Biogeography of Burkholderia pseudomallei in the Torres Strait Islands of Northern Australia

Abstract

It has been hypothesized that biogeographical boundaries are a feature of Burkholderia pseudomallei ecology, and they impact the epidemiology of melioidosis on a global scale. This study examined the relatedness of B. pseudomallei sourced from islands in the Torres Strait of Northern Australia to determine if the geography of isolated island communities is a determinant of the organisms' dispersal. Environmental sampling on Badu Island in the Near Western Island cluster recovered a single clone. An additional 32 clinical isolates from the region were sourced. Isolates were characterized using multilocus sequence typing and a multiplex PCR targeting the flagellum gene cluster. Gene cluster analysis determined that 69% of the isolates from the region encoded the ancestral Burkholderia thailandensis-like flagellum and chemotaxis gene cluster, a proportion significantly lower than that reported from mainland Australia and consistent with observations of isolates from southern Papua New Guinea. A goodness-of-fit test indicated that there was geographic localization of sequence types throughout the archipelago, with the exception of Thursday Island, the economic and cultural hub of the region. Sequence types common to mainland Australia and Papua New Guinea were identified. These findings demonstrate for the first time an environmental reservoir for B. pseudomallei in the Torres Strait, and multilocus sequence typing suggests that the organism is not randomly distributed throughout this region and that seawater may provide a barrier to dispersal of the organism. Moreover, these findings support an anthropogenic dispersal hypothesis for the spread of B. pseudomallei throughout this region.

Fig 1

Map of Australasia and the Torres Strait. (A) During the last glacial maximum (21,500 years ago). The shaded regions represent what was dry land during the period. Note that Australia and PNG composed a single continent (Sahul) and that most of Southeast Asia (Sunda) was linked by land bridges (30). (B) The Torres Strait lies between Australia and New Guinea and comprises some 274 individual islands. The number of isolates collected from each island (Is.) are shown in parentheses. Panel B was adapted from a map by the Department of Agriculture, Fisheries and Forestry, Australian Government, with permission.

Fig 2

eBURST diagram constructed using Burkholderia pseudomallei isolates from the Torres Strait. Burkholderia pseudomallei isolates from the Torres Strait region (in red, numbered with sequence type identifiers) are compared to those from the rest of the world (in black). Connecting lines indicate single-locus variants, while the circle size for each ST is indicative of the number of isolates of each sequence type recorded in the MLST database. Isolates in the outer ring differ from isolates in the central cluster by two loci, while ST598 and ST605 were not plotted because they were divergent from the central cluster at three or more loci. Each asterisk indicates a YLF genotype.