  Microorganisms, such as bacteria, are one of the first targets of
nanoparticles in the environment. In this study, we tested the effect of two
nanoparticles, ZnO and TiO2, with the salt ZnSO4 as the control, on the
Gram-positive bacterium Bacillus subtilis by 2D gel electrophoresis-based
proteomics. Despite a significant effect on viability (LD50), TiO2 NPs had no
detectable effect on the proteomic pattern, while ZnO NPs and ZnSO4
significantly modified B. subtilis metabolism. These results allowed us to
conclude that the effects of ZnO observed in this work were mainly attributable
to Zn dissolution in the culture media. Proteomic analysis highlighted twelve
modulated proteins related to central metabolism: MetE and MccB (cysteine
metabolism), OdhA, AspB, IolD, AnsB, PdhB and YtsJ (Krebs cycle) and XylA,
YqjI, Drm and Tal (pentose phosphate pathway). Biochemical assays, such as free
sulfhydryl, CoA-SH and malate dehydrogenase assays corroborated the observed
central metabolism reorientation and showed that Zn stress induced oxidative
stress, probably as a consequence of thiol chelation stress by Zn ions. The
other patterns affected by ZnO and ZnSO4 were the stringent response and the
general stress response. Nine proteins involved in or controlled by the
stringent response showed a modified expression profile in the presence of ZnO
NPs or ZnSO4: YwaC, SigH, YtxH, YtzB, TufA, RplJ, RpsB, PdhB and Mbl. An
increase in the ppGpp concentration confirmed the involvement of the stringent
response during a Zn stress. All these metabolic reorientations in response to
Zn stress were probably the result of complex regulatory mechanisms including
at least the stringent response via YwaC.
