Location: Outside U.S.
Date: Summer 2014
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.
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.
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Update: November 2011