how to avoid Non-homologous recombination in gene knock-out

[jizhiyuan]

I'm Knocking out of bacterial genes. The bacterial genome has 7 copies.
I chose a vector with SacB for marker-free knockout. The recombinants are positive for PCR detection. But all of the recombinants are incorrect for southern blotting, the gene still exist in genome. I think non-homologous recombination happened.
Anybody knows what can I do? _at_-_at_