The protected physiological state of intracellular Salmonella enterica persisters reduces host cell-imposed stress

Risk of acute myocardial infarction in patients with non-typhoidal Salmonella infection: A nationwide matched population-based cohort study

OBJECTIVES

We used a nationwide cohort to explore the association of non-typhoidal Salmonella (NTS) with risk of incident acute myocardial infarction (AMI).

STUDY DESIGN

Nationwide matched population-based cohort study.

METHODS

We retrospectively analyzed a national insurance database, in which the NTS cohort (N = 15,152) and non-NTS cohort (N = 60,608) were individually-matched at 1:4 ratio by the index date, age, gender, and propensity score of comorbidities. Participants were identified by International Classification of Diseases, 9th Revision (ICD-9) codes from January 1, 2000 to December 31, 2014 and follow-up to Dec 31, 2017. Cox proportional hazard-models taking competing risk of death into account were adopted to estimate hazard ratios (HRs) and 95 % confidence intervals (CIs) of AMI. Multiple sensitivity analyses were conducted focusing on gender and comorbidities.

RESULTS

The sub-distribution HR (sHR) of new-onset AMI was 1.63 (95 % CI, 1.32-2.01; p < 0.0001). Our findings were consistent across gender (female, sHR = 1.42; 0.98-2.07 and male, sHR = 1.72; 1.34-2.22). While a positive association between NTS infection and AMI risk was noted in individuals without comorbidities, the correlation persisted in those with diabetes (sHR = 1.85; 1.34-2.55), moderate to severe kidney disease (sHR = 3.02; 1.49-6.13), acquired immunodeficiency syndrome (sHR = 1.64; 1.33-2.02), gastroenteritis (sHR = 2.75; 1.32-5.74) and hypertension (sHR = 1.66; 1.28-2.15) CONCLUSIONS: The risk of AMI was significantly higher in the NTS cohort than that in the non-NTS cohort, indicating that NTS infection may be a potentially modifiable risk factor for AMI that warrants further studies for verification.

Antibiotic-recalcitrant Salmonella during infection

Antibiotic-recalcitrant infections, defined as the prolonged carriage of pathogenic bacteria even in the presence of antibiotics, are often caused by bacteria that are genetically susceptible to the drug. These recalcitrant bacteria fail to proliferate in the presence of antibiotics but remain viable such that they may recolonize their niche following antibiotic withdrawal. Significant progress has been made in our understanding of antibiotic-recalcitrant Salmonella, which are thought to be the source of infection relapse. In recent years, it has been shown that recalcitrant bacteria manipulate host immune defences and could directly contribute to the spread of antimicrobial resistance. In this Review, we provide an overview of what is currently known about the antibiotic recalcitrance of Salmonella during infection and highlight knowledge gaps requiring additional research in the future.

Disruption of sulfur transferase complex increases bacterial intramacrophage persistence

Bacterial persisters contribute significantly to clinical treatment failure and relapse. These cells could resist antibiotic treatment via transient phenotypic and gene expression alterations. We conducted a high-throughput screening of Salmonella Typhimurium transposon mutants to identify key genes for intramacrophage antibiotic persistence. The results show that a sulfur transferase complex encoded by yheM, yheL, yheN, trmU and yhhP are involved in bacterial intramacrophage antibiotic persistence. Salmonella could persist in macrophages by downregulating the expression of the sulfur transferase complex during exposure to high concentrations of antibiotics, and even in a persistent infection mouse model. Mechanistically, deletion of yheM increases reactive nitrogen species (RNS) in the exponential phase, which inhibits bacterial respiration and ATP generation. In contrast, absence of yheM promotes persister formation by elevating (p)ppGpp levels in the stationary phase. Taken together, our data demonstrate that bacteria use the sulfur transferase to coordinate intramacrophage replication and persistence for adaptation to various environmental stresses. These findings reveal the role of the sulfur transferase complex in bacterial intramacrophage persistence and provide a promising target for antibacterial infection therapy.

