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: Of those evaluated with posttherapy audiograms , three of four netilmicin and piperacillin - treated patients had auditory thresholds return to baseline compared with one of nine tobramycin and piperacillin - treated patients .
### Output Text:
Of those evaluated with posttherapy audiograms , three of four <span class="chemical">netilmicin</span> and <span class="chemical">piperacillin</span> - treated patients had auditory thresholds return to baseline compared with one of nine <span class="chemical">tobramycin</span> and <span class="chemical">piperacillin</span> - treated patients . 