F41 antigen as a virulence factor in the infant mouse model of Escherichia coli diarrhoea.

The properties responsible for the virulence in infant mice of the bovine enterotoxigenic Escherichia coli strain B41 were investigated. A B41K99- variant previously found to be nearly as virulent as the original strain B41 (B41K99+) possessed F41 antigen and haemagglutinating properties. Two variants that did not haemagglutinate sheep and human erythrocytes were isolated from strain B41K99-. These variants simultaneously lost their ability to agglutinate with F41 antiserum and their haemagglutinating properties. They still produced heat-stable enterotoxin. The first B41K99-F41- variant was much less virulent than strains B41K99+ and B41K99-, the second was not virulent at all. F41 properties were not acquired by other E. coli strains by plasmid transfers. Non-haemagglutinating variants could not be obtained from the original strain B41K99+. However, a B41K99+F41- strain was obtained by a four-step procedure: (i) spontaneous loss of the K99 plasmid, (ii) obtaining a nalidixic acid-resistant mutant, (iii) obtaining a non-haemagglutinating F41- variant, (iv) reacquisition of the K99 plasmid. This B41NalrK99+F41- strain, although producing heat-stable toxin, was not at all virulent, whereas reacquisition of the K99 plasmid by the strain B41NalrF41+ restored virulence. These results show that F41 antigen is an important virulence factor of strain B41 in the infant mouse model.