elegans

[17] or tomato plant [18] infection models. There were some important differences in the relative virulence of isolates within each species in our models which GS-9973 are not reflected in mouse virulence data. In our macrophage and G. mellonella models, B. pseudomallei 708a was highly attenuated, to a level similar to that of the least virulent B. thailandensis isolates and both of the B. oklahomensis isolates. However, B. pseudomallei 708a is reported to be significantly more virulent than any B. thailandensis and B. oklahomensis isolates in mice [7, 16, 23]. B. pseudomallei 708a is a naturally occurring gentamicin sensitive isolate that, when compared to B. pseudomallei K96243,

contains a 131-kb deletion within chromosome I [23]. This deletion removes the amrAB-oprA operon P-gp inhibitor providing aminoglycoside resistance, which explains the low MIC of kanamycin for this strain (Table 1). The deletion also results in loss of genes coding for the anaerobic arginine deiminase pathway, clusters encoding cobalamin GSK2118436 and malleobactin iron uptake systems, and a putative type-1 fimbrial gene cluster [23]. Transcriptome data obtained from B. pseudomallei K96243 at day three after intranasal infection of BALB/c mice showed that genes involved in iron acquisition, including the malleobactin operon, were induced in vivo compared to bacteria grown in vitro in LB broth (C. Müller, unpublished data). The same genes are also upregulated under low iron conditions [24, 25], which suggests that B. pseudomallei encounters iron limited conditions in the mouse model of infection. The absence of these siderophore systems in strain 708a might also partly explain the observed intracellular replication defect in macrophages (Figure 1B). Overall, and bearing in mind the genome plasticity of B. pseudomallei

[26], we cannot be certain that the B. pseudomallei 708a isolate we have used in our study was genetically similar to the isolate previously tested in mice. It would therefore be valuable to re-test the B. pseudomallei 708a isolate we have used for virulence in mice. We also Chloroambucil identified differences in the virulence of B. thailandensis isolates, which were consistent between our macrophage growth, macrophage killing and G. mellonella models, but not with previously reported data on virulence in mice or hamsters. In our models, CDC301 and CDC272 were the most virulent isolates, whereas CDC301, E264 and Phuket were most virulent in mouse and hamster infection models [16]. A recent study revealed that both CDC strains belong to the same sequence type and are part of a distinct phylogenetic subgroup of B. thailandensis isolates that is separate from strains isolated in Thailand [27].