L. johnsoniiL531 pretreatment decreased the abundance of total bacteria in the ileal mucosa. Mucosal tissues (midjejunum, distal ileum, caecum, and colon) and systemic organs (liver, spleen, and MLNs) were collected from the indicated piglets 10 days after S. Infantis challenge. The number of lactobacilli (A–C) in intestinal mucosal tissues and systemic organs after S. Infantis challenge was monitored using culture-based enumeration. Bacterial DNA isolated from 200 mg of intestinal mucosal tissues from piglets of each of the three groups was analyzed by quantitative PCR using universal primers for bacterial 16S rRNA genes (D–F). Data are presented as mean ± SEM. *P < 0.05; **P < 0.01.