Investigation of a pseudo-outbreak of Ralstonia pickettii in Sterile Tissue

<>Title: Investigation of a pseudo-outbreak of Ralstonia pickettii in Sterile Tissue Culture samples <>UMass Memorial Medical Center, Worcester, Massachusetts, USA <> 

Background: December 6, 2009 routine surveillance by Infection Control staff found 4 positive microbiology cultures for Ralstonia pickettii from OR tissue samples.  R. pickettii had only rarely been identified by the laboratory.

Objective: To undertake an investigation of a potential outbreak due to R.pickettii.

Setting: A 780-bed tertiary care hospital with 3 separate campuses. The institution's clinical laboratories also provide services to outside facilities.

Methods: A retrospective review was performed of medical records, operative reports, and microbiology laboratory data; as well as prospective surveillance for new cases. Environmental culturing in the operating rooms (ORs) and the clinical microbiology laboratory was performed, as was pulse-field gel electrophoresis (PFGE) of available isolates.

Results: Initially it was found that in 2009 there had been 15 cultures positive from 11 patients for R. pickettii; 1 in June 2009, 1 from an outside facility in September 2009, and 13 from the institution's ORs that had been collected between October 22 and November 25, 2009. The OR specimens had been obtained at 2 different campuses, from multiple different OR rooms, and patients treated by multiple different surgical services; 12 of the 13 were tissue samples. There were no common personnel or equipment; samples were obtained on different weekdays, at different times of day, and processed by differing lab personnel. Initial PFGE analysis of 4 tissue samples found that all were identical. There were no other positive cultures until February 2010 when there was a second cluster of X positive OR tissue cultures from 7 patients, again from differing campuses, OR rooms, and surgical services. On February 25th, 4 of 18 submitted OR tissue cultures grew R. pickettii. Except for cultures from 2 OR sinks, all cultures from the OR environment, the microbiology lab environment, equipment and media were negative for R. pickettii. PFGE analysis of 11 tissue cultures (7 from first and 4 from the second cluster) found 10 were identical and 1 was highly related; the OR sink isolates were unrelated to the clinical isolates and each other (figure). There was 1 additional R. pickettii isolate identified in March 2010 from a non-OR bronchoscopy specimen, and no further positive cultures through October 15, 2010. Infectious Disease physicians were notified in late February 2010 of an apparent pseudo-outbreak. Six patients received antibiotics for potential R. pickettii infection; 5 of these patients were from the first cluster.  No patient was found to have a definitive R. pickettii infection.

Conclusions: While no source was defined, molecular typing results are indicative of a pseudo-outbreak, most likely due to occult contamination of samples during tissue sample processing. Our findings emphasize the need for early identification and management of such events.