High genetic diversity among Stenotrophomonas maltophilia strains despite their originating at a single hospital

Abstract

The levels of genetic relatedness of 139 Stenotrophomonas maltophilia strains recovered from 105 hospitalized non-cystic fibrosis patients (51% from medical wards, 35% from intensive care units, and 14% from surgical wards) and 7 environmental sources in the same hospital setting during a 4-year period were typed by the pulsed-field gel electrophoresis (PFGE) technique. A total of 99 well-defined distinct XbaI PFGE patterns were identified (Simpson's discrimination index, 0.996). The dendrogram showed a Dice similarity coefficient ranging from 28 to 80%. Two major clusters (I and II), three minor clusters (III, IV, and V), and two independent branches were observed when using a 36% Dice coefficient, indicating a high diversity of genetic relatedness. It is of note that 84% of strains were grouped within two major clonal lineages. No special cluster gathering was found among strains belonging to the same sample type specimen, patients' infection or colonization status, and ward of precedence. Despite this fact, three different clones (A, B, and C) recovered from respiratory samples from six, three, and two patients, respectively, and two clones, D and E, in two bacteremic patients each, were identified. Isolation of an S. maltophilia strain belonging to the clone A profile in a bronchoscope demonstrated a common source from this clone. This study revealed a high genetic diversity of S. maltophilia isolates despite their origin from a single hospital, which may be related to the wide environmental distribution of this pathogen. However, few clones could be transmitted among different patients, yielding outbreak situations.

FIG. 1.

Phylogenetic analysis of digitized 99 PFGE XbaI profiles of 132 clinical and 7 environmental S. maltophilia isolates recovered in our hospital during a 4-year period. The dendrogram was constructed with PFGE data by similarity and clustering analysis by using the Dice coefficient and UPGMA with the Molecular Analysts software. A percent genetic similarity scale is shown above the dendrogram. Profile types are marked on the left in arabic numerals, and the clusters (cutoff value of 36% similarity) are marked on the right in roman numerals. The clinical origins of the isolates are as follows: respiratory specimens, profiles 1 to 51; blood, profiles 52 to 67; wounds, profiles 70 to 77; organic fluid, profiles 69 and 79; ocular specimens, profiles 68 and 78; urine, profiles 80 to 83; stool, profiles 84 and 85; environmental isolates, profiles 1, 86 to 91; and others, profiles 92 to 99.

FIG. 2.

Progression of clones A, B, and C of S. maltophilia. Boxes represent one isolate each; the number below corresponds to the number of the patient. The time period, ward of precedence, bronchoscopy procedure, infection, and patient deaths are also indicated.

FIG. 3.

XbaI profiles obtained in S. maltophilia isolates from blood from 16 patients. Two episodes of cross-transmission were suspected (clone E, lanes 8 and 9, and clone D, lanes 14 and 15). Lane M, molecular size marker.