LETTER
The timely identification of Burkholderia pseudomallei is important to optimize the clinical management of the frequently lethal conditions it causes (1) and to prevent laboratory-acquired infections.
A 53-year-old fisherman from Da Nang, Vietnam, presented to an Australian hospital with a ruptured mycotic aortic aneurysm between the celiac trunk and superior mesenteric artery. Past history was significant for type 2 diabetes mellitus and untreated chronic hepatitis B infection.
After 36 h of incubation, the aerobic peripheral blood culture (Bactec BD, USA) flagged with a Gram-negative bacillus with bipolar staining. At 24 h of incubation at 35°C in atmospheric oxygen, there was equal growth on horse blood and MacConkey agar (bioMérieux, Australia) of moist cream-colored non-lactose-fermenting oxidase-positive colonies. The bacteria were also cultured from aortic tissue. The isolate was identified using matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS; Bruker Daltonics) using the standard database (version 7.0) as Burkholderia thailandensis, with an identification score of 1.97 (genus-level identification, 1.7 to 2.0; species-level identification, >2.0). B. pseudomallei was not a listed organism within the probability results. Given the suggestive epidemiological and clinical features for B. pseudomallei, the isolate was referred to the Menzies School of Health Research (Darwin, Australia) for definitive identification. A rapid lateral flow immunoassay (LFA) against the B. pseudomallei capsular polysaccharide (CPS) antigen (2) was positive (Fig. 1), and, likewise, the isolate was identified as B. pseudomallei using the Vitek 2 (93% probability) and API 20E systems after 48 h of growth. A B. pseudomallei species-specific PCR targeting the type III secretion system gene (3) was positive, providing conclusive identification. Interestingly, repeat mass spectrometry using the bioMérieux Vitek MS with the standard database also incorrectly identified the isolate as B. multivorans (99.9% probability).
Lateral flow immunoassay against B. pseudomallei capsular polysaccharide. Arrow 1, patient isolate; arrow 2, positive control.
Increasingly, MALDI-TOF MS is being used as a rapid and reliable technique for bacterial identification; however, the absence of reference spectra for rare and high-risk pathogens, such as B. pseudomallei, within the standard Bruker and Vitek MS databases may lead to misidentification of B. pseudomallei as the genetically similar but much less pathogenic B. thailandensis (4–7). Use of in-house-curated databases with B. pseudomallei isolates from a broad geographic distribution or commercial “security relevant” databases is associated with improved diagnostic accuracies of up to 100% (4, 5, 8–10). However, these databases are not routinely available to most clinical laboratories, and misidentifications can still occur if the security relevant databases are not fully operational.
This case demonstrates the potential utility of the recently developed LFA against the B. pseudomallei CPS antigen (2) as a rapid point-of-care test and adjunct to MALDI-TOF MS in routine clinical microbiology laboratories. The LFA is highly sensitive (98.7%) and specific (97.2%). It is also rapid (15 min) and can be performed on bacterial cultures or direct clinical samples (2), although its sensitivity is significantly lower (40%) on culture-unenriched whole blood (11). Although the LFA remains experimental and is yet to be FDA approved, its ability to resolve identification dilemmas renders it a highly useful diagnostic assay. The rapid latex agglutination assay (12) is another highly sensitive (99.1%) potential adjunctive test for B. pseudomallei identification.
- Copyright © 2018 American Society for Microbiology.
