DNA array based detection of Legionella species reveals selectivity of the conventional culture based detection method

Background and aims - Legionnaires' disease is a form of pneumonia caused by legionellae, which are ubiquitous in aquatic environments. Out of more than 50 Legionella species, L. pneumophila is reported as the most common cause of legionellosis. Nevertheless, in addition to L. pneumophila, 20 other species have been reported as human pathogens. The aim of this study was to develop and validate a DNA array for the simultaneous detection and identification of all Legionella species that have been associated with human infections. Methods - Based on macrophage infectivity potentiator and 16S rDNA sequences a DNA array was developed for 20 Legionella spp. associated with human disease. Specificity and sensitivity of the assay was evaluated using a large collection of reference cultures. In addition, 183 water samples were processed using the array and classical plating on GVPC agar as described in the ISO 11731 standard protocol. Results - A highly specific DNA array was developed, enabling accurate detection of potentially pathogenic Legionella species. Validation of the assay using environmental samples revealed consistent differences with the results obtained by classical plating on GVPC agar. Whereas Legionella DNA was detected in approximately 80 % of the samples, plating recovered Legionella colonies in only 32 % of the samples. All isolates obtained by culturing were identified as L. pneumophila or L. anisa. Using the DNA array, a variety of additional Legionella species were detected, including L. birminghamiensis, L. bozemanii, L. cincinnatiensis, L. gormanii, L. longbeachae and L. sainthelensii. Consequently, this study illustrates the usefulness of novel detection tools in legionella monitoring. Conclusions - The currently used GVPC agar plating method for detection of Legionella spp. selects for detection of L. anisa and L. pneumophila. Screening of a large numer of water samples using the DNA array revealed the occurence of other Legionella spp.