Unsaturated long-chain free fatty acids are input signals of the Salmonella enterica PhoP/PhoQ regulatory system

VIARENGO, GASTÓN; SALAZAR, MARIO O.; FURLÁN, RICARDO L.E.; GARCÍA VÉSCOVI, ELEONORA

Congreso; IX Congreso de Microbiología General - Sociedad Argentina de Microbiología General (SAMIGE); 2013

Salmonella is an enteropathogen that causes a wide range of diseases in humans and animals. During infection, bacteria continuously interact with the surrounding media, in order to coordinate the expression of genes required for invasion and colonization of the host. The PhoP/PhoQ system consists of an orthodox two component regulatory system that serves as a master regulator of Salmonella virulence. The regulon governed by the PhoP/PhoQ system includes genes that are critical for Mg2+ homeostasis and those that provoke modifications of the LPS. This regulatory system is also involved in the bacterial entry mechanism into host cell, modulating the expression of the injectisome and of translocated effectors. Once inside the cell, PhoP-modulated genes contribute to define the intracellular survival of Salmonella. In this work we demonstrate that long chain unsaturated free fatty acids (LCUFAs) could reversibly down-regulate PhoP/PhoQ activity at the transcriptional level, both for the phoPQ operon and for PhoP-activated genes, while these compounds exerted no effect on genes controlled by other signal transduction mechanisms. When the autophosphorylation ability of PhoQ-harboring vesicles obtained from bacteria grown in LB supplemented with LCUFAs was measured, we found that the sensor protein autokinase activity was severely repressed, while no effect was detected when saturated fatty acids of equal chain length were tested. We also demonstrate that the integrity of the fatty acids β-oxidation pathway is dispensable for LCUFAs to exert their repressive action on the PhoP/PhoQ system. In consequence, when LCUFAs are provided in the bacterial growth medium, it is reasonable to speculate that the interaction of these fatty acids with PhoQ would be responsible for the induction of a conformational change that switches off the autokinase activity of the sensor protein, turning down the system. Further work is underway in our laboratory to determine the biochemical basis of the LCUFAs-PhoQ interaction. Finally, we hypothesize that the presence of LCUFAs such as linoleic acid might function as a signal that, in combination with divalent cations, aids Salmonella to distinguish between extracellular from intracellular environments. In response to these signals, the PhoP/PhoQ system will remain turned off. Conversely, once inside the intravacuolar ambient of the host cell, the absence of repressing cues together with triggering conditions such as acidic pH and the presence of cationic peptides will turn on the system. This will allow Salmonella to induce the expression of virulence factors that counteract the defense mechanisms of the infected cell, favoring bacterial survival and dissemination. Our finding puts forth the complexity of input signals that can converge to finely tune the activity of the PhoP/PhoQ system, reflecting the variety of ambient conditions that Salmonella faces in host and non-host environments.