Morphogenetic penicillin-binding proteins control virulence-associated type III secretion systems in Salmonella

Type III protein secretion systems (T3SSs) function as multiprotein devices that span the envelope of Gram-negative bacteria using the peptidoglycan (PG) layer as scaffold. This spatial arrangement explains why modifications in PG structure can alter T3SS activity. In Salmonella, incorporation of non-canonical D-amino acids in the PG was shown to decrease the activity of the T3SS encoded by the pathogenicity island-1 (SPI-1) without affecting other T3SS, like the flagellum apparatus. Enigmatically, following invasion of host cell Salmonella enterica serovar Typhimurium modifies PG synthesis by upregulating two pathogen-specific enzymes, the penicillin-binding proteins PBP2_SAL and PBP3_SAL, with roles in cell elongation and division, respectively. In the mouse typhoid model, the amount of PBP2_SAL and PBP3_SAL produced by the pathogen exceeds by large those of the canonical enzymes PBP2 and PBP3. This change responds to acidity and high osmolarity, the same cues that intra-phagosomal S. Typhimurium perceives to switch the SPI-1 T3SS by that encoded in SPI-2. Using isogenic mutants lacking each of the four morphogenetic PBPs, we tested whether their activities and those of the T3SS encoded by SPI-1 and SPI-2, are interconnected. Our data show that PBP2 is required for proper function of SPI-1 T3SS but dispensable for motility, whereas the lack of any of the morphogenetic PBPs increases SPI-2 T3SS activity. The positive control exerted by PBP2 on SPI-1 takes place via the SPI-1-specific regulators HilA and InvF. To our knowledge, these findings provide the first evidence linking morphogenetic enzymes that synthesize PG with T3SS associated to virulence.