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: Continuous infusion tobramycin combined with carbenicillin for infections in cancer patients .
### Output Text:
Continuous infusion <span class="chemical">tobramycin</span> combined with <span class="chemical">carbenicillin</span> for infections in cancer patients . 