determination of microbial numbers:-"COLONY HYBRIDIZATION"

A method has been developed whereby a very large numbers of colonies of E.coli carry different hybrid plasmids can be rapidly screened to determine which hybrid hybrid plasmids contain a specified DNA sequence or genes.The colonies to be screened are formed on nitrocellulose filters and after a reference set of this colonies has been prepared by replica plating are lysed and their DNA is denatured and fixer to the filter in situ.The resulting DNA prints of the colonies are then hybridized to  radioactive RNA that defines the sequence or gene of assayed by autoradiograph.Colonies whose DNA prints exhibit hybridization can then picked from the reference plate.