Agrobacteria do not directly harm the best beneficial bacteria for plants but indirectly because they help in increasing plant size through crown galls or knobs on the roots. These knob-like projections caused by agrobacteria hurt the best beneficial bacteria for plants because it becomes difficult to penetrate the best beneficial bacteria cells in soils.
However, when these best beneficial bacteria for plants are killed, harmful microbes take their place and infect the best beneficial bacteria for plants making it difficult for them to survive and thrive well in soil.