General Guidelines for Handling DNA
Working with DNA: Good microbiological practice
Growth of E. coli strains
The E. coli growth curve
Storage of E. coli strains
Generating liquid cultures from bacterial stocks
The figure, Essential steps for storage and handling of E. coli shows the sequence of steps necessary to go from a stored stock of bacteria to a liquid culture for plasmid isolation. Bacterial stocks should always be streaked onto selective plates prior to use, to check that they give rise to healthy colonies carrying the appropriate antibiotic resistance. Stocks can potentially contain mutants arising from the cultures used to prepare them, or can deteriorate during storage.
Inoculate liquid cultures from a healthy, well-isolated colony, picked from a freshly streaked selective plate. This will ensure that cells growing in the culture are all descended from a single founder cell, and have the same genetic makeup.
Tip: Culture volumes >10 ml should not be inoculated directly from a plate, but diluted 1/500 to 1/1000 from a pre-culture of 2–5 ml.