Nonrandom distribution of Burkholderia pseudomallei clones in relation to geographical location and virulence

Abstract

Burkholderia pseudomallei is a soil-dwelling saprophyte and the causative agent of melioidosis, a life-threatening human infection. Most cases are reported from northeast Thailand and northern Australia. Using multilocus sequence typing (MLST), we have compared (i) soil and invasive isolates from northeast Thailand and (ii) invasive isolates from Thailand and Australia. A total of 266 Thai B. pseudomallei isolates were characterized (83 soil and 183 invasive). These corresponded to 123 sequence types (STs), the most abundant being ST70 (n=21), ST167 (n=15), ST54 (n=12), and ST58 (n=11). Two clusters of related STs (clonal complexes) were identified; the larger clonal complex (CC48) did not conform to a simple pattern of radial expansion from an assumed ancestor, while a second (CC70) corresponded to a simple radial expansion from ST70. Despite the large number of STs, overall nucleotide diversity was low. Of the Thai isolates, those isolated from patients with melioidosis were overrepresented in the 10 largest clones (P<0.0001). There was a significant difference in the classification index between environmental and disease isolates (P<0.001), confirming that genotypes were not distributed randomly between the two samples. MLST profiles for 158 isolates from Australia (mainly disease associated) contained a number of STs (96) similar to that seen with the Thai invasive isolates, but no ST was found in both populations. There were also differences in diversity and allele frequency distribution between the two populations. This analysis reveals strong genetic differentiation on the basis of geographical isolation and a significant differentiation on the basis of virulence potential.

FIG. 1.

eBURST of 266 Thai B. pseudomallei isolates obtained from the environment (n = 83) or associated with human melioidosis (n = 183), together with 158 isolates from northern Australia which were mainly disease associated. Isolates from Thailand are labeled in blue, and those from Australia are labeled in black. The majority of Thai isolates correspond to a single large clonal complex (CC48) which does not conform to a simple pattern of radial expansion. A second clonal complex (CC70) much more closely corresponds to a simple radial expansion from a founding genotype, ST70. A much higher degree of variation is observed for the Australian population.

FIG. 2.

Neighbor-joining tree using the concatenated sequences of all seven loci for Thai and Australian isolates (n = 266 and n = 158, respectively). Isolates from Thailand are labeled in red; those from Australia are labeled in blue. The majority of Australian isolates appear to belong to a single clade, although a small number of Thai isolates also cluster within this group, possibly reflecting recent migration events. A clade composed entirely of Thai isolates is also apparent.