Question

Describe two ways beneficial bacteria help defend against pathogenic bacteria

Answer 1

They fight against the pathogenic ones.

Answer 2

it is a fact that in microbial ecology, populations of different kind of bacteria compete over resources such as a place to colonize or for the source of nutrients. one example of this could be the intestinal bacteria populations that naturally abide there and that because these are adapted to the conditions of the intestines, expel any other microorganism that would try to take its place.
another way could be through the release of different chemical compounds that inhibit growth of other kind of microorganisms. for example the streptomycetes release diverse kinds of antibiotics for this purpose.