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: Physicians should recognise the possibility of fatal bacterial infections related to bortezomib plus high - dose dexamethasone in elderly patients , and we believe this case warrants further investigation .
### Output Text:
Physicians should recognise the possibility of fatal bacterial infections related to <span class="chemical">bortezomib</span> plus high - dose <span class="chemical">dexamethasone</span> in elderly patients , and we believe this case warrants further investigation . 