File:800px-Escherichia coli.jpg

Escherichia coli: Scanning electron micrograph of Escherichia coli, grown in culture and adhered to a cover slip. Credit: Rocky Mountain Laboratories, NIAID, NIH

Source: NIAID: These high-resolution (300 dpi) images may be downloaded directly from this site. All the images, except specified ones from the World Health Organization (WHO), are in the public domain. For the public domain images, there is no copyright, no permission required, and no charge for their use.

http://www3.niaid.nih.gov/biodefense/public/images.htm

This image is a work of the National Institutes of Health, part of the United States Department of Health and Human Services. As a work of the U.S. federal government, the image is in the public domain.