Endogenous H₂O₂ production by intestinal cells and commensal lactic acid bacteria plays an important role in reducing pathogenic infection. Few reports have addressed the influence of catalase-mimetic gold nanoparticles (AuNPs) on intestinal barrier function against invading pathogens. In this study, citrate- and tannic acid-stabilized 5 nm-sized AuNPs were orally administered to C57BL/6 mice daily for 21 day before and during 6 day of Salmonella enterica serovar Typhimurium (S. Typhimurium) exposure. High-throughput sequencing of fecal 16S rRNA revealed that AuNPs did not significantly alter gut microbiota diversity and community before S. Typhimurium challenge. Fecal bacterial enumeration with selective media during the 6-day S. Typhimurium challenge period unveiled that AuNPs markedly induced the increased Salmonella colonization and the decreased Lactobacillus abundance. AuNPs exacerbated S. Typhimurium-induced food intake reduction, body weight loss, cecum/colon histopathological lesions, and mortality. In vitro growth kinetic and virulence assays demonstrated that AuNPs notably counteracted the H₂O₂-induced suppression of S. Typhimurium growth in liquid medium, migration on agar plate, and adhesion and invasion of Caco-2 cell monolayers. In comparison with AuNPs coated by tannic acid, citrate-coated AuNPs exerted significantly greater effects on S. Typhimurium-induced weight loss and mortality in vivo, as well as S. Typhimurium growth, migration, and invasion in the presence of H₂O₂ in vitro, which correlated well with the superior catalase-mimetic activity of citrate-coated AuNPs than tannic acid-coated ones in serum-supplemented cell culture media according to the results of electron spin resonance oximetry. Overall, AuNPs may increase the risk of pathogenic Salmonella infection through catalytic decomposition of endogenous H₂O₂.