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: Five of 8 patients ( 63 % ) improved during fusidic acid treatment : 3 at two weeks and 2 after four weeks .
### Output Text:
Five of 8 patients ( 63 % ) improved during <span class="chemical">fusidic acid</span> treatment : 3 at two weeks and 2 after four weeks . 