Normally, both Trichoderma reesei and E. coli exist in communities of other microbes, which form a complex ecosystem rich with organisms that cooperate and compete. But with just the two of them, the relationship would be simpler. Trichoderma reesei would be what‘s termed a cooperator, producing material that could be used by other organisms by releasing glucose from the cellulose it digests. E. coli, in contrast, would be a cheater, leeching off the fungus without producing anything useful for it. And those properties put the system within the realm of game theory, which typically examines how people interact in situations that could involve cooperation or competition.