. Stage III's have colors other than pink in them because they extend into the subq.
To answer your initial question, the best way to do a culture that identifies the organisms causing pathogenic changes is to do a tissue culture, which technically involves sharps debridement, and your ability to perform this depends on your licensure/regulations of your facility, etc. Get an order for a tissue culture maybe? Swab cultures or drainage cultures usually give you every color of the rainbow as far as organisms go, so you're smart to know you need a closer look! GL