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: To overcome the adverse effects of neutropenia , tobramycin was given by continuous infusion and combined with intermittent carbenicillin .
### Output Text:
To overcome the adverse effects of neutropenia , <span class="chemical">tobramycin</span> was given by continuous infusion and combined with intermittent <span class="chemical">carbenicillin</span> . 