Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Purity Confirmation

 Name: Joyce
Status: student
Grade: other
Location: Outside U.S.
Country: Fiji
Date: Summer 2014

Question: You are given a "pure" culture of E.Coli. What can you do to verify its purity?

Since you can't obviously test every organism in your "E coli" culture, you will have to test a representative sample for purity. Grow a broth media sample culture from your source. Take a sample-a loopful sample is OK and plate out on different types of agar medias (that would be capable of growing different species of bacteria other than E Coli). including EMB agar Then examine colony morphology and perform diagnostic tests (sugar fermentation, etc) on samples of colonies that grow on the different media to see if only consistent with characteristics of E coli). Much more complicated to test if you have only one pure particular strain of E Coli since there can be different strains of E Coli (phage types) which won't be discussed here.

Dr JD Cowsar San Antonio, Texas USA

I would probably spread some of the culture out onto an agar plate, allowing for single E. coli cells to grow up isolated from one another, resulting in little dots of colonies growing on the plate. Any one of these colonies could be considered a "pure" culture in that they all came from a single genetic ancestor. If you're looking to evaluate the genotype of the culture to determine the types and subspecies you have in the culture, you could try one of the various high-throughput sequencing techniques, or deep-seq (high-throughput RNA-sequencing), which, once analyzed, would give you an idea of the different species you have in the culture, and potentially even the percentages each make up the total population of E. coli. If these advanced sequencing techniques were unavailable, simple PCR for known markers on suspected contaminants would at least tell you whether or not the contaminating bacteria were present or not.

Sincerely, Rick Armstrong

With conventional methods, one could reculture it on different media, stain and examine under microscope. DNA could be run on a gel with comparison to pure culture.

With new molecular techniques, genomic identification is rather routine; expensive but easy.

Steve Dunn

Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 223
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: November 2011
Weclome To Newton

Argonne National Laboratory