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: Cotrimoxazole , metronidazole , and erythromycin were involved in 15 reported manic episodes .
### Output Text:
<span class="chemical">Cotrimoxazole</span> , <span class="chemical">metronidazole</span> , and <span class="chemical">erythromycin</span> were involved in 15 reported manic episodes . 