Burkholderia pseudomallei is the causative agent of the disease melioidosis. Melioidosis is considered endemic to Southeast Asia and northern Australia. However, sporadic cases do occur elsewhere in the world, especially in tropical areas (1).

The predominant method of molecular subtyping of B. pseudomallei is multilocus sequence typing (MLST), which is based on a comparison of the alleles of 7 housekeeping genes to generate a sequence type (ST) (2). These data can then be analyzed by using tools such as eBURST, which is used to infer phylogenetic patterns (3).

As of May 30, 2013, a total of 3,028 B. pseudomallei isolates were listed in the MLST database (www.mlst.net); these isolates are predominantly from Southeast Asia (1,036 isolates) and Australia (1,776). Some entries are from other Pacific areas (e.g., New Caledonia [9 isolates] and Hong Kong [39]) and other parts of the world (e.g., Africa [8], Europe [15], and the Western Hemisphere [30]) or of unknown origin (32).

For a population study, Pearson et al. analyzed the STs in the B. pseudomallei MLST database along with other data, such as single-nucleotide polymorphisms from whole-genome sequencing data (4). Their study indicated some population structures associated with geographic origin, in particular, clades associated with isolates from Southeast Asia or northern Australia. The data were used to support a hypothesis that B. pseudomallei originated on the Australian continent, spread to Southeast Asia, and from there spread throughout the world (4).

For an isolate of unknown origin, however, MLST alone may not provide information about geographic origin. For example, isolates from the Western Hemisphere have yielded STs unique to that hemisphere, but when these isolates were analyzed by eBURST or other methods, no distinct clade was found. In the clusters in which these STs appear, STs are intermixed with those from regions in which melioidosis is traditionally endemic, such as Southeast Asia (2,4). Therefore, a method for determining whether an isolate of unknown origin is from the Western Hemisphere is desirable.

Recently, Ligouri et al. developed a typing scheme by measuring the length polymorphisms in the 16S–23S internal transcribed spacer (ITS) of Burkholderia spp. (5). The typing scheme consists of 10 types: A, B, C, D, E, F, G, CE, GE, and GC. Ligouri et al. found that some types are unique to a given Burkholderia species (i.e., A = B. thailandensis, B = B. humptydooensis, D = B. oklahomensis, and F = B. cepacia). They also determined that type C could be found in B. mallei and in B. pseudomallei. The remaining 5 types were exclusive to B. pseudomallei (5). Ligouri et al. determined that types C, E, GE, and CE were the predominant types for isolates from northern Australia and Southeast Asia and that type G was rare in Australia (4 isolates) and Southeast Asia (3). They noted, on the basis of a limited number of strains from these regions, that type G was overrepresented in isolates from other parts of the world: Madagascar (1 isolate), Ecuador (2), Puerto Rico (2), Venezuela (1), and Kenya (1). They hypothesized that a genetic bottleneck occurred during the dispersal of type G to regions outside of Southeast Asia and Australia (5).

ITS typing of B. pseudomallei might be a powerful tool for linking cases of melioidosis to regions outside of those in which melioidosis in highly endemic, such as Southeast Asia and northern Australia. To further investigate this trend, we assessed the ITS types of B. pseudomallei from the Western Hemisphere.

The Study

Figure 1

Figure 1. . Diagram of eBURST (3) analysis of multilocus sequence types (STs) of Burkholderia pseudomallei isolates; default settings were used. Dots represent sequence types. Blue dot represents the calculated primary...

All tested Western Hemisphere isolates from our collection were ITS type G (Table). In addition to those isolates tested, we performed in silico analysis of whole-genome sequencing data from other B. pseudomallei isolates in our collection or publicly available data with origins in the Western Hemisphere and found them to also be ITS type G (Table). As expected, eBURST analysis of the STs from the type G strains that originated in the Western Hemisphere did not yield a discrete clade. These STs are interspersed with other STs that are predominantly from Southeast Asia (Figure). Our results support the findings of Ligouri et al. (5). The predominance of type G in isolates in our panel of isolates from the Western Hemisphere is consistent with a hypothesis that these isolates are derived from a bottleneck that occurred during dispersal to the rest of the world from Southeast Asia.

Other supporting evidence is provided by testing for the Yersinia-like fimbrial (YLF) gene by YLF PCR. The presence of the YLF gene is associated with B. pseudomallei isolates from Southeast Asia (6). For those strains tested, all were positive for YLF or the YLF gene was present according to in silico analysis of whole-genome sequencing data when available (Table).

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