Molecular phylogeny of Burkholderia pseudomallei from a remote region of Papua New Guinea

Abstract

Background: The island of New Guinea is located midway between the world's two major melioidosis endemic regions of Australia and Southeast Asia. Previous studies in Papua New Guinea have demonstrated autochthonous melioidosis in Balimo, Western province. In contrast to other regions of endemicity, isolates recovered from both environmental and clinical sources demonstrate narrow genetic diversity over large spatial and temporal scales.

Methodology/principal findings: We employed molecular typing techniques to determine the phylogenetic relationships of these isolates to each other and to others worldwide to aid in understanding the origins of the Papua New Guinean isolates. Multi-locus sequence typing of the 39 isolates resolved three unique sequence types. Phylogenetic reconstruction and Structure analysis determined that all isolates were genetically closer to those from Australia than those from Southeast Asia. Gene cluster analysis however, identified a Yersinia-like fimbrial gene cluster predominantly found among Burkholderia pseudomallei derived from Southeast Asia. Higher resolution VNTR typing and phylogenetic reconstruction of the Balimo isolates resolved 24 genotypes with long branch lengths. These findings are congruent with long term persistence in the region and a high level of environmental stability.

Conclusions/significance: Given that anthropogenic influence has been hypothesized as a mechanism for the dispersal of B. pseudomallei, these findings correlate with limited movement of the indigenous people in the region. The palaeogeographical and anthropogenic history of Australasia and the results from this study indicate that New Guinea is an important region for the further study of B. pseudomallei origins and dissemination.

Figure 1. Map of Australasia.

a) 21,500 years ago during the last glacial maximum. The shaded regions represent what was dry land during the period. Note that Australia and PNG comprised a single continent (Sahul) and that most of Southeast Asia (Sunda) was linked by land bridges . b) The Balimo region of the Western Province of Papua New Guinea on the Aramia River floodplain.

Figure 2. Maximum likelihood tree of B. pseudomallei isolates using MLST data.

Two individual regions from a maximum likelihood tree constructed using the entire multi-locus sequence typing database showing the relationships of Papua New Guinean B. pseudomallei isolates (highlighted in grey) to other sequence types (STs). All three PNG STs fell into regions of the tree heavily dominated by Australian STs.

Figure 3. Neighbor joining tree constructed from B. pseudomallei MLVA data.

The isolates in box A represent those with very short branch lengths and resistance to chloramphenicol, suggesting a high level of relatedness.