Sterility testing of platelet concentrates prepared from deliberately infected blood donations

Three different bacterial detection systems for platelet concentrates under inter-laboratory conditions

BACKGROUND

A variety of screening methods are currently used worldwide in order to decrease the risk of transfusion-transmitted sepsis and improve the safety of PCs.

METHODS/MATERIALS

PCs inoculated with five different transfusion-relevant species of bacteria at concentrations of 1, 10, and 100 colony-forming units (CFU)ml(-1) were stored at 22°C for 7 days. Flow cytometry (FACS), BacT/Alert automated culturing, and a quantitative real-time PCR (Q-PCR) were then used to detect the presence of bacteria in samples prepared from these PCs.

RESULTS

At the initial spiking concentrations of 1, 10, and 100 CFU ml(-1), Q-PCR detected all five bacterial species tested. Screening with the BacT/Alert culture-based system allowed bacterial detection (inoculated on day 0) within a mean time of 15.13 h for all three spiking concentrations. Using FACS, positive signals were obtained for all three concentrations of Escherichia coli and Bacillus cereus on day 1 and for initial spiking concentrations of Pseudomonas aeruginosa and Staphylococcus aureus of 1 CFU ml(-1) on day 2. For Staphylococcus epidermidis, detection of an initial inoculum of 1 CFU ml(-1) was possible only beginning on day 6.

CONCLUSION

This study shows that under standard laboratory conditions the sensitivity of FACS in the detection of bacterial contamination of PCs was lower than that of either the BacT/Alert automated culturing method or Q-PCR.

Laboratory Evaluation of the Effectiveness of Pathogen Reduction Procedures for Bacteria

SUMMARY: Bacterial contamination remains a leading factor for transfusion-associated serious morbidity and mortality. Pathogen reduction procedures offer a pro-active approach to prevent bacterial contamination of cellular blood components and especially of platelet concentrates. In the past, the laboratory evaluation of the effectiveness of the pathogen reduction procedures to minimise the bacterial load of blood components has been primarily based on log reduction assays similar to the assessment of antiviral activities. Bacteria strains with the ability to multiply in the blood components are seeded in highest possible cell numbers, the pathogen reduction procedure is applied, and the post-treatment number of bacteria is measured. The effectiveness of the procedure is characterised by calculating the log reduction of the post- to pre-treatment bacteria titres. More recently, protocols have been developed for experiments starting with a low bacteria load and monitoring the sterility of the blood component during the entire storage period of the blood component. Results for 3 different pathogen reduction technologies in these experimental models are compared and critical determinants for the results are addressed. The heterogeneity of results observed for different strains suggests that the introduction of international transfusion-relevant bacterial reference strains may facilitate the validity of findings in pathogen reduction experiments.

Implementation of Bacterial Detection Methods into Blood Donor Screening - Overview of Different Technologies

SUMMARY: BACKGROUND: Through the implementation of modern technology, such as nucleic acid testing, over the last two decades, blood safety has improved considerably in that the risk of viral infection is less than 1 in a million blood transfusions. By contrast, the residual risk of transfusion-associated bacterial infection is stable at approximately 1 in 2,000 to 1 in 3,000 in platelets. To improve blood safety with regard to bacterial infections, many countries have implemented bacterial screening methods as part of their blood donor screening programmes. METHODS: BACTERIAL DETECTION METHODS ARE CLUSTERED INTO THREE GROUPS: i) culture methods in combination with the 'negative-to-date' concept, ii) rapid detection systems with a late sample collection, and iii) bedside screening tests. RESULTS: The culture methods are convincing because of their very high analytical sensitivity. Nevertheless, false-negative culture results and subsequent fatalities were reported in several countries. Rapid bacterial systems are characterised as having short testing time but reduced sensitivity. Sample errors are prevented by late sample collection. Finally, bedside tests reduce the risk for sample errors to a minimum, but testing outside of blood donation services may have risks for general testing failures. CONCLUSION: Bacterial screening of blood products, especially platelets, can be performed using a broad range of technologies. Each system exhibits advantages and disadvantages and offers only a temporary solution until a general pathogen inactivation technology is available for all blood components.

Methods for the detection of bacterial contamination in blood products

Culture-based and molecular assays have been developed for the screening of platelet concentrates and other blood components for bacterial contaminations. In this review, the principles of the assays are outlined. The focus of this review is the assessment of the analytical qualities of the methods. Spiking studies by adding defined levels of a wide range of bacteria to the complex biological matrix provide the first basis to evaluate and compare the qualities of methods for bacterial detection. The sensitivity acceptable for reliable screening for bacteria critically depends on the timing of either early sampling (within a period of up to 24 h after preparation of the blood component) or late sampling (a few hours before issuing the blood component). Large screening studies are essential to confirm both adequate sensitivity and specificity of the testing. In the ideal setting, these studies are prospectively planned and include systematic surveillance of adverse events in response to the administration of the screened products. The findings from sterility testing (predominantly with automated systems for detection of bacteria based on CO(2) generation) of more than 550,000 platelet concentrates in 13 studies are summarised. The limitations of the early sampling and the "negative-to-date" strategy to issue platelet concentrates are addressed. A few reported cases of probable transmission of bacteria by platelet transfusion despite negative screening tests emphasise the need to further develop optimised methods for testing of bacteria blood components.

