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: We conclude that aminoglycoside - associated ototoxicity was less severe and more often reversible with netilmicin than with tobramycin .
### Output Text:
We conclude that <span class="chemical">aminoglycoside</span> - associated ototoxicity was less severe and more often reversible with <span class="chemical">netilmicin</span> than with <span class="chemical">tobramycin</span> . 