Unexpected Similarity of Pulsed-Field Gel Electrophoresis Patterns of Unrelated Clinical Isolates of Legionella pneumophila, Serogroup 1

Phenotypic and genotypic methods identify subtypes of Legionella pneumophila, serogroup 1, and match patient and environmental isolates from suspected sources. The strength of this association is limited by the lack of information regarding the frequency and distribution of isolates belonging to various subtypes. In this study, 62 clinical isolates of L. pneumophila, serogroup 1, were subtyped by using pulsed-field gel electrophoresis (PFGE), to determine the distribution and degree of diversity of PFGE patterns among monoclonal antibody (MAb) subtypes. Unexpectedly, 8 of 21 MAb Philadelphia 1 isolates had a common PFGE pattern, and, among 12 MAb OLDA isolates, only 2 PFGE patterns were seen. Our hypothesis was that PFGE patterns were distributed randomly; however, statistical analysis showed that the distribution of subtypes was not random (Fisher’s exact test 0.13; ). In light of these P 1 .05 results, researchers who do epidemiological investigations should use caution when interpreting the significance of matching PFGE patterns of L. pneumophila, serogroup 1. Making the epidemiological link between cases of legionnaires disease and a suspected environmental source can be difficult, because of the sporadic nature of the disease and the variety of sources from which the organism can be isolated [1]. In epidemiological investigations, both phenotypic and genotypic methods have been used to demonstrate identity among strains of Legionella pneumophila. These methods include serotyping, monoclonal antibody (MAb) subtyping, isoenzyme analysis, protein and carbohydrate profiling, plasmid analysis, restriction endonuclease analysis, restriction fragment length polymorphism (RFLP) analysis of rRNA (ribotyping) or chromosomal DNA, amplified fragment length polymorphism analysis, restriction endonuclease analysis of whole-cell DNA with or without pulsedfield gel electrophoresis (PFGE), arbitrarily primed (AP) polymerase chain reaction (PCR), repetitive element (RE) PCR, and infrequent-restriction-site PCR [1‐10]. MAb subtyping is a useful phenotypic method for subdividing strains of L. pneumophila, serogroup 1. Genotypic methods, such as PFGE, have become increasingly valuable for epidemiological investigation, because of their high discriminatory power, broad application, and speed of results [11]. One limitation of PFGE and other subtyping methods, however, is the lack of information regarding the frequency and distribution of the isolates belonging to the various subtypes or patterns