Vin­cent, Elio, and Michele re­view how hor­i­zon­tal gene trans­fer from bac­te­ria to an in­sect genome en­ables a tri­par­tite nested mealy­bug sym­bio­sis, and how pro­bi­otic bac­te­ria work by com­pet­ing for iron in the in­tes­tine.