Molecular epidemiology of Pseudomonas aeruginosa colonization in a burn unit: persistence of a multidrug-resistant clone and a silver sulfadiazine-resistant clone

Abstract

To study the epidemiology of Pseudomonas aeruginosa colonization in a 32-bed burn wound center (BWC), 321 clinical and 45 environmental P. aeruginosa isolates were collected by prospective surveillance culture over a 1-year period and analyzed by serotyping, drug susceptibility testing, and amplified fragment length polymorphism (AFLP) analysis. Among 441 patients treated at the center, 70 (16%) were colonized with P. aeruginosa, including 12 (17%) patients who were colonized on admission and 58 (83%) patients who acquired the organism during their stay. Of the 48 distinct AFLP genotypes found, 21 were found exclusively in the environment, 15 were isolated from individual patients only, and 12 were responsible for the colonization of 57 patients, of which 2 were also isolated from the environment, but secondary to patient carriage. Polyclonal P. aeruginosa colonization with strains of two to four genotypes, often with different antibiotic susceptibility patterns, was observed in 19 patients (27%). Two predominant genotypes were responsible for recurrent outbreaks and the colonization of 42 patients (60% of all colonized patients). The strain with one of those genotypes appeared to be endemic to the BWC and developed multidrug resistance (MDR) at the end of the study period, whereas the strain with the other genotype was antibiotic susceptible but resistant to silver sulfadiazine (SSD(r)). The MDR strain was found at a higher frequency in sputum samples than the SSD(r) strain, which showed a higher prevalence in burn wound samples, suggesting that anatomic habitat selection was associated with adaptive resistance to antimicrobial drugs. Repeated and thorough surveys of the hospital environment failed to detect a primary reservoir for any of those genotypes. Cross-acquisition, resulting from insufficient compliance with infection control measures, was the major route of colonization in our BWC. In addition to the AFLP pattern and serotype, analysis of the nucleotide sequences of three (lipo)protein genes (oprI, oprL, and oprD) and the pyoverdine type revealed that all predominant strains except the SSD(r) strain belonged to recently identified clonal complexes. These successful clones are widespread in nature and therefore predominate in the patient population, in whom variants accumulate drug resistance mechanisms that allow their transmission and persistence in the BWC.

FIG. 1.

Percentages of burn wound patients colonized with P. aeruginosa by age (A), duration of hospitalization (B), and TBSA (C).

FIG. 2.

Normalized AFLP patterns and dendrogram for the type isolates of the 48 genotypes from patients and/or the environment of the BWC during a 1-year period according to the differentiation criteria proposed by Speijer et al. (53). AFLP types are indicated in parentheses. Cluster analysis was performed with BioNumerics software by using the Pearson correlation and the unweighted pair group method with arithmetic averages. Percent similarities are shown above the dendrogram. ▪, isolated exclusively from clinical samples; •, isolated exclusively from the hospital environment; ⧫, isolated from clinical samples as well as from the hospital environment (secondary to patient carriage).

FIG. 3.

Time course of P. aeruginosa AFLP35 and AFLP8 colonization in the BWC. P, patient.

FIG. 4.

Colonization of different sites of patient P13 with four P. aeruginosa genotypes. The horizontal bars represent the length of time that patient P13 was colonized.