Answer: Strongyloides stercoralis
The blood agar and MacConkey plates clearly show bacterial growth tracks that correlate with the migration of larvae through the agar. As noted by Adam, Florida Fan and Arthur, the small size of the larvae, lack of sheath and short buccal cavity are consistent with the rhabditiform larvae of S. stercoralis. Particularly worrying is the fact that the patient is immunosuppressed and thus is at risk of hyperinfection syndrome. Lee commented that the diagnosis is now known and the patient is receiving treatment, so hopefully he will recover.
Thanks again to Lee for sharing this interesting case with us. It's an important reminder to always be on the look-out for larval tracks on bacterial stool cultures. There are a number of organisms that can cause these tracks including hookworm larvae, free-living nematodes and even arthropods (e.g. mites) and it's therefore important to perform a wet prep from the plate to determine the etiology of the tracks.