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Probiotic could be a promising alternative to antibiotics for the prevention of enteric infections; however, further information on the dose effects is required. Therefore, in this study, a low- and high-dose pre-treatment of Lactobacillus rhamnosus ATCC 7469 (1010 and 1012 CFU/d, respectively) was used to elucidate dose effects of L. rhamnosus on the gut microbiota and mucosal immune responses in F4+ enterotoxigenic E. coli (F4+ETEC) challenged pigs. Twenty crossbred (Landrace x Large White) piglets weaned at 21 days of age and weighting 5.2 ± 0.2 kg were orally administered low- or high-dose L. rhamnosus ACTT 7469 (1010 CFU/d or 1012 CFU/d) for 1 week before F4 (K88)-positive Escherichia coli challenge. The compositions of faecal and gastrointestinal microbiota were recorded; gene expression in the intestines was assessed by real-time PCR; serum tumour necrosis factor-α (TNF-α) concentrations and intestinal Toll-like receptor 4 (TLR4) were detected by ELISA and immunohistochemistry, respectively.
Unexpectedly, high-dose administration increased the incidence of diarrhoea before F4+ETEC challenge, despite the fact that both doses ameliorated F4+ETEC-induced diarrhea with increased Lactobacillus and Bifidobacterium counts accompanied by reduced coliform shedding in faeces. Interestingly, L. rhamnosus administration reduced Lactobacillus and Bifidobacterium counts in the colonic contents, and the high-dose piglets also had lower Lactobacillus and Bacteroides counts in the ileal contents. An increase in the concentration of serum TNF-α induced by F4+ETEC was observed, but the increase was delayed by L. rhamnosus. In piglets exposed to F4+ETEC, jejunal TLR4 expression increased at the mRNA and protein levels, while jejunal interleukin (IL)-8 and ileal porcine β-defensins 2 (pBD2) mRNA expression increased; however, these increases were attenuated by administration of L. rhamnosus. Notably, expression of jejunal TLR2, ileal TLR9, Nod-like receptor NOD1 and TNF-α mRNA was upregulated in the low-dose piglets after F4+ETEC challenge, but not in the high-dose piglets.
These findings indicate that pre-treatment with a low dose of L. rhamnosus might be more effective than a high dose at ameliorating diarrhoea. There is a risk that high-dose L. rhamnosus pre-treatment may negate the preventative effects, thus decreasing the prophylactic benefits against potential enteric pathogens.
XQ Li, YH Zhu, HF Zhang, Y Yue, ZX Cai, QP Lu, L Zhang, XG Weng, FJ Zhang, D Zhou, JC Yang, JF Wang. 2012. Risks associated with high-dose Lactobacillus rhamnosus in an Escherichia coli model of piglet diarrhoea:intestinal microbiota and immune imbalances. PLoS ONE, 7(7): e40666. doi:10.1371/journal.pone.0040666.
