Biological relevance of colony morphology and phenotypic switching by Burkholderia pseudomallei

Faculty of Tropical Medicine, and Faculty of Medical Technology, Mahidol University, Bangkok, Thailand; Faculty of Allied Health Sciences, Chulalongkorn University, Bangkok, Thailand; Melioidosis Research Center, Faculty of Medicine, Khon Kaen University, Khon Kaen, Thailand; Center for Clinical Vaccinology and Tropical Medicine, Nuffield Department of Clinical Medicine, University of Oxford, Churchill Hospital, Oxford, UK

^* To whom correspondence should be addressed. Email: sharon{at}tropmedres.ac.

	Abstract

Melioidosis is notoriously protracted and difficult to cure. We hypothesize that the causative organism B. pseudomallei undergoes a process of adaptation involving altered expression of surface determinants which facilitates persistence in vivo, and that this is reflected by changes in colony morphology. A colony morphotyping scheme and typing algorithm was developed using clinical B. pseudomallei isolates. Morphotype was divided into seven Types (denoted I to VII). Type I gave rise to other morphotypes (most commonly Type II or III) by a process of switching in response to environmental stress including starvation, iron limitation and growth at 42°C. Switching was associated with complex shifts in phenotype, one of which (Type I to Type II), was associated with a marked increase in production of factors putatively associated with in vivo concealment. Isogenic Types II and III derived from Type I were examined using several experimental models. Switching between isogenic morphotypes occurred in a mouse model, in which Type II appeared to become adapted for persistence in a low virulence state. Isogenic Type II demonstrated a significant increase in intracellular replication fitness compared with parental Type I after uptake by epithelial cells in vitro. Isogenic Type III demonstrated a higher replication fitness following uptake by macrophages in vitro, associated with a switch to Type II. Mixed B. pseudomallei morphology was common in individual clinical specimens, and was significantly more frequent in samples of blood, pus and respiratory secretions compared with urine and surface swabs. These findings have major implications for therapeutics and vaccine development.