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: Gentamicin sulfate and tobramycin sulfate continue to demonstrate ototoxicity and nephrotoxicity in both animal and clinical studies .
### Output Text:
<span class="chemical">Gentamicin sulfate</span> and <span class="chemical">tobramycin sulfate</span> continue to demonstrate ototoxicity and nephrotoxicity in both animal and clinical studies . 