A hospital-associated outbreak of Legionnaires' disease caused by Legionella pneumophila serogroup 1 is characterized by stable genetic fingerprinting but variable monoclonal antibody patterns

Abstract

An outbreak of 18 pneumonia cases caused by Legionella pneumophila serogroup 1 occurred at a Swedish university hospital 1996 to 1999. Eight clinical isolates obtained by culture from the respiratory tract were compared to 20 environmental isolates from the hospital and to 21 epidemiologically unrelated isolates in Sweden, mostly from patients, by using pulsed-field gel electrophoresis (PFGE), amplified fragment length polymorphism analysis (AFLP), and monoclonal antibody (MAb) typing. All patients and most environmental isolates from the outbreak hospital belonged to the same genotypic cluster in both PFGE and AFLP. This genotype was distinctly different from other strains, including a cluster from a second hospital in a different part of the country. The MAb subtype of the outbreak clone was Knoxville except for three isolates that were Oxford. A variation in the MAb reactivity pattern was also found in a second genotypic cluster. These changes in the MAb reactivity pattern were due to the absence or presence of the lag-1 gene coding for an O-acetyltransferase that is responsible for expression of the lipopolysaccharide epitope recognized by MAb 3/1 of the Dresden Panel. In all MAb 3/1-positive strains, the lag-1 gene was present on a genetic element that was bordered by a direct repeat that showed a high degree of sequence homology. Due to this homology, the lag-1 gene region seemed to be an unstable element in the chromosome. MAb patterns are thus a valuable adjunct to genotyping methods in defining subgroups inside a genotypic cluster of L. pneumophila sg 1.

FIG. 1.

(A) PFGE. Lanes 2 to 6 represent strains from cluster A (hospital I), two patient isolates (isolates 3 [lane 2] and 138 [lane 4], both Knoxville strains) and three environmental (isolates 10 [lane 3] and 139 [lane 5], both Oxford strains, and isolate 83 [lane 6], a Knoxville strain). The patient isolates in lanes 7 (Philadelphia, isolate 59) and 8 (OLDA, isolate 60) show a genotypic similarity with other strains from cluster B (hospital II) in lanes 9 (Philadelphia, isolate 21) and 10 (OLDA, isolate 22) but are epidemiologically unrelated. The strains in lanes 11 to 13 (Benidorm, isolates 28 and 56, and Philadelphia, isolate 54) have been isolated from travelers' pneumonia and are completely unrelated epidemiologically. (B) AFLP. The same series of strains as in Fig. 1 is shown. Note the similarities between isolates in cluster 1(lanes 2 to 6) and between the isolates in lanes 7 to 10.

FIG. 2.

Dendrogram of PFGE, with a Dice coefficient with 2% tolerance and UPGMA clustering of 50 isolates. The isolate number is shown, (though not in numerical ordrer), and the MAb subtype. Note the agreement of cluster A (31 isolates) in Fig. 2 with cluster 1 (27 isolates) in Fig. 3. Similarly, cluster B (7 isolates) agrees with cluster 2 (7 isolates). Patient isolates are marked by “#” symbols. Also, note the variation between the serotypes Oxford/OLDA and Knoxville/Philadelphia inside the same genotypic clusters.

FIG. 3.

Dendrogram of AFLP, with a Dice coefficient with 2% tolerance and UPGMA clustering of 50 isolates. See Fig. 2 legend for more details.

FIG. 4.

Schematic representation of the gene organization in the lag-1 region in the MAb 3/1-positive patient strain 3. Open reading frames are represented as open boxes, with arrowheads indicating the orientation. The design of the open reading frame was based on published sequences (21). The closed box indicates the 2,857-bp fragment that was deleted in the MAb 3/1-negative strain 10, which has the same genotype as strain 3. Two repeats of 797 bp that showed an identity of 99.6% are also shown.