Elucidating the origins of nosocomial infections with Candida albicans by DNA fingerprinting with the complex probe Ca3

Abstract

Computer-assisted DNA fingerprinting with the complex probe Ca3 has been used to analyze the relatedness of isolates collected from individuals with nosocomial bloodstream infections (BSIs) and hospital care workers (HCWs) in the surgical and neonatal intensive care units (ICUs) of four hospitals. The results demonstrate that for the majority of patients (90%), isolates collected from commensal sites before and after collection of a BSI isolate were highly similar or identical to the BSI isolate. In addition, the average similarity coefficient for BSI isolates was similar to that for unrelated control isolates. However, the cluster characteristics of BSI isolates in dendrograms generated for each hospital compared to those of unrelated control isolates in a dendrogram demonstrated a higher degree of clustering of the former. In addition, a higher degree of clustering was observed in mixed dendrograms for HCV isolates and BSI isolates for each of the four test hospitals. In most cases, HCW isolates from an ICU were collected after the related BSI isolate, but in a few cases, the reverse was true. Although the results demonstrate that single, dominant endemic strains are not responsible for nosocomial BSIs in neonatal ICUs and surgical ICUs, they suggest that multiple endemic strains may be responsible for a significant number of cases. The results also suggest that cross-contamination occurs between patients and HCWs and between HCWs in the same ICU and in different ICUs. The temporal sequence of isolation also suggests that in the majority of cases HCWs are contaminated by isolates from colonized patients, but in a significant minority, the reverse is true. The results of this study provide the framework for a strategy for more definitive testing of the origins of Candida albicans strains responsible for nosocomial infections.

FIG. 1

Examples of the Southern blot hybridization patterns obtained with the complex probe Ca3. (A) Isolates from patient P5 and the reference strain 3153A; (B) isolates from patient P17; (C) isolates from patient P4; (D) isolates from patient P28. Isolate labels are explained in Materials and Methods. Molecular weights (in kilobases) are noted to the left of each Southern blot hybridization pattern.

FIG. 2

Dendrogram of a control collection of 29 unrelated BSI isolates each collected from a different hospital across the continental United States. a through e, clusters of two or more isolates with S_ABs of ≥ 0.90. S_AB thresholds for cluster analysis are drawn at 0.80 (straight line) and 0.90 (dashed line).

FIG. 3

Examples of dendrograms for collections of isolates from patients in which all isolates are identical (A), isolates show some variability reflecting microevolution (B), and isolates separate into unrelated clusters of isolates (C and D). Horizontal models of the Ca3 hybridization patterns are displayed next to the respective isolates in the dendrograms.

FIG. 4

Dendrograms of the BSI isolates from the patients in hospitals A (A), B (B), C (C), and D (D). Only one BSI isolate from each patient was incorporated. The type of ICU and the date of collection are noted to the right of each isolate. Arbitrary S_AB thresholds are drawn at 0.80 (straight line) and 0.90 (dashed line).

FIG. 5

Dendrograms of the isolates from HCWs of hospitals A (A), B (B), C (C), and D (D). The type of HCW is noted immediately to the right of health care worker number. The type of ICU and the date of collection are noted to the right of each isolate. Arbitrary S_AB thresholds are drawn at 0.80 (straight line) and 0.90 (dashed line). Abbreviations for HCWs are provided in Materials and Methods.

FIG. 6

Dendrograms for combined BSI isolates and HCW isolates from hospitals A (A), B (B), C (C), and D (D). Only one BSI isolate from each patient was used. Isolate labels are explained in Materials and Methods. Arbitrary S_AB thresholds are drawn at 0.80 (straight line) and 0.90 (dashed line). Clusters determined by a threshold of 0.90 are delineated to the right of each dendrogram.