Bacterial safety of blood products

Project leader: Dirk de Korte PhD

Contamination of platelets with bacteria is a major microbiological risk of blood transfusion. This applies especially for platelet concentrates (PCs) because their storage conditions, at room temperature and under constant agitation, support bacterial growth. Therefore both the prevention of contamination and gathering knowledge on the type of contamination are subject of research within the Sanquin. In 2010 a study on the frequency of false negatives was completed as well as a study on the origin of Propionibacterium species isolated from blood products.

Frequency of false negatives in the Dutch screening system
In the Netherlands, platelet concentrates (PCs) are screened with the BacT/ALERT culturing system and released as ‘negative to date’. From all whole blood derived PCs in the Netherlands, approximately 0.37% are tested positive for bacterial growth with the BacT/ALERT culture system. Recently there were several reports showing the risk of false negatives to be around 1 in 1000 PC. For the Netherlands, no accurate numbers were available. To determine the frequency of false negative culture results for the screening system as used in the Netherlands, an ‘outdated’ study was initiated, in which five (in PASII) or seven day old (in plasma; outdated) PCs were re-cultured with the BacT/ALERT. Over the period of April 2009 to October 2010, 4082 outdated buffy coat PC units were re-tested with the BacT/ALERT. The 16S rRNA gene was sequenced for species confirmation and strains from the same species were analyzed by amplified fragment length polymorphism (AFLP) analysis to determine if the strains found in the PCs and the accompanying culture bottles were identical. During the study period 4 false negative PCs were found leading to a frequency of false negative BacT/ALERT results of 0.10%. Three PCs were contaminated with Staphylococcus epidermidis and one with S. hominis. AFLP analysis showed that all strains that were cultured from the PC and its accompanying culture bottles were identical but the S. epidermidis strains from the different PC bags did not show identical patterns. The bacterial concentration of the false negative PCs on the day of re-testing was estimated to be at least 105 CFU/ml. It was concluded that, although screening of PCs with the BacT/ALERT prevents the transfusion of PC contaminated with bacteria, not all contaminated PCs are detected. These false negative PCs can be a risk for patients receiving transfusion, but it has to be kept in mind that bacterial transmission by BacT/Alert tested PC’s is a very rare event in the Netherlands; during the last 3 years only 2 cases were reported.

Molecular relatedness of Propionibacterium species isolated from blood products and on the skin of blood donors
From all the bacteria that are found with the BacT/ALERT more than half are Propionibacterium acnes, gram positive, anaerobic bacteria that are part of the normal resident skin flora. This bacterium is rarely implicated in transfusion related reactions. However, several studies suggested its involvement in various clinical conditions and infections such as spondylodiscitis, endocarditis, and it is the second most frequent bacterium colonizing catheter tips. It is generally assumed that the bacteria found in blood products originate from the skin of the donor but this has not been examined properly. The aim of this study was to investigate whether the P. acnes strains present in the PCs and related RBCs, originate from the skin of the donor. Therefore, strains that were found throughout 2007 and 2008 in PCs and RBCs with the BacT/ALERT and strains that were cultured in 2010 from the phlebotomy site (antecubital fossa) of the donors of these blood products were analyzed. The strains were determined to species level by sequencing of the 16S rRNA and recA genes and typed by AFLP. A part of the strains was also determined to species level by sequencing of the 16S rRNA and recA genes. Based on this sequencing, three different phylogenetic groups of P. acnes were found. All strains that were found in PCs and their accompanying RBCs were identical, which indicates that the strain is already present in the whole blood donation. P. acnes could be found on the skin of almost all screened donors. In eight out of twenty-two cases (36.4%), one of the strains from the donor skin was identical to the strains found in PCs and their accompanying RBCs. In two other cases the strains belonged to the same phylogenetic group. These results support the theory that the source of P. acnes contamination is in many cases the skin of the donor. However, further study is necessary to rule out other sources of contamination. Because it is difficult to prevent bacterial contamination by P. acnes completely, it is necessary to further investigate the clinical significance of blood products contaminated with P. acnes. This work was performed in collaboration with Paul Savelkoul and Annika Peterson from the VU University Medical Center.

Key publications