A risk associated with transfusion of platelet products is transfusion-transmitted bacterial infection as platelet concentrates are stored at 22 ºC with gentle shaking, suitable conditions to promote bacterial growth. In Australia, this risk is minimised by bacterial contamination screening (BCS), however false positive results can result in component recall. Cumulative data in Australia from 2014 to 2017 demonstrated the rate of BCS initially reactive was 6 per 1000 platelet components comprised of confirmed positive 1/1000, false positive 4/1000 and indeterminate (unconfirmed initially reactive samples) 1/1000.
Using next generation sequencing (NGS) targeting the 16S rRNA gene, we aim to investigate whether BCS indeterminate results are signals resulting from the presence of microorganisms that do not grow under culture conditions or low concentrations of slow growing microorganisms. As a proof of principle, we examined the microbiome in plasma samples from 10 blood donations. DNA extracted from E.coli culture and plasma spiked with Coxiella burnetti were used as positive controls.
Signals for the E.coli and C.burnetti DNA were as expected providing evidence that the processing and analytical procedures were suitable for purpose. Amongst the other 10 samples, evidence for low levels of two bacterial genera was found. Using 95% sequence similarity as a threshold, 2 out of the 10 samples were positive for Stenotrophomonas spp., and 1 was positive for Bosea spp. These are environmental microorganisms and are unlikely to be human pathogens, although nosocomial infection related to Stenotrophomonas sp. has been reported.
These preliminary results demonstrate that 16S rRNA NGS is a highly sensitive tool for detection of bacterial contamination; however, procedures for validation of low level signals may be required. The application of 16S metagenomics has the potential to provide rapid identification of bacterial types and raises new questions in relation to policies for platelet BCS, including how to manage units with indeterminate results and those with low levels of environmental organisms that are not detected by BCS.