Distribution of Burkholderia pseudomallei in northern Australia, a land of diversity

Abstract

Burkholderia pseudomallei is a Gram-negative soil bacillus that is the etiological agent of melioidosis and a biothreat agent. Little is known about the biogeography of this bacterium in Australia, despite its hyperendemicity in the northern region of this continent. The population structure of 953 Australian B. pseudomallei strains representing 779 and 174 isolates of clinical and environmental origins, respectively, was analyzed using multilocus sequence typing (MLST). Bayesian population structure and network SplitsTree analyses were performed on concatenated MLST loci, and sequence type (ST) diversity and evenness were examined using Simpson's and Pielou's indices and a multivariate dissimilarity matrix. Bayesian analysis found two B. pseudomallei populations in Australia that were geographically distinct; isolates from the Northern Territory were grouped mainly into the first population, whereas the majority of isolates from Queensland were grouped in a second population. Differences in ST evenness were observed between sampling areas, confirming that B. pseudomallei is widespread and established across northern Australia, with a large number of fragmented habitats. ST analysis showed that B. pseudomallei populations diversified as the sampling area increased. This observation was in contrast to smaller sampling areas where a few STs predominated, suggesting that B. pseudomallei populations are ecologically established and not frequently dispersed. Interestingly, there was no identifiable ST bias between clinical and environmental isolates, suggesting the potential for all culturable B. pseudomallei isolates to cause disease. Our findings have important implications for understanding the ecology of B. pseudomallei in Australia and for potential source attribution of this bacterium in the event of unexpected cases of melioidosis.

FIG 1

Origin of Burkholderia pseudomallei isolates used in this study. (Adapted from a map from the University of Melbourne Library Map Collection [http://www.lib.unimelb.edu.au/collections/maps/digital/outline-maps/aust-l.gif].)

FIG 2

Relative contribution of each Burkholderia pseudomallei sequence type (ST) to the average group similarity in various geographic regions across the Northern Territory and Queensland, Australia. The relative contribution of an ST is averaged across all pairs of isolates within a group over its standard deviation. The Bray-Curtis (or Sorensen) similarity of the ith ST between the jth and kth isolates [S_jk(i)] equals 100[2min(y_ij,y_ik)/Σ_i=1^p(y_ij, y_ik)] or, simply, it is 100 if both isolates share the same ST or 0 if they do not. The more abundant an ST is in a group, the higher its contribution to the within-group similarity. The combined STs shown make up approximately 90% of the overall ST contribution for each sampled geographic region. As shown by the average group similarity (gray line), the larger the region sampled, the smaller the group similarity and greater the ST diversity.

FIG 3

Rarefaction curves reflecting the sampling effort of Burkholderia pseudomallei isolates compared to the sequence type diversity for the Darwin region and Remote Top End. Vertical bars extending from each curve represent 95% confidence intervals.

FIG 4

Burkholderia pseudomallei population structure based on Bayesian analysis of MLST-concatenated sequences. Each colored vertical line represents a B. pseudomallei isolate. Green, population 1; red, population 2. Isolates are in order of location, as indicated by the labeled braces. NT, Northern Territory; QLD, Queensland.