Out of the ground: aerial and exotic habitats of the melioidosis bacterium Burkholderia pseudomallei in grasses in Australia

Abstract

Summary Melioidosis is an emerging infectious disease of humans and animals in the tropics caused by the soil bacterium Burkholderia pseudomallei. Despite high fatality rates, the ecology of B. pseudomallei remains unclear. We used a combination of field and laboratory studies to investigate B. pseudomallei colonization of native and exotic grasses in northern Australia. Multivariable and spatial analyses were performed to determine significant predictors for B. pseudomallei occurrence in plants and soil collected longitudinally from field sites. In plant inoculation experiments, the impact of B. pseudomallei upon these grasses was studied and the bacterial load semi-quantified. Fluorescence-in-situ-hybridization and confocal laser-scanning microscopy were performed to localize the bacteria in plants. B. pseudomallei was found to inhabit not only the rhizosphere and roots but also aerial parts of specific grasses. This raises questions about the potential spread of B. pseudomallei by grazing animals whose droppings were found to be positive for these bacteria. In particular, B. pseudomallei readily colonized exotic grasses introduced to Australia for pasture. The ongoing spread of these introduced grasses creates new habitats suitable for B. pseudomallei survival and may be an important factor in the evolving epidemiology of melioidosis seen both in northern Australia and elsewhere globally.

Unprecedented Melioidosis Cases in Northern Australia Caused by an Asian Burkholderia pseudomallei Strain Identified by Using Large-Scale Comparative Genomics

“…The second scenario involves the inadvertent importation of B. pseudomallei-contaminated plant or soil material. B. pseudomallei can infect the rhizosphere, roots, and aerial parts of certain grasses, including exotic species (50). However, imported soil was ruled out as a source of B. pseudomallei infection in a highly unusual southern Arizonan melioidosis case (14); in addition, the high salinity of potting mix is not conducive to B. pseudomallei survival (51).…”

Burkholderia pseudomallei Genotype Distribution in the Northern Territory, Australia

“…This bacterium is also able to move in the water table 11 and presumably to track along waterways, to be transported in soil used for gardening, building and farming, and to live in the aerial parts of grasses. 12 In addition, B. pseudomallei has been isolated from the beak of a healthy bird, suggesting another possible mode of longdistance dissemination. 13 B. pseudomallei can also seed new endemic areas and cause long-term contamination of soil and water outside of the tropics.…”

Seasonal variation of soil environmental characteristics affect the presence of Burkholderia pseudomallei in Khon Kaen, Thailand

“…B. pseudomallei have been revealed to be associated with rhizosphere or root zone (Inglis et al, 2000;Levy et al, 2003). Recently, Kaestli et al (2011) revealed the localization of B. pseudomallei in exotic grasses in Australia which could provide hydrophilic environments to facilitate the spreading of the pathogens in soil in endemic area. These observations suggest that environmental factors apart from physico-chemical factors of soil are also involved in the survival of B. pseudomallei.…”