The largest EHEC outbreak up to now in Germany occurred in 2011. It was caused by the non-O157:H7 Shiga-toxinogenic enterohemorrhagic E. coli strain O104:H4. 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 O104:H4 EHEC isolates from the 2011 outbreak and on the classical O157:H7 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 1:1 or 10:1 (
EcN:EHEC). 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 1:1 ratio and very dramatically at the 10:1 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 O104:H4 showed reduced growth in the presence of
EcN (10:1 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 (
EcN:EHEC ratio 10:1) 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 O104:H4 and O157:H7 tested here regarding adherence to human gut epithelial cells, bacterial growth, and Stx2 production in vitro.