cell 16785615 line-dependent behavior. 20567609 Intriguingly, the supernatant from L. reuteri, although it caused only a moderate pH reduction, was still able to reduce host cell proliferation. This observation indicates that other factors are The Secretion of Lactic Acid Reduces Cell Cycle MedChemExpress GW 501516 Progression When ME-180 cells were colonized with the different Lactobacillus strains, the cell media underwent a reduction in pH due to the lactic acid production by the bacteria. To investigate whether the pH change alone could affect the cell cycle, we adjusted the DMEM with lactic acid to pH 7 and pH 6 and added this medium to ME-180 cells for 24 hours. Although the same amounts of lactic acid were added in three different experiments, the pH varied during the BrdU incorporation due to the buffering capacity of the cell culture medium. The pH 6 treatments significantly reduced cell proliferation, producing a relative BrdU incorporation of 76% relative to control cells; this pH is achieved through the addition of 2.7 mg/ ml of lactic acid. No reduction in cell proliferation was observed with cell culture medium that was adjusted to pH 7. Next, we measured the pH of the cell supernatants after 24 hours of colonization by lactobacilli. The ME-180 cells alone displayed an average of pH of 8.1, and colonization by L. rhamnosus caused a pH reduction to 6.7 in the cell supernatant, whereas L. reuteri and L. crispatus caused only moderate reductions in the cell culture pH. Thus, only L. rhamnosus was able to reduce the supernatant pH to levels approaching pH values known to decrease cell proliferation rates. Subsequently, the 24-hour supernatants from the colonized cells were sterile-filtered and transferred to a new set of cell culture dishes of ME-180 cells, and the S phase progression of these cells was investigated for 24 hours. Supernatants were diluted 1:2 in fresh medium to avoid possible nutrient reduction. Supernatants from cells colonized with L. rhamnosus reduced the new cells to a BrdU incorporation of 28% relative to control cells. In addition, the L. reuteri supernatant reduced the number of cells progressing through S phase. As expected, compared with control cells, the supernatant Lactobacilli Influence the Human Cell Cycle involved in the induction of the reduced proliferation. It is likely that the heavy colonization of cells by L. reuteri, which contrasts with the moderate colonization of cells by L. crispatus, mimics the cellular confluence signals on the epithelial cell surface, resulting in a slowdown of the proliferation speed. Due to their differences in lactobacillus morphology and aggregation, the coat over and/or around epithelial cells that different lactobacilli can form varies greatly. However, the epithelial monolayer beneath the carpet of lactobacilli appears healthy, and no increased apoptosis of the epithelial cells is observed. During experimental assays, great care was taken to perform all assays at low confluence, ensuring that cell-to-cell contact inhibition was never the cause for cell cycle arrest. In combination, the results of this study demonstrate that the properties of each Lactobacillus strain are unique. Certain Lactobacillus strains have been found to possess anti-proliferative effects on human gastric and colon cancer cells. In this study, we reveal the differences between L. rhamnosus, which possesses anti-proliferative qualities for cervical cells, and L. crispatus, which lacks this quality but has the ability to transie