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: The most common pathogen was Klebsiella pneumoniae and this , together with Escherichia coli and Pseudomonas aeruginosa , accounted for 74 % of all gram - negative bacillary infections .
### Output Text:
The most common pathogen was Klebsiella pneumoniae and this , together with Escherichia coli and Pseudomonas aeruginosa , accounted for 74 % of all gram - negative bacillary infections . 