SGM Journals
Case Reports

Misidentification of Burkholderia pseudomallei as Burkholderia cepacia by the VITEK 2 system

Zhiyong Zong, Xiaohui Wang, Yiyun Deng, Taoyou Zhou

  • 1Center of Infectious Diseases, West China Hospital, Sichuan University, Chengdu, PR China
  • 2State Key Laboratory of Biotherapy, Sichuan University, Chengdu, PR China
  • 3Intensive Care Unit, West China Hospital, Sichuan University, Chengdu, PR China
  • Correspondence
    Zhiyong Zong zongzhiy{at}scu.edu.cn

    • Journal of Medical Microbiology 2012; 61(Pt 10):1483–1484 · https://doi.org/10.1099/jmm.0.041525-0

    View at publisher PubMed

    Abstract

    A previously healthy Chinese male returned from working in the Malaysian jungle with a fever. A blood culture grew Gram-negative bacilli that were initially identified as Burkholderia cepacia by the VITEK 2 system but were subsequently found to be Burkholderia pseudomallei by partial sequencing of the 16S rRNA gene. The identification of B. pseudomallei using commercially available automated systems is problematic and clinicians in non-endemic areas should be aware of the possibility of melioidosis in patients with a relevant travel history and blood cultures growing Burkholderia spp.

    Case report

    A previously healthy Chinese male who had been working in the Malaysian jungle returned with a high fever. Blood cultures were taken on admission to hospital and grew non-fermenting Gram-negative rods that were identified as Burkholderia cepacia (91 % probability) by the VITEK 2 compact semi-automated system using the GN (Gram-negative) card and the Advanced Expert System version VT2-R4.01 (bioMérieux). As melioidosis was suspected based on the presence of disseminated abscesses and the travel history, the 16S rRNA gene of this isolate (designated WCBP1) was amplified with the universal primers 27F and 1492R (Lane, 1991). Amplicons were purified using a commercial kit (Omega) and sequenced on an ABI 3730xl DNA Analyser (Applied Biosystems) at the Beijing Genomics Institute (Beijing, China). Similarity searches of the sequences obtained were carried out against the GenBank, EzTaxon and LeBIBI databases. The 1011 bp partial 16S rRNA gene sequence of WCBP1 was identical to Burkholderia pseudomallei strain Had_B73694 (GenBank accession no. FJ426359), which was recovered in a patient from Thailand (Cahn et al., 2009). WCBP1 was therefore actually B. pseudomallei and a diagnosis of acute melioidosis was made.

    Discussion

    Both B. pseudomallei and B. cepacia can cause life-threatening infections but the two species present different levels of risk to human health. B. pseudomallei is a Centers for Disease Control and Prevention category B agent with biological warfare potential that is classified as biosafety level 3 and can cause laboratory-acquired infections in exposed workers, while B. cepacia is not classified as a biological threat agent and is classified as biosafety level 2. Rapid and correct identification of both Burkholderia species is important for making therapeutic decisions and taking safety measures. However, commercially available automated microbiological systems that are widely used in diagnostic laboratories, VITEK 2, Pheonix (BD) and MicroScan WalkAway 96 (Siemens), do not reliably identify B. pseudomallei.

    The misidentification of B. pseudomallei by VITEK 2 has been described in several reports (Kiratisin et al., 2007; Lowe et al., 2002). The earlier fluorometric-based GN bacterial identification card of the VITEK 2 was only able to correctly identify a minority (19 %) of B. pseudomallei isolates (Lowe et al., 2002). The later colorimetric-based GN identification card has a better performance for the identification of B. pseudomallei but the accuracy is still suboptimal, varying from 63 to 81 % (Deepak et al., 2008; Kiratisin et al., 2007; Lowe et al., 2006), depending on the media used for culture (Lowe et al., 2006). B. pseudomallei was not in the database of the BD Phoenix System and in a previous study most isolates (34 of 47) were misidentified as B. cepacia by this system (Koh et al., 2003). MicroScan WalkAway 96 exhibited 96 % accuracy for identifying B. pseudomallei (Kiratisin et al., 2007). However, WalkAway 96 is not acceptable for the identification of B. pseudomallei as it gave low discrimination for B. pseudomallei with only 81.8 % probability and had a poor specificity with 57 % of B. cepacia isolates being incorrectly identified as B. pseudomallei (Kiratisin et al., 2007).

    The accuracy of the widely used API 20NE manual system for the identification of B. pseudomallei varied significantly, from 37 to 99 %, in different studies. Combining the results of eight published studies (Amornchai et al., 2007; Dance et al., 1989; Deepak et al., 2008; Glass & Popovic, 2005; Inglis et al., 1998, 2005; Kiratisin et al., 2007; Lowe et al., 2002), API 20NE correctly identified 1490 out of 1598 isolates, corresponding to 93 % accuracy. However, API 20NE has a longer turnaround time (up to 48 h) than automated systems and some isolates are still not correctly identified by this system, so API 20NE is not also ideal for the identification of B. pseudomallei.

    Other methods including fluorescence in situ hybridization (FISH), matrix-assisted laser desorption ionization time-of-flight MS technology (Giebel et al., 2010), latex agglutination tests and GLC have been developed to improve the identification of B. pseudomallei. The FISH method exhibited 100 % sensitivity and 100 % specificity in the identification of B. pseudomallei in a small-scale study but requires further validation (Hagen et al., 2011). FISH also requires fluorescence microscopy equipment and is not practical for many diagnostic laboratories. The latex agglutination test was highly sensitive (99.5 % sensitivity) and specific (100 %) in identifying B. pseudomallei isolates but requires specific antibodies that are not commonly available in non-endemic areas (Amornchai et al., 2007). GLC analysis of bacterial fatty acid methyl esters correctly identified 98 % of B. pseudomallei isolates but requires specific equipment (Inglis et al., 2005). Correct identification of B. pseudomallei remains a challenge in non-endemic areas, but PCR-based methods appear to be a reliable and practical choice for the identification of B. pseudomallei, as described by Kiratisin et al. (2007) and shown here.

    In conclusion, the identification of B. pseudomallei by commercially available automated systems is problematic. Clinicians and microbiologists should be aware of the possibility of misidentification of B. pseudomallei by automated systems. In particular, physicians in regions where melioidosis is not endemic should consider the possibility of melioidosis based on travel history and clinical manifestations when encountering isolates from blood samples that are identified as B. cepacia.

    Acknowledgements

    The authors are grateful to Dr Sally Partridge, Center of Infectious Diseases and Microbiology, Westmead Hospital, The University of Sydney, Australia, for the critical reading of this paper.

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