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: Further research clearly is required to determine the incidence of antimicrobial - induced mania , the relative risk factors of developing an antimicrobial - induced manic episode among various demographic populations , and the incidence of patients who continue to have persistent affective disorders once the initial episode , which occurs while the patient is taking antibiotics , subsides .
### Output Text:
Further research clearly is required to determine the incidence of antimicrobial - induced mania , the relative risk factors of developing an antimicrobial - induced manic episode among various demographic populations , and the incidence of patients who continue to have persistent affective disorders once the initial episode , which occurs while the patient is taking antibiotics , subsides . 