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: Drug studies using the indirect antiglobulin test were strongly positive with trimethoprim and trimethoprim - sulfamethoxazole but negative with sulfamethoxazole .
### Output Text:
Drug studies using the indirect antiglobulin test were strongly positive with <span class="chemical">trimethoprim</span> and <span class="chemical">trimethoprim - sulfamethoxazole</span> but negative with <span class="chemical">sulfamethoxazole</span> . 