Burkholderia pseudomallei Type G in Western Hemisphere

Burkholderia pseudomallei isolates from the Western Hemisphere are difficult to differentiate from those from regions in which melioidosis is traditionally endemic. We used internal transcribed spacer typing to determine that B. pseudomallei isolates from the Western Hemisphere are consistently type G. Knowledge of this relationship might be useful for epidemiologic investigations.

B urkholderia 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 (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. 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
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 wholegenome 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).

Conclusions
In melioidosis cases for which the origin is unclear, such as for patients with unknown exposure histories or histories of travel to multiple regions to which melioidosis is endemic, ITS typing along with other molecular epidemiologic tools might be useful for assessing the B. pseudomallei origins. More insight into the relationship of Western Hemisphere isolates to isolates from regions to which melioidosis is highly endemic might come from wholegenome sequencing. As more isolates are analyzed, such studies might enable higher confidence in the geographic origin of isolates.
A.T. is supported by the United States Department of Homeland Security grant no. HSHQDC-10-C-00135. This study used the Multi Locus Sequence Typing website (www.mlst.net) at Imperial College London, developed by David Aanensen and funded by the Wellcome Trust.