Antagonistic effects of probiotic Escherichia coli Nissle 1917 on EHEC strains of serotype O104:H4 and O157:H7.

ABSTRACT

The largest EHEC outbreak up to now in Germany occurred in 2011. It was caused by the non-O157H7 Shiga-toxinogenic enterohemorrhagic E. coli strain O104H4. This strain encodes in addition to the Shiga toxin 2 (Stx2), responsible for the hemolytic uremic syndrome (HUS), several adhesins such as aggregative adherence fimbriae. Currently, there is no effective prophylaxis and treatment available for EHEC infections in humans. Especially antibiotics are not indicated for treatment as they may induce Stx production, thus worsening the symptoms. Alternative therapeutics are therefore desperately needed. We tested the probiotic Escherichia coli strain Nissle 1917 (EcN) for antagonistic effects on two O104H4 EHEC isolates from the 2011 outbreak and on the classical O157H7 EHEC strain EDL933. These tests included effects on adherence, growth, and Stx production in monoculture and co-culture together with EcN. The inoculum of each co-culture contained EcN and the respective EHEC strain either at a ratio of 11 or 101 (EcNEHEC). Adhesion of EHEC strains to Caco-2 cells and to the mucin-producing LS-174T cells was reduced significantly in co-culture with EcN at the 11 ratio and very dramatically at the 101 ratio. This inhibitory effect of EcN on EHEC adherence was most likely not due to occupation of adhesion sites on the epithelial cells, because in monocultures EcN adhered with much lower bacterial numbers than the EHEC strains. Both EHEC strains of serotype O104H4 showed reduced growth in the presence of EcN (101 ratio). EHEC strain EDL933 grew in co-culture with EcN only during the first 2h of incubation. Thereafter, EHEC counts declined. At 24h, the numbers of viable EDL933 was at or slightly below the numbers at the time of inoculation. The amount of Stx2 after 24h co-incubation with EcN (EcNEHEC ratio 101) was for all 3 EHEC strains tested significantly reduced in comparison to EHEC monocultures. Obviously, EcN shows very efficient antagonistic activity on the EHEC strains of serotype O104H4 and O157H7 tested here regarding adherence to human gut epithelial cells, bacterial growth, and Stx2 production in vitro.