That will depend on the concentration of DMSO that you will use for the dilution. Normally, DMSO at 2-10 % (v/v) is used to dissolve the plant extracts and should not have any effect on the bacterial growth. However, you can make a negative control where you can only use DMSO (at the desired concentration) without extract to check whether your antibacterial effect is due to the extract or DMSO.