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: The number of greater than or equal to 15 - dB increases in auditory threshold as a proportion of total greater than or equal to 15 - dB changes ( increases and decreases ) was significantly lower in netilmicin and piperacillin - vs tobramycin and piperacillin - treated patients ( 18 of 78 vs 67 of 115 ) .
### Output Text:
The number of greater than or equal to 15 - dB increases in auditory threshold as a proportion of total greater than or equal to 15 - dB changes ( increases and decreases ) was significantly lower in <span class="chemical">netilmicin</span> and <span class="chemical">piperacillin</span> - vs <span class="chemical">tobramycin</span> and <span class="chemical">piperacillin</span> - treated patients ( 18 of 78 vs 67 of 115 ) . 