Elevated activities of matrix metalloproteinase-9 (MMP-9) were detected in the conditioned medium of NE6-T and KYSE30 cells (S1D Fig) following challenge with P. gingivalis. Since Escherichia coli is frequently used as a negative control bacterium in studies of colorectal carcinogenesis promoted by certain gut bacteria or microbiota [15], we probed the effects of E. coli DH5α on ESCC cells compared to colon cancer cells. Here, MMP9 is linked to esophageal squamous cell carcinoma.