Intestinal dysbiosis has been reported in patients with colorectal cancer, and there is a high prevalence of Escherichia coli belonging to B2 phylogroup and producing a genotoxin, termed colibactin. Macrophages are one of the predominant tumor-infiltrating immune cells supporting key processes in tumor progression by producing protumoral factors such as cyclooxygenase-2 (COX-2). Here, we investigated whether B2 E. coli colonizing colon tumors could influence protumoral activities of macrophages. In contrast to commensal or nonpathogenic E. coli strains that were efficiently and rapidly degraded by macrophages at 24 h after infection, colon cancer-associated E. coli were able to resist killing by human THP-1 macrophages, to replicate intracellularly, and to persist inside host cells until at least 72 h after infection. Significant increases in COX-2 expression were observed in macrophages infected with colon cancer E. coli compared with macrophages infected with commensal and nonpathogenic E. coli strains or uninfected cells at 72 h after infection. Induction of COX-2 expression required live bacteria and was not due to colibactin production, as similar COX-2 levels were observed in macrophages infected with the wild-type colon cancer-associated E. coli 11G5 strain or a clbQ mutant unable to produce colibactin. Treatment of macrophages with ofloxacin, an antibiotic with intracellular tropism, efficiently decreased the number of intracellular bacteria and suppressed bacteria-induced COX-2 expression. This study provides new insights into the understanding of how tumor- infiltrating bacteria could influence cancer progression through their interaction with immune cells. Manipulation of microbes associated with tumors could have a deep influence on the secretion of protumoral molecules by infiltrating macrophages.