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: Levofloxacin , a fluoroquinolone , is one of the most commonly prescribed antibiotics in clinical practice and is effective against Gram - positive , Gram - negative , and atypical bacteria .
### Output Text:
<span class="chemical">Levofloxacin</span> , a <span class="chemical">fluoroquinolone</span> , is one of the most commonly prescribed antibiotics in clinical practice and is effective against Gram - positive , Gram - negative , and atypical bacteria . 