Pathogenic bacteria can modulate and interfere with human cell cycle progression. Here we study the human pathogen Neisseria gonorrhoeae and its ability to influence and affect the cell cycle in two human target cell lines. We found that bacteria adhere equally well to cells synchronized into the different cell cycle phases of G1, S, and G2, but were unable to adhere to cells in M phase or G0 phase. In addition, using Western blot and/or flow cytometry analysis we demonstrate that bacterial infection for 24 h results in decreased levels of the cell cycle regulatory proteins cyclin B1, cyclin D1, and cyclin E. Further studies in N. gonorrhoeae-infected epithelial cells involving analysis of DNA content, bromodeoxyuridine (BrdU) incorporation, quantification of late mitotic cells and analysis of nuclear phenotype provide compelling evidence that a 24 h gonococcal infection arrests the cells in early G1 phase of the cell cycle. In summary, we present data showing that MS11 P⁺ strain of N. gonorrhoeae can down-regulate cyclins, important modulators of the cell cycle, and result in a G1 arrest.--Jones, A., Jonsson, A-B., Aro, H. Neisseria gonorrhoeae infection causes a G1 arrest in human epithelial cells.