Co-colonization of ApcMinΔ716/+ mice or AOM-treated wild-type mice with pathogenic strains of both pks+ E. coli and ETBF significantly increased colon tumor multiplicity relative to co-colonization with a single pathogenic strain of either E. coli or ETBF and the nonpathogenic strain of the other. The gene discussed is ARAF; the disease is infectious otitis media.