Salmonella effector SteE converts the mammalian serine/threonine kinase GSK3 into a tyrosine kinase to direct macrophage polarization.

Many Gram-negative bacterial pathogens antagonize anti-bacterial immunity through translocated effector proteins that inhibit pro-inflammatory signaling. In addition, the intracellular pathogen Salmonella enterica serovar Typhimurium initiates an anti-inflammatory transcriptional response in macrophages through its effector protein SteE. However, the target(s) and molecular mechanism of SteE remain unknown. Here, we demonstrate that SteE converts both the amino acid and substrate specificity of the host pleiotropic serine/threonine kinase GSK3. SteE itself is a substrate of GSK3, and phosphorylation of SteE is required for its activity. Remarkably, phosphorylated SteE then forces GSK3 to phosphorylate the non-canonical substrate signal transducer and activator of transcription 3 (STAT3) on tyrosine-705. This results in STAT3 activation, which along with GSK3 is required for SteE-mediated upregulation of the anti-inflammatory M2 macrophage marker interleukin-4Rα (IL-4Rα). Overall, the conversion of GSK3 to a tyrosine-directed kinase represents a tightly regulated event that enables a bacterial virulence protein to reprogram innate immune signaling and establish an anti-inflammatory environment

Single-cell RNA-seq ties macrophage polarization to growth rate of intracellular Salmonella

Intracellular bacterial pathogens can exhibit large heterogeneity in growth rate inside host cells, with major consequences for the infection outcome. If and how the host responds to this heterogeneity remains poorly understood. Here, we combined a fluorescent reporter of bacterial cell division with single-cell RNA-sequencing analysis to study the macrophage response to different intracellular states of the model pathogen Salmonella enterica serovar Typhimurium. The transcriptomes of individual infected macrophages revealed a spectrum of functional host response states to growing and non-growing bacteria. Intriguingly, macrophages harbouring non-growing Salmonella display hallmarks of the proinflammatory M1 polarization state and differ little from bystander cells, suggesting that non-growing bacteria evade recognition by intracellular immune receptors. By contrast, macrophages containing growing bacteria have turned into an anti-inflammatory, M2-like state, as if fast-growing intracellular Salmonella overcome host defence by reprogramming macrophage polarization. Additionally, our clustering approach reveals intermediate host functional states between these extremes. Altogether, our data suggest that gene expression variability in infected host cells shapes different cellular environments, some of which may favour a growth arrest of Salmonella facilitating immune evasion and the establishment of a long-term niche, while others allow Salmonella to escape intracellular antimicrobial activity and proliferate

Salmonella persisters undermine host immune defenses during antibiotic treatment

Many bacterial infections are hard to treat and tend to relapse, possibly due to the presence of antibiotic-tolerant persisters. In vitro, persister cells appear to be dormant. After uptake of Salmonella species by macrophages, nongrowing persisters also occur, but their physiological state is poorly understood. In this work, we show that Salmonella persisters arising during macrophage infection maintain a metabolically active state. Persisters reprogram macrophages by means of effectors secreted by the Salmonella pathogenicity island 2 type 3 secretion system. These effectors dampened proinflammatory innate immune responses and induced anti-inflammatory macrophage polarization. Such reprogramming allowed nongrowing Salmonella cells to survive for extended periods in their host. Persisters undermining host immune defenses might confer an advantage to the pathogen during relapse once antibiotic pressure is relieved

The <i>Salmonella</i> Effector SteE Converts the Mammalian Serine/Threonine Kinase GSK3 into a Tyrosine Kinase

Many Gram-negative bacterial pathogens antagonize anti-bacterial immunity through translocated effector proteins that inhibit pro-inflammatory signaling. In addition, the intracellular pathogen Salmonella enterica serovar Typhimurium initiates an anti-inflammatory transcriptional response in macrophages through its effector protein SteE. However, the target(s) and molecular mechanism of SteE remain unknown. Here, we demonstrate that SteE converts both the amino acid and substrate specificity of the pleiotropic serine/threonine kinase GSK3. SteE itself is a substrate of GSK3 and phosphorylation of SteE is required for its activity. Remarkably, phosphorylated SteE then forces GSK3 to phosphorylate the non-canonical substrate STAT3 on tyrosine-705. This results in STAT3 activation and both GSK3 and STAT3 are required for SteE-mediated upregulation of the anti-inflammatory M2 macrophage marker IL-4Rα. Overall, the conversion of GSK3 to a tyrosine-directed kinase represents an unprecedented example of how a bacterial virulence protein reprograms innate immune signalling to establish an anti-inflammatory environment

Evolutionary causes and consequences of bacterial antibiotic persistence

Antibiotic treatment failure is of growing concern. Genetically encoded resistance is key in driving this process. However, there is increasing evidence that bacterial antibiotic persistence, a non-genetically encoded and reversible loss of antibiotic susceptibility, contributes to treatment failure and emergence of resistant strains as well. In this Review, we discuss the evolutionary forces that may drive the selection for antibiotic persistence. We review how some aspects of antibiotic persistence have been directly selected for whereas others result from indirect selection in disparate ecological contexts. We then discuss the consequences of antibiotic persistence on pathogen evolution. Persisters can facilitate the evolution of antibiotic resistance and virulence. Finally, we propose practical means to prevent persister formation and how this may help to slow down the evolution of virulence and resistance in pathogens