Tumor colonization of Salmonella depends on intracellular accumulation in tumor masses. a) Control Salmonella (Sal) intracellularly invaded MCF7 cells more than the minimally invasive ΔsipB Salmonella (S-Sal). Green fluorescence indicates induction of GFP expression by the PSSEJ promoter, which is activated intracellularly. Scale bar is 100 μm. b) The ΔsipB mutant (S-Sal) intracellularly invaded tumor cells ten-fold less than control Salmonella in monolayer (*, P<0.05). c) The sipB knockout reduced the amount of intracellular Salmonella (green) in devices at 30 and 48 h after administration. Scale bar is 100 μm. d, e) Compared to control Sal, S-Sal (d) accumulated in tumor cells in devices 2.5 fold less (*, P<0.05, n = 6) and (e) had a four-fold slower rate of fluorescence increase (*, P<0.05). f) The sipB knockout also reduced the total density of colonized Salmonella (red) in devices at 30 and 48 h after administration. Scale bar is 100 μm. g, h) Compared to control (Sal), S-Sal (G) colonized tumors 2.5 fold less (*, P<0.05) and (h) grew in tumors four-fold slower (*, P<0.05).