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: These studies suggest that an antimetabolite - induced leukopenia predisposes to severe Pseudomonas sepsis and that such monkeys may serve as a biological model for study of comparative efficacy of antimicrobial agents .
### Output Text:
These studies suggest that an antimetabolite - induced leukopenia predisposes to severe Pseudomonas sepsis and that such monkeys may serve as a biological model for study of comparative efficacy of antimicrobial agents . 