Hospital acquired pneumonia: Comparison of culture and real-time PCR assays for detection of Legionella pneumophila from respiratory specimens at Tehran Hospitals

Legionella pneumophila is an important etiological agent in both hospital and community acquired pneumonia. The sensitivity of culture for isolation of L. pneumophila from clinical specimens is low and time consuming. Similar problem also exists when the method of direct immunofluorescence is used. To detect this organism quantitatively from respiratory specimens, a Taq Man based real-time PCR targeting the mip sequence was developed. Both real-time PCR and culture methods were applied on 262 respiratory specimens from 262 ICU patients with pneumonia admitted to 5 different hospitals in Tehran. The results of real-time PCR were compared with those obtained by culture. Real-time PCR and culture found 12 and 4 specimens, respectively, as positive for L. pneumophila. Its technical specificity (100%) was checked against a panel of microorganisms consisting of both Gram-positive and Gram-negative bacteria. Our real-time PCR assay showed high sensitivity (100%) and specificity (96.9%) and could detect 200 organisms per ml from respiratory specimens. Using real-time PCR as a screening method, the frequency of nosocomial pneumonia with L. pneumophila at Tehran hospitals was estimated as 4.58%.