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: Metformin prevents experimental gentamicin - induced nephropathy by a mitochondria - dependent pathway .
### Output Text:
<span class="chemical">Metformin</span> prevents experimental <span class="chemical">gentamicin</span> - induced nephropathy by a mitochondria - dependent pathway . 