TABLE 2.

Comparison of antitoxic activity among several strains of bifidobacteria with natural resistance to SM sulfate^a

Treatment	STEC viable counts/g of feces 18 h after infection (log10, mean ± SD)	No. of deaths/total no. of mice (survival time [days, mean ± SD])	Concn in cecal contents ofb:
			Stx1	Stx2
None (untreated control)	9.2 ± 0.2	8/10 (8.0 ± 1.2)	0.7 ± 0.5	200.0 ± 162.1
B. breve strain Yakult	9.0 ± 0.3	0/10c	0.1 ± 0.1c	1.9 ± 1.2d
B. pseudocatenulatum DSM 20439	9.1 ± 0.3	0/10c	0.1 ± 0.1c	1.4 ± 1.4d
B. bifidum ATCC 15696	9.1 ± 0.2	8/10 (8.6 ± 1.4)	0.6 ± 0.6	141.3 ± 99.7
B. catenulatum ATCC 27539T	9.0 ± 0.3	7/10 (8.6 ± 1.4)	0.8 ± 0.3	155.7 ± 128.6

^aSM sulfate at a concentration of 5 mg/ml in drinking water was given to mice from day −6 to day 16. Bifidobacterial strains (1 × 10⁸ to 4 × 10⁸ CFU/mouse/day) at an inoculum size of 0.1 ml/mouse were administered to separate groups of mice (10 mice/group) once a day from day −5 to day −3. Population levels of bifidobacteria at the time of STEC infection (log₁₀; mean ± standard deviation) are as follows: B. breve strain Yakult, 9.7 ± 0.3; B. pseudocatenulatum DSM 20439, 9.7 ± 0.1; B. bifidum ATCC 15696, 9.6 ± 0.2; B. catenulatum ATCC 27539^T, 9.8 ± 0.3. Mice were orally infected with STEC at a dose of 8.1 × 10³ CFU on day 6 after starting SM treatment. MMC (0.25 mg/kg of body weight) was administered a total of three times, once each at 18, 21, and 24 h postinfection. Mice were observed for survival for 16 days after STEC infection. Mice were sacrificed 6 h after the last MMC shot, and Stx concentrations in the cecal contents were determined by the RPLA test as described in the text.

^bResults are expressed as mean Stx concentrations (micrograms per gram of cecal contents) ± standard deviations for the results from 6 mice.

^cSignificant difference between the Bifidobacterium-treated mice and the untreated control mice (P < 0.05).

^dSignificant difference between the Bifidobacterium-treated mice and the untreated control mice (P < 0.01).