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: Treatment was empirically initiated with vancomycin , levofloxacin , and piperacillin / tazobactam .
### Output Text:
Treatment was empirically initiated with <span class="chemical">vancomycin</span> , <span class="chemical">levofloxacin</span> , and <span class="chemical">piperacillin</span> / <span class="chemical">tazobactam</span> . 