I'm trying to modify a bacterial artificial chromosome and to do so, need to prepare electrocompetent cells, mix with a PCR product (designed to undergo homologous recombination with the BAC) and electroporate. Immediately upon addition of the PCR product, the bacterial suspension gets expelled up the walls of the eppendorf tube. Subsequent electroporation, recovery and plating of such samples never produces any colonies. This expulsion phenotype seems to be independent of the bacterial preparation: when several electrocompetent samples are prepared simultaneously, addition of the exact same PCR product causes expulsion in some eppendorf tubes but not others.
The electrocompetent bacteria are suspended in water and the PCR products are also suspended in water. Any ideas what might be causing this and how to fix it?