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: Six patients in the conventional group had a positive culture , compared with none in the extended - interval group ( P = 0 . 002 ) .
### Output Text:
Six patients in the conventional group had a positive culture , compared with none in the extended - interval group ( P = 0 . 002 ) . 