PRJNA559002
Investigating human-to-human transmission event of Burkholderia pseudomallei
Melioidosis, a tropical disease with high mortality, is caused by the environmental bacterium Burkholderia pseudomallei. Infections are almost exclusively acquired through contact with contaminated environmental sources, with confirmed human-to-human transmission being exceedingly rare. In one such transmission event, we used whole-genome sequencing (WGS) to characterize a B. pseudomallei transmission event from a mother with mastitis to her child. Specimens from each patient were examined by plating sputum, blood, breast milk and cerebrospinal fluid (CSF) onto selective Ashdown's agar. For each sample, DNA was extracted from pure colonies and from the primary culture to identify putative mixtures. A mixture of two strains was identified in the mother's sputum, with two identifiable multilocus sequence types (STs 259 and 261) present in this specimen and in purified colonies. The mother's breast milk contained only ST259, which was subsequently transmitted to the suckling child, as confirmed by MLST and WGS of the clinical samples (CSF and blood specimens). WGS confirmed that only two STs were present in the mother, with no evidence of ST261 in the child. Analysis of purified ST259 isolates identified identical isolates from the mother and child, confirming transmission to the child via breastfeeding. Phylogenomic analysis of all isolates, including the mixed sputum sample, was performed to investigate the effects of mixtures on phylogenetic inference. The inclusion of the ST261/ST259 mixture reduced the number of available informative SNPs to construct the tree, which resulted in branch collapse of ST259 isolates. Additionally, incorrect topology with the erroneous placement of ST261 isolates was observed. Using current bioinformatic tools, we demonstrate a robust method for bacterial mixture identification and deconvolution. These results show that, while phylogenomic analysis can provide clues to the presence of mixtures within WGS samples, such methodology is error-prone if mixtures are not anticipated and omitted.