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: Microbiologic eradication rates ( presumed plus documented ) of the most common pathogens ranged from 93 % ( M . catarrhalis ) to 100 % ( S . pneumoniae ) at the post - therapy visit .
### Output Text:
Microbiologic eradication rates ( presumed plus documented ) of the most common pathogens ranged from 93 % ( M . catarrhalis ) to 100 % ( S . pneumoniae ) at the post - therapy visit . 