Accidental Exposure to Burkholderia pseudomallei in the Laboratory in the Era of Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry

LETTER

Burkholderia pseudomallei is the causative agent of melioidosis, a disease that can vary in severity from fulminant sepsis to chronic infection with fatality rates of up to 42% (1, 2). As world travel has increased, so have melioidosis rates in travelers to countries where melioidosis is endemic, particularly Thailand and northern Australia (3). B. pseudomallei is considered a category B bioterrorism (BT) agent, and despite efforts of clinical microbiology laboratories to quarantine potential B. pseudomallei isolates, laboratory worker exposures have been reported (4, 5). Here, we report a case of accidental laboratory exposure to B. pseudomallei following genus-level identification by matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS).

A 65-year-old male was vacationing in Thailand for 3 days, hiking in rural areas and near waterfalls, when he suffered a myocardial infarction. He was hospitalized for 7 days prior to returning to the United States. Approximately 1 month later, he developed dysuria and urinary frequency and was prescribed nitrofurantoin. His symptoms did not improve after 4 days of antibiotics, and a urine specimen was then obtained for culture. After 24 h of incubation, growth of small colonies was noted on sheep blood agar, with no growth evident on MacConkey agar, and was initially reported as 1,900 CFU/ml of Gram-positive flora. At 48 h, a pure culture of gray, medium-sized colonies was growing equally well on blood and MacConkey agars. The oxidase-positive Gram-negative rod (GNR) was identified by MALDI-TOF MS (Bruker Daltonics database v3.3; Bruker, Billerica, MA) as Burkholderia thailandensis with a log score value of 1.864 (genus-level identification [ID] score value of 1.7 to 2.0), while concurrent susceptibility testing was performed (TREK; Thermo Fisher Scientific, Oakwood Village, OH). The following day, the culture was discussed during daily rounds, at which point the culture plates were sealed over concern for the possibility of B. pseudomallei. The Washington State Public Health Laboratory identified the organism as B. pseudomallei with a species-specific PCR assay. An extensive investigation by the University of Washington Employee Health Center and King County Public Health ensued to determine the extent of employee exposure in the laboratory. Following recommendations by Peacock et al. (5) for assessment of B. pseudomallei exposure risk, 21 employees were categorized into high- and low-risk exposure groups. Regardless of the exposure risk, all 21 employees were offered antibiotic prophylaxis. Serology testing was performed on exposed individuals, and no employees seroconverted.

BT organisms have been successfully identified by MALDI-TOF MS using custom databases (6–9); however, the Bruker Biotyper database v4.0 (as well as earlier database versions) contains no spectra from BT agents. Although Bruker has developed a database containing 53 spectra from 6 BT agents, it currently has limited availability. The Vitek MS IVD database contains spectra for Bacillus anthracis and Yersinia pestis, but these are not FDA cleared. The Vitek MS RUO database v4.12 contains a number of BT agent spectra (including B. pseudomallei), but this database is also not FDA cleared. Therefore, the ability of technologists to recognize and quarantine BT agents is imperative. Nevertheless, because B. pseudomallei grows on both blood and MacConkey agars and is nonpigmented and nonhemolytic, it can be difficult to distinguish from other non-lactose-fermenting Gram-negative rods, particularly in young cultures where the characteristic wrinkled colony morphology is not yet apparent (10). With the increasing use of MALDI-TOF MS for bacterial identifications, it is likely that this pathogen may continue to go unrecognized. Until such time that MALDI databases for BT agents have more widespread accessibility, clinical laboratory scientists and directors should be wary of genus-level identification of Burkholderia species by MALDI-TOF MS.