Detection of bovine enterotoxigenic Escherichia coli: a comparative study of a direct fluorescent antibody technique and conventional culturing methods

Experiments were conducted to evaluate the sensitivity and specificity of a direct fluorescent antibody technique (DFAT) in detecting bovine enterotoxigenic Escherichia coli possessing the K99 adherence antigen. A monospecific K99 antiserum was tested by DFAT against K99-positive and negative control strains and against faecal smears from 257 calves with diarrhoea. Conventional bacteriological cultures were also carried out. The isolated colonies were tested for the presence of the K99 antigen by slide agglutination using an identical K99 antiserum.Good correlation existed between the DFAT and the slide agglutination as applied to control strains. All strains which were K99 positive by slide agglutination scored positive by DFAT; non-agglutinating colonies showed no fluorescence. The agreement between the DFAT and the slide agglutination method was 75% in the studies involving the faecal specimens. Twenty-four per cent of the faecal specimens scored positive by DFAT but isolated colonies did not agglutinate; slide agglutination was positive while no fluorescence was seen in 1% of specimens.It is concluded that the DFAT involving a monospecific K99 antiserum is a sensitive screening method to detect K99-positve E. coli in faecal specimens.