• Cancer Control and Population Health Sciences Program, Duke Cancer Institute and Department of Population Health Sciences, Duke University, Durham, NC 27705, USA.
• Epidemiology Program, University of Hawai'i Cancer Center, Honolulu, HI 96813, USA.
• Department of Medicine and Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
• Department of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA 02215, USA.
• Division of Epidemiology, Vanderbilt University Medical Center, Nashville, TN 37203, USA.
• Behavioral and Epidemiology Research Group, American Cancer Society, Atlanta, GA 30303, USA.
• Department of Epidemiology, Johns Hopkins School of Public Health, and Department of Oncology, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA.
• Department of Population Health, New York University Grossman School of Medicine, New York, NY 10016, USA.
• Occupational and Environmental Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Rockville, MD 20850, USA.
• Centre for Public Health Research, Massey University, Wellington 6140, New Zealand.

Previously, we found that risk of colorectal cancer (CRC) is increased in individuals with serum antibody response to both Helicobacter pylori (HP) Vacuolating Cytotoxin (VacA) toxin or Streptococcus gallolyticus (SGG) pilus protein Gallo2178. In the present analysis, we tested the hypothesis that combined seropositivity to both antigens is a better indicator of CRC risk than seropositivity to single antigens. We used multiplex serologic assays to analyze pre-diagnostic serum for antibody responses from 4063 incident CRC cases and 4063 matched controls from 10 US cohorts. To examine whether combined SGG Gallo2178 and HP VacA sero-status was associated with CRC risk, we used conditional logistic regression models to estimate odds ratios (ORs) and 95% confidence intervals (CIs). Compared to dual sero-negative individuals, there was no increased risk for individuals sero-positive to SGG Gallo2178 only (OR: 0.93; 95% CI: 0.66-1.31) or to HP VacA only (OR: 1.08; 95% CI: 0.98-1.19). However, dual sero-positive individuals had a >50% increased odds of developing CRC (OR: 1.54; 95% CI: 1.16-2.04), suggesting an interaction between antibody responses to these two pathogens and CRC risk (p_interaction = 0.06). In conclusion, this study suggests that dual sero-positivity to HP VacA and SGG Gallo2178 is an indicator of increased risk of CRC.