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: RESULTS : High frequency hippocampal DBS suppressed the acute penicillin - induced cortical epileptic activity independent from stimulus intensity .
### Output Text:
RESULTS : High frequency hippocampal DBS suppressed the acute <span class="chemical">penicillin</span> - induced cortical epileptic activity independent from stimulus intensity . 