Complementation strain - (Jun/04/2009 )

When I come across some papers about deletion of a gene in bacterium, they usually make a complementation strain, why such strain is needed? Moreover, there are two types of complementation (chromsomal and episomal), are there any differences between them?

If you see an effect (different phenotype) of your gene deletion you can't be absolutely sure that it is due to the deletion of your gene or due to other effects. Maybe there was a readthrough of your gene, or some element was placed on the complementary DNA strand or something went wrong with your homologous recombination. Whatever.
If you make a complementation, it should restore your WT phenotype. If it's chromosomal it should look identical to your WT, if it's episomal you would probably see that it is even better than just restored, because of the higher copy number and/or better promotor.