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 antiinfective group of drugs was the more frequently incriminated , amoxicillin - clavulanate accounting for the 12 . 8 % of the whole series .
### Output Text:
The antiinfective group of drugs was the more frequently incriminated , <span class="chemical">amoxicillin - clavulanate</span> accounting for the 12 . 8 % of the whole series . 