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: A total of 675 patients were included ; 163 of these had therapy with vancomycin , gentamicin , or tobramycin ; were > 18 years ; and treated in the ICU for > 24 hours .
### Output Text:
A total of 675 patients were included ; 163 of these had therapy with <span class="chemical">vancomycin</span> , <span class="chemical">gentamicin</span> , or <span class="chemical">tobramycin</span> ; were > 18 years ; and treated in the ICU for > 24 hours . 