If the patient is responding to the antibiotics you need not change them .Merpenam and linozolid are higher antibiotics .

The treatment going on now is fine ...but sputum culture  based antibiotucs will be verified once treating doc gets reports...

Antibiotics use should very much judicial in immunocompromised patients. Meropenam will cover gram negative bacteria and linezolid will cover gram positive unfection particularly staphyloccus bacteria. Colistin and fosfomycin, polymyxin b are other higher antibiotics against gram negative but these drugs carefully used unless it is much necessary. Otherwise antibiotic resistance will increase. Teiciplanin and daptomycin are other higher antibiotic for gram positive infection. Fungal infection also common in immunocompromised patients. Depending upon clinical condition antibiotics should be decided