The highest reported incidence worldwide of Burkholderia pseudomallei,
which is the causative agent of melioidosis, is in the Northeast of Thailand, where
saline soil and water are prevalent. Moreover, recent reports indicate a potential
pathogenic role for B. pseudomallei in cystic fibrosis lung disease, in which the airway
surface liquid has an increased concentration of sodium chloride (NaCl). These
observations raise the possibility that high salinity may represent a favorable niche for
B. pseudomallei. We performed a proteomics study to examine alterations in secreted
proteins (secretome) and a microarray analysis to investigate the global transcriptional
response of B. pseudomallei under salt stress (320 mM NaCl) compared to the normal
cultured condition in LB broth (170 mM NaCl). A comparison of secretome of
B. pseudomallei proteins during the salt stress revealed 42 protein spots representing
37 unique proteins, such as a drug resistant protein (beta-lactamase-like protein),
chaperones (i.e., GroEL) and potential virulence factors (i.e., peptidases in the M1
family). The increased level of a beta-lactamase-like protein suggested this bacterium
in a high salt environment might be resistant to beta-lactam antibiotics. Functional
analysis revealed that the B. pseudomallei grown under salt stress had significantly
greater resistance to the antibiotic ceftazidime. Additionally, transcriptome analysis of
B. pseudomallei under salt stress indicated several genes significantly up-regulated in
the presence of 320 mM NaCl, including genes associated with the bsa-encoded type
III secretion system (T3SS). Microarray data were verified by reverse transcriptasepolymerase
chain reactions (RT-PCR). Western blot analysis confirmed the increased
expression and secretion of the invasion-associated type III secreted protein BopE in
B. pseudomallei cultures at 320 mM NaCl. Furthermore, salt-treated B. pseudomallei
exhibited greater invasion efficiency into the lung epithelial cell line A549. These
findings suggest that salinity has the potential to influence the virulence and adaptation
of B. pseudomallei by modulating the transcription and secretion of a particular set of
elements, among which are the Bsa T3SS associated with virulence and the betalactamase-like
protein which contributes to its resistance to ceftazidime.