Several groups of bacterial protein toxins and effectors act on eukaryotic cells by cytosolic mono-O-glucosylation. One group comprises clostridial glucosylating exotoxins, which are produced by Clostridium difficile, C. sordellii, C. novyi and C. perfringens. The large protein toxins (200-300 kDa) modify Rho- and Ras-subfamily proteins at Thr35/37 by O-glucosylation or GlcNAcylation thereby inhibiting the signaling functions of the eukaryotic GTPases. Rho proteins are also the targets of PaTox, an exotoxin produced by entomopathogenic Photorhabdus asymbiotica. PaTox GlcNAcylates Rho proteins at Tyr32/34. Novel bacterial glucosyltransferase toxins and cytosolic factors essential for up-take of glucosylating toxins will be discussed in detail.