Rapid detection ofShigella dysenteriae andShigella flexneri in faeces by an immunomagnetic assay with monoclonal antibodies

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Abstract
A rapid and sensitive method for the detection ofShigella dysenteriae type 1 andShigella flexneri serotypes in faeces based on capture of the bacteria with immunomagnetic particles is described. The particles were coated with either of two different monoclonal antibodies specific for the O-antigens ofShigella dysenteriae type 1 andShigella flexneri serotypes. Captured bacteria were detected by an enzyme immunoassay with O-antigen specific rabbit antiserum. The whole assay required 2 to 3 hours to perform and the sensitivity limit was 103 cfu/ml as determined by viable cell counting. One hundred and fifty enterobacteria strains, including 100Shigella strains from a strain collection, and 302 fresh faecal samples were used for the study. AllShigella dysenteriae type 1 andShigella flexneri culture-positive faecal samples were positive in the immunomagnetic assay. In addition 18 of 252 culture-negative faecal samples were positive. The immunomagnetic assay was compared with latex agglutination and indirect immunofluorescence using culture as the reference method. The immunomagnetic assay had a sensitivity of 100%, latex agglutination a sensitivity of 72% with 28% false-negative results, and indirect immunofluorescence a sensitivity of 95%. The immunomagnetic assay was superior in sensitivity since it also detectedShigella in faecal samples up to two days after antibiotic therapy had been started, which both latex agglutination and indirect immunofluorescence failed to do. The high sensitivity in detecting live and dead bacteria, and the ease of performance of the immunomagnetic assay render it an attractive method for detection ofShigella.