Summary: The properties responsible for the virulence in infant mice of the bovine enterotoxigenic strain B41 were investigated. A B41K99 variant previously found to be nearly as virulent as the original strain B41 (B41 K99) 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 B41K99F41 variant was much less virulent than strains B41K99 and B41K99, the second was not virulent at all. F41 properties were not acquired by other strains by plasmid transfers. Non-haemagglutinating variants could not be obtained from the original strain B41K99. However, a B41K99F41 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 B41NalK99F41 strain, although producing heat-stable toxin, was not at all virulent, whereas reacquisition of the K99 plasmid by the strain B41NalF41 restored virulence. These results show that F41 antigen is an important virulence factor of strain B41 in the infant mouse model.


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