Virulence from the rhizosphere: ecology and evolution of Burkholderia pseudomallei-complex species
Affiliation:
1. California NanoSystems Institute, UCLA, 570 Westwood Plaza Bldg. 114, 4538 West, Los Angeles, CA 90095, United States;2. Molecular Biology Institute, UCLA, 611 Charles E. Young Drive East, Los Angeles, CA 90095, United States;3. Department of Microbiology, Immunology, and Molecular Genetics, UCLA, 609 Charles E. Young Drive East, Los Angeles, CA 90095, United States;4. Department of Pathology, Stanford University, Lane Building, L235, 300 Pasteur Drive, Stanford, CA, 94305, United States;5. Northern Arizona University, Department of Biological Sciences, Pathogen and Microbiome Institute 1395 S Knoles Drive, Flagstaff, AZ 86011, United States
Abstract:
Burkholderia pseudomallei (Bp) is an environmental organism that inhabits the rhizosphere, a rich and diverse zone of soil supported by a network of plant roots. In the rhizosphere, Bp faces intense competition with other bacterial species and fungi, as well as predation by protozoa, nematodes, and insects. Our hypothesis is that Bp virulence determinants that promote disease in mammals arose to defend against environmental predators. In this context, infections of humans and animals are likely to be incidental occurrences. We speculate that most Bp virulence determinants that are critical for intracellular survival could have relevant roles for interactions with ecological species, including the Bsa type III secretion system (T3SSBsa), BimA-mediated actin polymerization and type VI secretion (T6SS-5) in promoting intracellular survival and cell-cell spread in nematodes, grazing insects and social amoebae. Bp possesses two more T3SSs and numerous polyketide/non-ribosomal peptide synthetases (PKS/NRPS) that resemble systems in phytopathogenic bacteria, hinting at its ability to interact with plants or fungi. Bp is capable of adopting a lifestyle of obligate parasitism, as shown by the divergence of the equine host-restricted B. mallei. Some environmental species that prey on Bp may double as host organisms, and although not proven, it is conceivable that Bp may use them as a survival niche. Understanding the lifestyle of Bp in an ecological context will shed light on the mechanisms that promote human infections, and may potentially help explain the broad differentials in virulence and the genetic diversity that is a hallmark of the species.
