Date Published: January 23, 2017
Publisher: Public Library of Science
Author(s): Adi Haber, Sivan Friedman, Lior Lobel, Tamar Burg-Golani, Nadejda Sigal, Jessica Rose, Nurit Livnat-Levanon, Oded Lewinson, Anat A. Herskovits, Marcel A. Behr.
The high environmental adaptability of bacteria is contingent upon their ability to sense changes in their surroundings. Bacterial pathogen entry into host poses an abrupt and dramatic environmental change, during which successful pathogens gauge multiple parameters that signal host localization. The facultative human pathogen Listeria monocytogenes flourishes in soil, water and food, and in ~50 different animals, and serves as a model for intracellular infection. L. monocytogenes identifies host entry by sensing both physical (e.g., temperature) and chemical (e.g., metabolite concentrations) factors. We report here that L-glutamine, an abundant nitrogen source in host serum and cells, serves as an environmental indicator and inducer of virulence gene expression. In contrast, ammonia, which is the most abundant nitrogen source in soil and water, fully supports growth, but fails to activate virulence gene transcription. We demonstrate that induction of virulence genes only occurs when the Listerial intracellular concentration of L-glutamine crosses a certain threshold, acting as an on/off switch: off when L-glutamine concentrations are below the threshold, and fully on when the threshold is crossed. To turn on the switch, L-glutamine must be present, and the L-glutamine high affinity ABC transporter, GlnPQ, must be active. Inactivation of GlnPQ led to complete arrest of L-glutamine uptake, reduced type I interferon response in infected macrophages, dramatic reduction in expression of virulence genes, and attenuated virulence in a mouse infection model. These results may explain observations made with other pathogens correlating nitrogen metabolism and virulence, and suggest that gauging of L-glutamine as a means of ascertaining host localization may be a general mechanism.
Listeria monocytogenes is a Gram-positive facultative intracellular bacterial pathogen and the causative agent of listeriosis in humans, a disease with deleterious impacts, such as increased risk of meningitis and miscarriage . L. monocytogenes invades mammalian cells by expressing surface proteins named internalins, that bind host proteins to induce active bacterial uptake . Upon entry, L. monocytogenes escapes the vacuole (phagosome) by producing a pore-forming hemolysin, listeriolysin O (LLO, encoded by the hly gene) and two additional phospholipases, PlcA and PlcB , . Once in the host cytosol, L. monocytogenes multiplies rapidly and expresses ActA, which recruits the host actin polymerization machinery to propel the bacteria within the cytosol and facilitate its spread from cell to cell . Most of the known virulence factors involved in internalization, vacuolar escape and cell-to-cell spread are positively regulated by PrfA, the master virulence activator of L. monocytogenes , .
L. monocytogenes is a highly adaptive bacterium that successfully inhabits diverse environmental niches. It is routinely isolated from soil, silage, decaying plants, and water reservoirs. It is also known to infect at least 50 different animals, and to invade almost all types of mammalian cells. Clearly, each niche presents different challenges and therefore, to survive L. monocytogenes must constantly adjust its lifestyle. When the bacterium is free-roaming, the environments it traverses are usually spatially continuous (e.g., a water reservoir and its banks) and present overlapping metabolic requirements. From a temporal perspective, the environmental changes encountered by the freely roaming bacterium are likely to be gradual (e.g., temperature, pH, salinity). Yet, when entering a host, environmental changes are both spatially and temporally abrupt. The host’s environment is spatially isolated from the external environment, it presents very different metabolic requirements, and poses unique survival challenges mounted by the immune system. Therefore, a facultative pathogen such as L. monocytogenes must feature tools to rapidly and efficiently react to such conditions. Indeed, upon infection, L. monocytogenes activates elaborate survival machineries that power altered motility, metabolism, mammalian cell penetration, phagosomal escape and cell-to-cell spreading. However, to activate these machineries the bacterium must be able to sense that it is no longer free roaming, and is inside a host. Several such host-based cues have been previously identified, including temperature (37°C), pH, carbon source identity and availability, and specific metabolites such as branched-chain amino acids and glutathione , , [45–47]. As activation of the entire host-related survival machinery on the basis of a single cue would clearly be detrimental, the bacteria integrate multiple signals before switching to a virulent state. This work identified and characterized the role of L-glutamine levels in bacterial sensing of the host environment.