Abstract
The inhibitory effect of Bifidobacterium longum and Bifidobacterium infantis isolated from a Lac-B preparation on the growth of Escherichia coli O157: H7 strains, NK2, No.61468, and No.33009, and the production of verotoxins by these strains in vitro was investigated. In the control experiment, all three strains of E. coli O157: H7 grew well in GAM broth under aerobic incubation (35°C, 18 hr), and strains NK2 and No.61468 produced high levels of VT2 (400 ng/ml each) and of VT1 (25 and 3.13 ng/ml, respectively); however, strain No.33009 produced only VT2 (25 ng /ml), but no VT1 at any detectable level. The growth of these strains and their production of verotoxins in vitro were strongly inhibited by their incubation (35°C, 18 hr) in culture broths of B. longum or B. infantis, or in their supernatants obtained by centrifugation. When the strains of bifidobacteria were cultured in GAM broth by the conventional method, the pH of the culture broth shifted from the neutral region to the acidic region (about pH 5.0). The growth and verotoxin production of E. coli O157: H7 NK2 in GAM broth acidified by three types of low-molecular-weight fatty acids and hydrochloric acid were tested. It was found that the growth and verotoxin production of the test strain were inhibited to different degrees by the acids used; they were only slightly inhibited by lactic acid and hydrochloric acid, but strongly inhibited by acetic acid and butyric acid. The in vitro activity of bifidobacteria, the main components in a Lac-B preparation, corresponded to their excellent efficacy in intestinal infection with E. coli O157: H7 previously observed in germ-free mice. These results suggest that the oral administration of bifidobacteria could prevent the development of intestinal infections by enterohemorrhagic E. coli, including E. coli O157: H7.
