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: METHODS : We present a case of presumed amikacin retinal toxicity following treatment with amikacin and vancomycin for alpha - haemolytic streptococcal endophthalmitis .
### Output Text:
METHODS : We present a case of presumed <span class="chemical">amikacin</span> retinal toxicity following treatment with <span class="chemical">amikacin</span> and <span class="chemical">vancomycin</span> for alpha - haemolytic streptococcal endophthalmitis . 