@oristarA,
I can't answer questions on microbiology, but "engage" is quite an interesting word.
A fighting ship can "engage" with the enemy, which means it joins battle.
Engage and join in that sense ( and in every other sense I can think of) mean exactly the same.
Except maybe....join usually has the sense of "join with".
....engage can have the sense of "engage against".
(And this thought takes us into the field of matrimony
)