Oxidoreductases and RNA degradosome controlling virulence-associated traits of Salmonella enterica serovar Typhimurium

The oxidative stress response is a fundamental and primitive mode of innate immune defense in nearly all forms of life. The host oxidative stress response becomes truly important and effective especially against intracellular pathogens such as Salmonella. Pathogens have evolved diverse mechanisms to withstand the oxidative responses. These include the use of oxidoreductases to neutralize oxidative products and repair oxidative damages, rapid alteration in their transcriptome and diversion of such oxidative products in cellular signaling, for their survival and successful infection.

In the first study in this thesis, we have introduced the putative ScsABCD oxidoreductases of the thioredoxin superfamily for their role in oxidative stress tolerance and in virulence of S. Typhimurium. We demonstrated that ScsABCD proteins are dispensable for invasion in cultured epithelial cells under normal invasive conditions, although ScsABCD acts as a suppressor of SPI-1 mediated invasion upon oxidative stress. Our results have further shown a functional association between ScsABCD and thiroredoxin 1 (TrxA) oxidoreductase of S. Typhimurium. In this, we demonstrated that absence of ScsABCD restored the invasiveness of a trxA mutant in epithelial cells and its virulence in C. elegans. (Paper I).

Next, we present the analyses on the role of periplasmic Dsb oxidoreductase system in S. Typhimurium’s biofilm-development, specifically under redox stress. In this, we show that DsbA and DsbB act as suppressors of rdar-morphotype development and affect biofilm-regulation using either Csg-dependent or -independent mechanism, respectively. Our results further reveal that oxidative stress abrogates rdar-morphotype of S. Typhimurium, whereas reductive stress reduces rdar-morphotype with concomitant plentiful release of extracellular slimy material containing, notably, the extracellular DNA (eDNA). Furthermore, we have demonstrated the oxidative recovery of swimming motility defects of a dsbA mutant. (Paper II).

Finally, we have demonstrated that exoribonuclease; PNPase and its genetic associate membrane lipoprotein NlpI constitute an operon and are functionally connected (Papers III and IV). PNPase was required for rdar-morphotype development whereas, NlpI suppresses the biofilm formation. In addition, we established the association of PNPase with c-di-GMP metabolism in biofilm regulation. Moreover, we showed that both PNPase and NlpI are required, independently, for cold adaptation of S. Typhimurium.

List of scientific papers

I. NAEEM ANWAR, Xiao Hui Sem and Mikael Rhen. Oxidoreductases that act as conditional virulence suppressors in Salmonella enterica serovar Typhimurium. PLoS One. 2013 Jun; 8(6):e64948.
https://doi.org/10.1371/journal.pone.0064948

II. NAEEM ANWAR, Ute Römling and Mikael Rhen. Redox-sensitivity of biofilm-formation in Salmonella enterica serovar Typhimurium – A particular impact of the DsbA/DsbB redox system. [Manuscript]

III. Syed Fazle Rouf, Irfan Ahmad, NAEEM ANWAR, Suman Kumar Vodnala, Abdul Kader, Ute Römling and Mikael Rhen. Opposing contributions of polynucleotide phosphorylase and the membrane protein NlpI to biofilm formation by Salmonella enterica serovar Typhimurium. Journal of Bacteriology. 2011 Jan; 193(2): 580-2.
https://doi.org/10.1128/JB.00905-10

IV. Syed Fazle Rouf, NAEEM ANWAR, Mark O Clements and Mikael Rhen. Genetic analysis of the pnp-deaD genetic region reveals membrane lipoprotein NlpI as an independent participant in cold acclimatization of Salmonella enterica serovar Typhimurium. FEMS Microbiology Letters. 2011 Dec; 325(1): 56-63.
https://doi.org/10.1111/j.1574-6968.2011.02416.x