The remarkable flexibility of the T6SS is demonstrated by its ability to target both eukaryotic and prokaryotic cells. T6SSmediated virulence of V52 toward eukaryotic cells is known to depend on the ACD (16). Although it is plausible that the ACD could use virulence toward E. coli by targeting MreB, the ACD was not found to be essential for V52 T6SS-mediated bacterial virulence (Fig. 5). This result suggests that MreB is not a target of the T6SS and that the T6SS likely accommodates separate virulence factors depending on the target cell.