OmpR and Prc contribute to switch the Salmonella morphogenetic program in response to phagosome cues

In Vivo Cross-Linking Sheds Light on the Salmonella Divisome in Which PBP3 and PBP3SAL Compete for Occupancy

Bacterial cell division is orchestrated by proteins that assemble in dynamic complexes collectively known as the divisome. Essential monofunctional enzymes with glycosyltransferase or transpeptidase (TPase) activities, FtsW and FtsI respectively, engage in the synthesis of septal peptidoglycan (sPG). Enigmatically, Salmonella has two TPases that can promote cell division independently: FtsI (PBP3) and the pathogen-specific paralogue PBP3SAL. How Salmonella regulates the assembly of the sPG synthase complex with these two TPases, is unknown. Here, we characterized Salmonella division complexes in wild-type cells and isogenic mutants lacking PBP3 or PBP3SAL. The complexes were cross-linked in vivo and pulled down with antibodies recognizing each enzyme. Proteomics of the immunoprecipitates showed that PBP3 and PBP3SAL do not extensively cross-link in wild type cells, supporting the presence of independent complexes. More than 40 proteins cross-link in complexes in which these two TPases are present. Those identified with high scores include FtsA, FtsK, FtsQLB, FtsW, PBP1B, SPOR domain-containing proteins (FtsN, DedD, RlpA, DamX), amidase activators (FtsX, EnvC, NlpD) and Tol-Pal proteins. Other cross-linked proteins are the protease Prc, the elongasome TPase PBP2 and, D,D-endo- and D,D-carboxypeptidases. PBP3 and PBP3SAL localize at midcell and compete for occupying the division complex in response to environmental cues. Thus, a catalytic-dead PBP3SAL-S300A variant impairs cell division in a high osmolarity and acidic condition in which it is produced at levels exceeding those of PBP3. Salmonella may therefore exploit an 'adjustable' divisome to exchange TPases for ensuring cell division in distinct environments and, in this manner, expand its colonization capacities.

Evidence of two differentially regulated elongasomes in Salmonella

Cell shape is genetically inherited by all forms of life. Some unicellular microbes increase niche adaptation altering shape whereas most show invariant morphology. A universal system of peptidoglycan synthases guided by cytoskeletal scaffolds defines bacterial shape. In rod-shaped bacteria, this system consists of two supramolecular complexes, the elongasome and divisome, which insert cell wall material along major and minor axes. Microbes with invariant shape are thought to use a single morphogenetic system irrespective of the occupied niche. Here, we provide evidence for two elongasomes that generate (rod) shape in the same bacterium. This phenomenon was unveiled in Salmonella, a pathogen that switches between extra- and intracellular lifestyles. The two elongasomes can be purified independently, respond to different environmental cues, and are directed by distinct peptidoglycan synthases: the canonical PBP2 and the pathogen-specific homologue PBP2SAL. The PBP2-elongasome responds to neutral pH whereas that directed by PBP2SAL assembles in acidic conditions. Moreover, the PBP2SAL-elongasome moves at a lower speed. Besides Salmonella, other human, animal, and plant pathogens encode alternative PBPs with predicted morphogenetic functions. Therefore, contrasting the view of morphological plasticity facilitating niche adaptation, some pathogens may have acquired alternative systems to preserve their shape in the host.

OmpR (TCS response regulator) of Aeromonas veronii plays a major role in drug resistance, stress resistance and virulence by regulating biofilm formation

Aeromonas veronii (A. veronii), a highly pathogenic bacteria with a wide range of hosts, widely exists in the environment of humans, animals and aquatic animals, and can cause a variety of diseases. In this study, the receptor regulator ompR in the envZ/ompR of two-component system was selected to construct a mutant strain (Δ ompR) and a complement strain (C-ompR) to explore the regulatory effect of ompR on the biological characteristics and virulence of TH0426. The results showed that the ability of biofilm formation and osmotic stress of TH0426 were significantly reduced (P < 0.001), the resistance to ceftriaxone and neomycin were slightly down-regulate when the ompR gene was deleted. At the same time, animal pathogenicity experiments showed that the virulence of TH0426 was significantly down-regulated (P < 0.001). These results indicated that ompR gene regulates the biofilm formation of TH0426, and regulates some biological characteristics of TH0426, including drug sensitivity, resistance to osmotic stress, and also affects its virulence.

OmpR and Prc contribute to switch the Salmonella morphogenetic program in response to phagosome cues

Salmonella enterica serovar Typhimurium infects eukaryotic cells residing within membrane-bound phagosomes. In this compartment, the pathogen replaces the morphogenetic penicillin-binding proteins 2 and 3 (PBP2/PBP3) with PBP2_SAL /PBP3_SAL , two proteins absent in Escherichia coli. The basis for this switch is unknown. Here, we show that PBP3 protein levels drop drastically when S. Typhimurium senses acidity, high osmolarity and nutrient scarcity, cues that activate virulence functions required for intra-phagosomal survival and proliferation. The protease Prc and the transcriptional regulator OmpR contribute to lower PBP3 levels whereas OmpR stimulates PBP2_SAL /PBP3_SAL production. Surprisingly, despite being essential for division in E. coli, PBP3 levels also drop in non-pathogenic and pathogenic E. coli exposed to phagosome cues. Such exposure alters E. coli morphology resulting in very long bent and twisted filaments indicative of failure in the cell division and elongation machineries. None of these aberrant shapes are detected in S. Typhimurium. Expression of PBP3_SAL restores cell division in E. coli exposed to phagosome cues although the cells retain elongation defects in the longitudinal axis. By switching the morphogenetic program, OmpR and Prc allow S. Typhimurium to properly divide and elongate inside acidic phagosomes maintaining its cellular dimensions and the rod shape.