Co-Culture of Acinetobacter calcoaceticus-baumannii complex and Staphylococcus saprophyticus Supports Simple Point Contamination Model in Recent Cases of Transfusion-Related Sepsis

The CDC recently reported a series of four septic transfusion reactions across three states resulting from contamination of apheresis platelet products. The apheresis platelets were contaminated with strains of both Acinetobacter calcoaceticus-baumannii complex (ACBC) and Staphylococcus saprophyticus (Ss). Two of the reported septic transfusion reactions occurred at our institution. The CDC investigation showed that isolates of each species were genetically related and suggested a point source of contamination. However, the contamination of blood products with ACBC is rare and the co-occurrence of these two species in all four cases was unusual. We hypothesized that there was an augmentative interaction between the clinical isolates of ACBC and Ss from these cases that contributed to their repeated co-occurrence. To test this hypothesis, we compared the growth characteristics of ACBC and Ss when cultured together versus independently. We used isolates from the contaminated platelets for our studies and performed experiments using both solid and liquid growth media. Experiments on solid media assessed density of growth and macroscopic morphology after cross-streaking on Columbia Blood Agar (CBA) and Luria-Bertani (LB) agar. Experiments in liquid media assessed growth by CFU counts in LB broth, platelet-poor plasma, and apheresis platelets. Growth in apheresis platelets was performed using standard, room temperature blood bank platelet storage conditions. Results of these experiments showed a higher CFU concentration of Ss when co-cultured with ACBC in LB broth only after several days, as compared to Ss alone under the same conditions. Otherwise, there was no evidence of augmented growth by either CFU concentration or growth rate in LB broth, plasma, or platelets at other time points. Similarly, there was no evidence of augmented growth by colony density or morphology when cross-streaking strains on either CBA or LB agar. As a result, we conclude that the co-occurrence of these two species in platelets is likely a coincidence of the point contamination suggested by the CDC investigation and not the result of growth augmentation between the two species.