False positive Legionella pneumophila direct immunofluorescent monoclonal antibody test caused by Bacillus cereus spores.

Direct immunofluorescent monoclonal antibody stain testing for Legionella pneumophila in Oklahoma lake water yielded an unknown bacillus with fluorescence intensity equal to that of L. pneumophila stock strains. The organism in question was identified as Bacillus cereus, a ubiquitous bacterium. When B. cereus cultures were studied, fluorescence was seen in spores but not in vegetative cells. Since a positive immunofluorescent monoclonal antibody test (alone) might be considered by some individuals as unequivocal to very good evidence for the presence of L. pneumophila, this finding emphasizes the importance of confirming positive stain results with cultures whenever possible.