If you are trained in microbiology you can tell by eye. Plate out the bacteria (for single colonies) onto a universal media that is suitable for your target bacteria, ie CLED, Maconkeys, Blood, Chocolate. Incubate until there is a growth (usually 24hrs at 37 degreees celcsius is aerobic). Look at the plate, are the colonies uniform in appearence.
If you are using liquid culture youu could use microscopy before plating out. Either a gram stain or a direct (wet prep). This would let you know if you had all gram positives or all gram negatives or all rods or all cocci or a mixture. You would still have to plate out to make sure only one colony type grows.
How would you determine that the bacteria cultures were pure?
You would compare randomly selected, uncomtaminated isolates of a qualitatively pure appearing culture with your standard of purity by genomic analysis in whatever detail satisfies your criteria.
Those that pass the test are the ones you deem pure.
If you have no standard, the ones that match each other are pure unto themselves.
A five year microbiology student will have a better answer, but will he be on Yahoo?
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