Pseudo-outbreak of α Proteobacteria associated with contaminated heparin-saline solution

Background: Afipia and Methylobacterium are fastidious gram-negative bacteria belonging to the α subgroup of Proteobacteria. They are environmental bacteria that have been isolated from hospital water systems but rarely cause clinical infection. In August 2010, the Microbiology Laboratory notified Hospital Epidemiology and Infection Control of a cluster of Afipia species isolated from bone marrow aspirates that were sent for mycobacterial culture.

Objective: To investigate an apparent outbreak of Afipia species involving patients who had undergone bone marrow aspiration in a tertiary care hospital in Baltimore, Maryland.

Methods: An epidemiologic investigation including case identification, review of medical records, observation of the related procedures and environmental cultures was performed. 16SrRNA gene sequencing (Seq) and cell wall fatty acid analysis using gas liquid chromatography (GLC) were performed to identify the isolates and to determine the clonal relationship.

Results: Five patients had mycobacterial cultures of bone marrow aspirates that grew fastidious gram-negative bacilli between May and August 2010. Four of the isolates were identified as Afipia genospecies 11 using GLC and Seq methods and the other one was Methylobacterium species. All 5 patients suffered from hematological malignancy and had bone marrow aspiration performed on the oncology service. None of them had symptoms consistent with a systemic gram-negative bacterial infection at the time of their cultures.

Observation of the procedure revealed that providers used a heparin saline solution prepared in the laboratory to coat the inside of the sterile syringes used for bone marrow aspiration. Cultures of the heparin saline solution grew both Afipia genospecies 11 and Methylobacterium species. The bottle of normal saline used for dilution of heparin grew Methylobacterium species. Cultures of unopened normal saline and heparin solutions with the same lot number did not grow. Cultures of tap water and swabs from three sinks (including one designated for hand washing) in the laboratory all grew Methylobacterium species.

All laboratory-prepared heparin saline solutions were removed and the pharmacy provided a commercial preparation of heparin saline solution. Bone marrow aspirate mycobacterial cultures for each procedure performed in the 10 days following these interventions did not grow any Afipia or Methylobacterium species.

Conclusions: The findings suggest that contamination of heparin saline solution prepared by the laboratory was the cause of a pseudo-outbreak of α Proteobacteria in patients undergoing bone marrow aspiration.  A point source was not identified, but findings suggest that tap water may have been the main reservoir for the organisms involved. Steps to review procedures and to ensure the use of appropriate aseptic and sterile technique and quality-controlled products ended the pseudo-outbreak.