Shigella flexneri evades LPS ubiquitylation through IpaH1.4-mediated degradation of RNF213

Pathogens have evolved diverse strategies to counteract host immunity. Ubiquitylation of lipopolysaccharide (LPS) on cytosol-invading bacteria by the E3 ligase RNF213 creates 'eat me' signals for antibacterial autophagy, but whether and how cytosol-adapted bacteria avoid LPS ubiquitylation remains poorly understood. Here, we show that the enterobacterium Shigella flexneri actively antagonizes LPS ubiquitylation through IpaH1.4, a secreted effector protein with ubiquitin E3 ligase activity. IpaH1.4 binds to RNF213, ubiquitylates it and targets it for proteasomal degradation, thus counteracting host-protective LPS ubiquitylation. To understand how IpaH1.4 recognizes RNF213, we determined the cryogenic electron microscopy structure of the IpaH1.4-RNF213 complex. The specificity of the interaction is achieved through the leucine-rich repeat of IpaH1.4, which binds the RING domain of RNF213 by hijacking the conserved RING interface required for binding to ubiquitin-charged E2 enzymes. IpaH1.4 also targets other E3 ligases involved in inflammation and immunity through binding to the E2-interacting face of their RING domains, including the E3 ligase LUBAC that is required for the synthesis of M1-linked ubiquitin chains on cytosol-invading bacteria downstream of RNF213. We conclude that IpaH1.4 has evolved to antagonize multiple antibacterial and proinflammatory host E3 ligases.

Tracking the progeny of bacterial persisters using a CRISPR-based genomic recorder

The rise of antimicrobial failure is a global emergency, and causes beyond typical genetic resistance must be determined. One probable factor is the existence of subpopulations of transiently growth-arrested bacteria, persisters, that endure antibiotic treatment despite genetic susceptibility to the drug. The presence of persisters in infected hosts has been successfully established, notably through the development of fluorescent reporters. It is proposed that infection relapse is caused by persisters resuming growth after cessation of the antibiotic treatment, but to date, there is no direct evidence for this. This is because no tool or reporter currently exists to track the extent to which infection relapse is initiated by regrowth of persisters in the host. Indeed, once they have transitioned out of the persister state, the progeny of persisters are genetically and phenotypically identical to susceptible bacteria in the population, making it virtually impossible to ascertain the source of relapse. We designed pSCRATCH (plasmid for Selective CRISPR Array expansion To Check Heritage), a molecular tool that functions to record the state of antibiotic persistence in the genome of Salmonella persisters. We show that pSCRATCH successfully marks persisters by adding spacers in their CRISPR arrays and the genomic label is stable in persister progeny after exit from persistence. We further show that in a Salmonella infection model the system enables the discrimination of treatment failure originating from persistence versus resistance. Thus, pSCRATCH provides proof of principle for stable marking of persisters and a prototype for applications to more complex infection models and other pathogens.

The macrophage-bacterium mismatch in persister formation

Many pathogens are hard to eradicate, even in the absence of genetically detectable antimicrobial resistance mechanisms and despite proven antibiotic susceptibility. The fraction of clonal bacteria that temporarily elude effective antibiotic treatments is commonly known as 'antibiotic persisters.' Over the past decade, there has been a growing body of research highlighting the pivotal role played by the cellular host in the development of persisters. In parallel, this research has also sought to elucidate the molecular mechanisms underlying the formation of intracellular antibiotic persisters and has demonstrated a prominent role for the bacterial stress response. However, questions remain regarding the conditions leading to the formation of stress-induced persisters among a clonal population of intracellular bacteria and despite an ostensibly uniform environment. In this opinion, following a brief review of the current state of knowledge regarding intracellular antibiotic persisters, we explore the ways in which macrophage functional heterogeneity and bacterial phenotypic heterogeneity may contribute to the emergence of these persisters. We propose that the degree of mismatch between the macrophage permissiveness and the bacterial preparedness to invade and thrive intracellularly may explain the formation of stress-induced nonreplicating intracellular persisters.

