Why Bacteria, But Not Humans, Can Live on Caffeine

When scientists discovered a new bacterium that can live on caffeine last month, it sparked many tongue-in-cheek headlines and ledes. "Bacteria survives on caffeine (like me)." "If you can’t live without a cup of coffee…" "Think you live on caffeine?" But after everyone had a good laugh about the extent to which modern existence is dependent on coffee and run through a few specifics about the bacterium, there was still one question left unanswered. When you get down to it, why can’t humans live on caffeine?