Burkholderia pseudomallei virulence: definition, stability and associationwith clonality

Clinical presentations of melioidosis, caused by Burkholderia pseudomallei are protean, but the mechanisms underlying development of the different for ms of disease remain poorly understood. In murine melioidosis, the level of virulence of B. pseudomallei is important in disease pathogenesis and prog ression. In this study, we used B. pseudomallei-susceptible BALB/c mice to determine the virulence of a library of clinical and environmental B. pseud omallei isolates from Australia and Papua New Guinea. Among 42 non-arabinos e-assimilating (ara(-)) isolates, LD,(, ranged from 10 to > 10(6) CFU. Ther e were numerous correlations between virulence and disease presentation in patients; however, this was not a consistent observation. Virulence did not correlate with isolate origin (i.e. clinical vs environmental), since nume rous ara- environmental isolates were highly virulent. The least virulent i solate was a soil isolate from Papua New Guinea, which was arabinose assimi lating (ara(+)). Stability of B. pseudomallei virulence was investigated by in vivo passage of isolates through mice and repetitive in vitro subcultur e. Virulence increased following in vivo exposure in only one of eight isol ates tested. In vitro subculture on ferric citrate-containing medium caused attenuation of virulence, and this correlated with changes in colony morph ology. Pulsed-field gel electrophoresis and randomly amplified polymorphic DNA typing demonstrated that selected epidemiologically related isolates th at had variable clinical outcomes and different in vivo virulence were clon al strains. No molecular changes were observed in isolates after in vivo or in vitro exposure despite changes in virulence. These results indicate tha t virulence of selected B. pseudomallei isolates is variable, being depende nt on factors such as iron bioavailability. They also support the importanc e of other variables such as inoculum size and host risk factors in determi ning the clinical severity of melioidosis. (C) 2001 Editions scientifiques et medicales Elsevier SAS.