Is pulsed-field gel electrophoresis a valuable tool to identify nosocomial cases of Legionella pneumophila disease?

Pulsed-field gel electrophoresis (PFGE) was used to type Legionella pneumophila isolates collected from the water systems of Besançon University Hospital and other hospitals in Franche-Comté region between January 2001 and December 2002 and compare them with patient isolates to identify hospital-acquired pneumonia. Genomic DNA was digested with SfiI and subjected to PFGE in a clamped, homogeneous electric field apparatus. Two of 11 Legionella infections were hospital-acquired. Both were with the same type, also present in the ward water. An environmental strain isolated from the water system of Vesoul Hospital, exhibited DNA pattern 6, also found in three patients with community-acquired pneumonia, who had never been in Vesoul hospital and lived in different towns located >60 km away. Patient 11, who lived in Besançon, was infected with a DNA pattern 12 strain. This patient had never been to the Besançon swimming pool, from which a similar strain was collected. Subtype matching of patient and environmental isolates should be interpreted with caution, and it is important to combine a molecular typing method with sound epidemiological data to ensure that the most stringent criteria are used to determine whether a hospital-reservoir is responsible for nosocomial pneumonia.