Assessment of biofilm-forming ability of coagulase-negative staphylococci isolated from contaminated platelet preparations in Canada

BACKGROUND

Coagulase-negative staphylococci (CoNS) are the most prevalent bacterial contaminants of platelet (PLT) preparations and have been implicated in adverse transfusion reactions worldwide. The most frequently identified contaminant is Staphylococcus epidermidis, which is noted for its ability to maintain chronic hospital-acquired infections by forming biofilms as a chief virulence mechanism.

STUDY DESIGN AND METHODS

Strains of S. epidermidis isolated from contaminated PLT preparations in Canada were distinguished via gene-specific polymerase chain reaction (PCR) with divIVA as a marker. Biofilm-forming ability was assessed by the presence of the gene icaD, slime production on Congo red agar, and biofilm formation on polystyrene surfaces. Production of polysaccharide intercellular adhesin (PIA) was resolved by immunofluorescence.

RESULTS

Eight of the 13 (62%) CoNS isolates under study were identified as S. epidermidis. Of these, four strains (50%) were classified as strong biofilm producers. Three of the four biofilm-positive strains (75%) produced slime, harbored the icaD gene, and had positive expression of PIA.

CONCLUSIONS

Despite the presumable commensal origin of the CoNS isolates, a large proportion of S. epidermidis strains demonstrated a potential for enhanced virulence. Identification of contaminant staphylococci as biofilm producers is thus relevant and informative with regard to treatment approach in the circumstance of inadvertent infection of a PLT recipient.

Real-Time Polymerase Chain Reaction in Transfusion Medicine: Applications for Detection of Bacterial Contamination in Blood Products

Bacterial contamination of blood components, particularly of platelet concentrates (PCs), represents the greatest infectious risk in blood transfusion. Although the incidence of platelet bacterial contamination is approximately 1 per 2,000 U, the urgent need for a method for the routine screening of PCs to improve safety for patients had not been considered for a long time. Besides the culturing systems, which will remain the criterion standard, rapid methods for sterility screening will play a more important role in transfusion medicine in the future. In particular, nucleic acid amplification techniques (NATs) are powerful potential tools for bacterial screening assays. The combination of excellent sensitivity and specificity, reduced contamination risk, ease of performance, and speed has made real-time polymerase chain reaction (PCR) technology an appealing alternative to conventional culture-based testing methods. When using real-time PCR for the detection of bacterial contamination, several points have to be considered. The main focus is the choice of the target gene; the assay format; the nucleic acid extraction method, depending on the sample type; and the evaluation of an ideal sampling strategy. However, several factors such as the availability of bacterial-derived nucleic acid amplification reagents, the impracticability, and the cost have limited the use of NATs until now. Attempts to reduce the presence of contaminating nucleic acids from reagents in real-time PCR have been described, but none of these approaches have proven to be very effective or to lower the sensitivity of the assay. Recently, a number of broad-range NAT assays targeting the 16S ribosomal DNA or 23S ribosomal RNA for the detection of bacteria based on real-time technology have been reported. This review will give a short survey of current approaches to and the limitations of the application of real-time PCR for bacterial detection in blood components, with emphasis on the bacterial contamination of PCs.

Staphylococcus epidermidis forms biofilms under simulated platelet storage conditions

BACKGROUND

Staphylococcus epidermidis grows slowly in platelet (PLT) preparations compared to other bacteria, presenting the possibility of missed detection by routine screening. S. epidermidis is a leading cause of nosocomial sepsis, with virulence residing in its ability to establish chronic infections through production of slime layers, or biofilms, on biomedical devices. This study aims to establish biofilm formation (BF) as a mode of growth by S. epidermidis in PLT preparations.

STUDY DESIGN AND METHODS

Biofilm-positive (BFpos) and -negative (BFneg) S. epidermidis strains were grown in whole blood-derived PLTs (WBDPs) and in glucose-rich medium (TSBg). An assay for BF was adapted for cultures grown in WBDPs or filtered WBDPs in polystyrene culture plates. Bacterial attachment to polyvinylchloride PLT bags and PLTs was examined by scanning electron microscopy.

