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

Abstract

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.