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: A total of 177 patients were diagnosed as allergic to beta - lactam antibiotics .
### Output Text:
A total of 177 patients were diagnosed as allergic to <span class="chemical">beta - lactam</span> antibiotics . 