Dietary fructo-oligosaccharides and inulin decrease resistance of rats to salmonella: protective role of calcium

Abstract

Background: We have shown recently that rapid fermentable fructo-oligosaccharides (FOS) decreased resistance of rats towards salmonella. It is not known whether inulin (which is fermented more gradually) has similar effects or whether buffering nutrients can counteract the adverse effects of rapid fermentation.

Aims: To compare the effects of dietary inulin and FOS on resistance of rats to Salmonella enterica serovar Enteritidis and to determine whether calcium phosphate counteracts the effects of fermentation.

Methods: Male Wistar rats (n = 8 per group) were fed a human "Western style diet". Diets with 60 g/kg cellulose (control), FOS, or inulin had either a low (30 mmol/kg) or high (100 mmol/kg) calcium concentration. After an adaptation period of two weeks, animals were orally infected with 2 x 10(9) colony forming units of Salmonella enterica serovar Enteritidis. Colonisation of salmonella was determined by quantification of salmonella in caecal contents. Translocation of salmonella was quantified by analysis of urinary nitric oxide metabolites in time.

Results: Inulin and FOS decreased intestinal pH and increased faecal lactobacilli and enterobacteria. Moreover, both prebiotics increased the cytotoxicity of faecal water and faecal mucin excretion. Both prebiotics increased colonisation of salmonella in caecal contents and enhanced translocation of salmonella. Dietary calcium phosphate counteracted most of the adverse effects of inulin and FOS.

Conclusions: Both inulin and FOS impair resistance to intestinal infections in rats. This impairment is partially prevented by dietary calcium phosphate. The results of the present study await verification in other controlled animal and human studies.

Figure 1

Effects of dietary inulin, fructo-oligosaccharides (FOS), and calcium on the cytotoxicity of faecal water of rats before infection. Cytotoxicity was determined using a haemolysis assay. Results are expressed as mean (SEM) (n = 7 in the high calcium FOS group and n = 8 in the other diet groups). *Significantly different from the low calcium/control group (p<0.05); †significantly different from its low calcium counterpart (p<0.05).

Figure 2

Effects of dietary inulin, fructo-oligosaccharides (FOS), and calcium on daily faecal mucin excretion of rats before infection. Mucins were measured fluorimetrically and expressed as μmol oligosaccharide equivalents. Results are expressed as mean (SEM) (n = 7 in the high calcium FOS group and n = 8 in the other diet groups). *Significantly different from the low calcium/control group (p<0.05); †significantly different from its low calcium counterpart (p<0.05).

Figure 3

Effects of dietary inulin, fructo-oligosaccharides (FOS), and calcium on salmonella numbers in caecal contents of rats six days after oral administration of 2×10⁹ colony forming units (CFU) of Salmonella enterica serovar Enteritidis. Salmonella was cultured aerobically on modified brilliant green agar. Results are expressed as mean (SEM) (n = 7 in the high calcium FOS group and n = 8 in the other diet groups). *Significantly different from the low calcium/control group (p<0.05).

Figure 4

Urinary sum of nitrate and nitrite (NO_x) excretion in the control, inulin, and fructo-oligosaccharide (FOS) groups after an oral challenge with 2×10⁹ colony forming units of Salmonella enterica serovar Enteritidis on day 0 in the (A) low calcium groups and (B) high calcium groups. Results are expressed as mean (SEM) (n = 7 in the high calcium FOS group and n = 8 in the other diet groups). *Significantly different from the low calcium/control group at that time point (p<0.05); †significantly different from its low calcium counterpart at that time point (p<0.05).