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 67 - year - old man who was treated with oxacillin for one week because of Staphylococcus aureus bacteremia , developed renal failure and diffuse , symmetric , palpable purpuric lesions on his feet .
### Output Text:
A 67 - year - old man who was treated with <span class="chemical">oxacillin</span> for one week because of Staphylococcus aureus bacteremia , developed renal failure and diffuse , symmetric , palpable purpuric lesions on his feet . 