With a little help from my friends

Shortly after the squid hatches, its sac is colonised by a Gram negative rod, Vibrio fischeri, a luminous bacterial symbiont selected by the squid for this purpose. What enables the squid to maintain such an organism without overgrowth or internal damage? Since there are organisms that are pathological to some species and symbionts to others, there may be much to learn from such co-evolved interaction.