Type III secretion system effector proteins have primarily been characterized for their interactions with host cell proteins and their ability to disrupt host signaling pathways. We are testing the hypothesis that some effectors are active within the bacterium, where they modulate bacterial signal transduction and physiology. We previously determined that the Citrobacter rodentium effector NleB possesses an intra-bacterial glycosyltransferase activity that increases glutathione synthetase activity to protect the bacterium from oxidative stress. Here we investigated the potential intra-bacterial activities of NleB orthologs in Salmonella enterica and found that SseK1 and SseK3 mediate resistance to methylglyoxal. SseK1 glycosylates specific arginine residues on four proteins involved in methylglyoxal detoxification, namely GloA (R9), GloB (R190), GloC (R160), and YajL (R149). SseK1-mediated Arg-glycosylation of these four proteins significantly enhances their catalytic activity, thus providing another important example of the intra-bacterial activities of type three secretion system effector proteins. These data are also the first demonstration that a Salmonella T3SS effector is active within the bacterium.