“…Of course, some marketed pharmaceutical drugs that are transported into cells are, in fact, naturally fluorescent, including molecules such as anthracyclines [ 56 , 57 , 58 ], mepacrine (atebrin, quinacrine) [ 59 ], obatoclax [ 60 , 61 ], tetracycline derivatives [ 57 , 62 ] and topotecan [ 63 ], The same is true of certain vitamins such as riboflavin [ 64 , 65 ] (that necessarily have transporters, as cells cannot synthesise them), as well as certain bioactive natural products (e.g., [ 66 , 67 , 68 ]). As per this Special Issue, and in an era of increasing antimicrobial resistance [ 69 , 70 , 71 , 72 ], this is very much the case for novel antimicrobials, which classically come from natural products (e.g., [ 73 , 74 , 75 , 76 , 77 , 78 ]). If so, they might then serve as surrogate transporter substrates for them.…”