Plating and transferring cosmid and plasmid libraries

There are two commonly used protocols for the screening of recombinant bacteria with hybridization probes. One method involves the spreading of bacteria on the surface of agar using a sterile spreader. A nitrocellulose membrane filter is then placed on top of the colonies and most of each colony is transferred to the filter. This method works well when relatively small numbers of positive colonies are being selected (up to several thousand). A second method, described in this unit, employs a matrix of some type (here nitrocellulose filters are used) upon which bacteria can be plated and grown when the filter is placed on top of a nutrient agar surface. Once the plated bacteria have grown into visible colonies, the filters can be used for replica plating and in situ hybridization analysis.