Analysis of a long-term outbreak of XDR Pseudomonas aeruginosa: a molecular epidemiological study.

Objectives: Here we report on a long-term outbreak from 2009 to 2012 with an XDR Pseudomonas aeruginosa on two wards at a university hospital in southern Germany.

Methods: Whole-genome sequencing was performed on the outbreak isolates and a core genome was constructed for molecular epidemiological analysis. We applied a time-place-sequence algorithm to improve estimation of transmission probabilities.

Results: By using conventional infection control methods we identified 49 P. aeruginosa strains, including eight environmental isolates that belonged to ST308 (by MLST) and carried the metallo-β-lactamase IMP-8. Phylogenetic analysis on the basis of a non-recombinant core genome that contained 22 outbreak-specific SNPs revealed a pattern of four dominant clades with a strong phylogeographic structure and allowed us to determine the potential temporal origin of the outbreak to July 2008, 1 year before the index case was diagnosed. Superspreaders at the root of clades exhibited a high number of probable and predicted transmissions, indicating their exceptional position in the outbreak.

Conclusions: Our results suggest that the initial expansion of dominant sublineages was driven by a few superspreaders, while environmental contamination seemed to sustain the outbreak for a long period despite regular environmental control measures.