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: Ototoxicity occurred in four ( 9 . 5 % ) of 42 netilmicin and piperacillin and in 12 ( 22 % ) of 54 tobramycin and piperacillin - treated patients .
### Output Text:
Ototoxicity occurred in four ( 9 . 5 % ) of 42 <span class="chemical">netilmicin</span> and <span class="chemical">piperacillin</span> and in 12 ( 22 % ) of 54 <span class="chemical">tobramycin</span> and <span class="chemical">piperacillin</span> - treated patients . 