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Investigating Human-to-Human Transmission Dynamics of Melioidosis Using Whole-Genome Sequencing
Conference poster   Open access

Investigating Human-to-Human Transmission Dynamics of Melioidosis Using Whole-Genome Sequencing

Ammar Aziz, Erin P Price, M Mayo, B J Currie and Derek S Sarovich
ASM Microbe Conference, 2019 (San Francisco, United States, 20-Jun-2019–24-Jun-2019)
American Society for Microbiology
2019
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Abstract

Medical Microbiology
Melioidosis, a tropical disease with high mortality, is caused by the environmental bacterium Burkholderia pseudomallei (Bp). Bp is recognized as a Tier 1 Select Agent in the United States due to its bioweaponization potential, combined with limited treatment options and no vaccine on the horizon. Infections are almost exclusively acquired through contact with contaminated environmental sources, with confirmed human-to-human transmission being exceedingly rare. In one such case, we used whole-genome sequencing (WGS) to characterize Bp transmission from a mother with mastitis to her baby. Three samples from each patient were examined by plating sputum, blood, breast milk and cerebrospinal fluid (CSF) onto selective Ashdown's agar. For each specimen, genomic DNA was extracted from the primary streak of the culture plate to identify putative mixtures. For the mother, a mixed specimen was identified from the primary streak of the sputum, with two identifiable multilocus sequence types (STs) present in the specimen (ST259 and ST261). The mother's breast milk contained only ST259 strains, which were subsequently transmitted to the suckling baby and confirmed by typing of the CSF and blood specimens. WGS of single colonies confirmed that only two STs were present in the mother, with no evidence of ST261 in the baby. WGS of the purified ST259 isolates revealed 0 and 1 single-nucleotide polymorphisms (SNPs) between mother and baby isolates, confirming transmission to the baby via breastfeeding. We next performed phylogenomic analysis of all isolates, including mixed specimens, to understand the effects of mixtures on tree topology. The inclusion of mixed specimens removed informative characters that resulted in branch collapse of all patient STs into a single clade. Additionally, incorrect topology with the erroneous placement of the ST261 isolate was observed. 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. Using current bioinformatics tools, we demonstrate a robust method for bacterial mixture identification and deconvolution.

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