acid
Explanation
The Joke
Two scientists are excited about a discovery: they have found species of "acid-loving bacteria." One exclaims "This is incredible!" and the other eagerly says "Let me see!" The final panel zooms in to show the bacterium itself, which is experiencing a psychedelic trip, saying "I can smell coloooooors like they're made of muuuuuuusic."
The joke is a pun on the word "acid." In biology, "acid-loving" (acidophilic) bacteria are real organisms that thrive in highly acidic environments, such as hot springs or stomach acid. But "acid" is also slang for LSD (lysergic acid diethylamide), the famous psychedelic drug. The comic switches from the scientific meaning to the slang meaning, depicting the bacterium not as an organism that thrives in low-pH environments, but as one that is tripping on psychedelic drugs and experiencing synesthesia -- the blending of senses where one can "smell colors" or "hear" music in visual form.
The Humor
The comedy relies on a clean bait-and-switch between two meanings of "acid." The scientific framing of the first two panels sets up the expectation that this is about microbiology, making the sudden shift to a stoned bacterium experiencing a psychedelic trip unexpected and silly. The bacterium's drawn-out, blissed-out speech ("coloooooors," "muuuuuuusic") perfectly captures the stereotypical way people describe hallucinogenic experiences, which is inherently funny when attributed to a single-celled organism. The visual of a bacterium having a spiritual experience adds an extra layer of absurdity.
References
Acidophilic organisms are indeed a real and well-studied category in microbiology. Species like Acidithiobacillus and Helicobacter pylori thrive in extremely acidic conditions. Synesthesia -- the cross-wiring of senses that allows someone to "smell colors" or "taste sounds" -- is a real neurological phenomenon that is also commonly reported during LSD experiences.