RESULTS

Both strains display similar growth profiles in WBDPs and TSBg. Unexpectedly, evidence of BF was observed on PLT bags and on PLTs directly, not only by the BFpos strain but also by the BFneg strain. The BFpos strain displayed greater plastic adherence than the BFneg strain in WBDPs (p < 0.05). BF by the BFneg strain was approximately 10-fold greater in WBDPs compared to TSBg (p < 0.05), likely by use of PLTs as a scaffold. Furthermore, BF by S. epidermidis was significantly decreased when PLT concentration was reduced 1000-fold.

CONCLUSIONS

S. epidermidis forms biofilms on PLT aggregates and on PLT bags under PLT storage conditions. Our results demonstrate that the PLT storage environment can promote a BF growth mechanism for contaminant bacteria.

Bacterial risk reduction by improved donor arm disinfection, diversion and bacterial screening

The Interventions of improved donor arm disinfection, diversion and bacterial screening have been implemented by blood services and shown to have substantial benefit. The major source of bacterial contamination is donor arm derived. Blood services are now introducing best practice donor arm disinfection techniques. Diversion has been shown to substantially reduce bacterial contamination in the order of 40-88%. Diversion, together with improved donor arm disinfection, has shown to improve the percentage of reduction in contamination from 47% to 77%. Residual contamination levels after the Introduction of diversion and improved donor arm disinfection may be in the order of 30-40%. Numerous countries have now implemented screen testing programmes for platelet concentrates, which are the major source of bacterial transfusion transmission. Pathogen reduction systems have been developed and are under development. At present, concerns remain with these systems regarding cost, process control, ability to inactivate high titres of viruses, killing of bacterial spores, product damage, genotoxicity and mutagenicity. The interventions of diversion, improved donor arm disinfection and bacterial screen testing are currently available, As such they can be implemented now to increase blood safety with no associated patient risk.

Detection of bacteria in platelet concentrates prepared from spiked single donations using cultural and molecular genetic methods

Bacteria show differences in their growth kinetics depending on the type of blood component. On to storage at 22 degrees C, platelet concentrates (PCs) seem to be more prone to bacterial multiplication than red cell concentrates. Knowledge of the potential for bacterial proliferation in blood components, which are stored at a range of temperatures, is essential before considering implementation of a detection strategy. The efficacy of bacterial detection was determined, using real-time reverse transcriptase-polymerase chain reaction (RT-PCR), following bacterial growth in blood components obtained from a deliberately contaminated whole-blood (WB) unit. Cultivation was used as the reference method. WB was spiked with 2 colony-forming units mL(-1)Staphylococcus epidermidis or Klebsiella pneumoniae, kept for 15 h at room temperature and component preparation was processed. Samples were drawn, at intervals throughout the whole separation process, from each blood component. Nucleic acids were extracted using an automated high-volume extraction method. The 15-h storage revealed an insignificant increase in bacterial titre. No bacterial growth was detected in red blood cell or plasma units. K. pneumoniae showed rapid growth in the pooled PC and could be detected immediately after preparation using RT-PCR. S. epidermidis grew slowly and was detected 24 h after separation. These experiments show that sampling is indicative at 24 h after preparation of PCs at the earliest to minimize the sampling error.

Sterility testing of platelet concentrates prepared from deliberately infected blood donations

BACKGROUND

In general the bacterial count in freshly donated blood is low and even lower in the corresponding platelet concentrates (PCs). By use of flow cytometry (FACS) for sterility testing, the reliability of early versus later sampling times was evaluated.

STUDY DESIGN AND METHODS

Blood donations were spiked with various numbers of Staphylococcus epidermidis, Staphylococcus aureus, Bacillus cereus, and Klebsiella pneumoniae. The corresponding PCs were prepared by the buffy-coat method and stored at 22 degrees C. A 20-mL sample was collected from each PC directly after preparation and after 8 hours. Samples were stored at 35 degrees C. Sterility testing of both PCs and samples was by FACS analysis at different time points.

RESULTS

All stored PCs were found positive by FACS analysis, with detection times ranging between 8 and 24 hours (K. pneumoniae, B. cereus), 8 and 91 hours (S. aureus), and 144 hours (S. epidermidis). In the samples incubated at 35 degrees C, bacteria were detected after 8 to 19 hours (K. pneumoniae, B. cereus), 8 to 67 hours (S. aureus), and 19 to 43 hours (S. epidermidis). Some of the samples did not contain bacteria.

CONCLUSION

Detection times for slow-growing bacteria are significantly shortened when PC samples are incubated at 35 degrees C: the numbers of bacteria in freshly prepared PCs may, however, be so low that the samples drawn for sterility testing do not contain a single bacterium. Our results do not support a shortening of the 24-hour or greater sampling time recommended by the manufacturers of established test systems, because also for consistent detection by FACS, bacteria need to grow in the PCs to sufficient numbers.