In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Five of 33 ( 15 % ) of the tobramycin - treated patients and 16 of 29 ( 55 . 2 % ) of the gentamicin - treated patients had renal failure .
### Output Text:
Five of 33 ( 15 % ) of the <span class="chemical">tobramycin</span> - treated patients and 16 of 29 ( 55 . 2 % ) of the <span class="chemical">gentamicin</span> - treated patients had renal failure . 