Bright individuals: Applications of fluorescent protein-based reporter systems in single-cell cellular microbiology

Activation and function of virulence functions of bacterial pathogens are highly dynamic in time and space, and can show considerable heterogeneity between individual cells in pathogen populations. To investigate the complex events in host-pathogen interactions, single cell analyses are required. Fluorescent proteins (FPs) are excellent tools to follow the fate of individual bacterial cells during infection, and can also be deployed to use the pathogen as a sensor for its specific environment in host cells or host organisms. This Resources describes design and applications of dual fluorescence reporters (DFR) in cellular microbiology. DFR feature constitutively expressed FPs for detection of bacterial cells, and FPs expressed by an environmentally regulated promoter for interrogation of niche-specific cues or nutritional parameters. Variations of the basic design allow the generation of DFR that can be used to analyze, on single cell level, bacterial proliferation during infection, subcellular localization of intracellular bacteria, stress response, or persister state. We describe basic considerations for DFR design and review recent applications of DFR in cellular microbiology.

In Vitro Persistence Level Reflects In Vivo Antibiotic Survival of Natural Pseudomonas aeruginosa Isolates in a Murine Lung Infection Model

Clinicians are increasingly confronted with the limitations of antibiotics to clear bacterial infections in patients. It has long been assumed that only antibiotic resistance plays a pivotal role in this phenomenon. Indeed, the worldwide emergence of antibiotic resistance is considered one of the major health threats of the 21st century. However, the presence of persister cells also has a significant influence on treatment outcomes. These antibiotic-tolerant cells are present in every bacterial population and are the result of the phenotypic switching of normal, antibiotic-sensitive cells. Persister cells complicate current antibiotic therapies and contribute to the development of resistance. In the past, extensive research has been performed to investigate persistence in laboratory settings; however, antibiotic tolerance under conditions that mimic the clinical setting remain poorly understood. In this study, we optimized a mouse model for lung infections with the opportunistic pathogen Pseudomonas aeruginosa. In this model, mice are intratracheally infected with P. aeruginosa embedded in seaweed alginate beads and subsequently treated with tobramycin via nasal droplets. A diverse panel of 18 P. aeruginosa strains originating from environmental, human, and animal clinical sources was selected to assess survival in the animal model. Survival levels were positively correlated with the survival levels determined via time-kill assays, a common method to study persistence in the laboratory. We showed that survival levels are comparable and thus that the classical persister assays are indicative of antibiotic tolerance in a clinical setting. The optimized animal model also enables us to test potential antipersister therapies and study persistence in relevant settings. IMPORTANCE The importance of targeting persister cells in antibiotic therapies is becoming more evident, as these antibiotic-tolerant cells underlie relapsing infections and resistance development. Here, we studied persistence in a clinically relevant pathogen, Pseudomonas aeruginosa. It is one of the six ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, P. aeruginosa, and Enterobacter spp.), which are considered major health threats. P. aeruginosa is mostly known to cause chronic lung infections in cystic fibrosis patients. We mimicked these lung infections in a mouse model to study persistence under more clinical conditions. It was shown that the survival levels of natural P. aeruginosa isolates in this model are positively correlated with the survival levels measured in classical persistence assays in vitro. These results not only validate the use of our current techniques to study persistence but also open opportunities to study new persistence mechanisms or evaluate new antipersister strategies in vivo.

Determinants of persistent Salmonella infections

Persistent bacterial infections constitute an enormous challenge for public health. Amongst infections with other bacteria, infections with typhoidal and nontyphoidal Salmonella enterica serovars can result in long-term infections of the human and animal host. Persistent infections that are asymptomatic are difficult to identify and thus can serve as a silent reservoir for transmission. Symptomatic persistent infections are often difficult to treat as they harbor a combination of antibiotic-tolerant and antibiotic-resistant bacteria and boost the spread of genetic antibiotic resistance. In the last couple of years, the field has made some major progress in understanding the role of persisters, their reservoirs as well as their interplay with host factors in persistent Salmonella infections.

Intracellular persister: A stealth agent recalcitrant to antibiotics

The bulk of bacteria transiently evading appropriate antibiotic regimes and recovered from non-resolutive infections are commonly refer to as persisters. In this mini-review, we discuss how antibiotic persisters stem from the interplay between the pathogen and the cellular defenses mechanisms and its underlying heterogeneity.

Exposure to stressors and antimicrobials induces cell-autonomous ultrastructural heterogeneity of an intracellular bacterial pathogen

Despite their clonality, intracellular bacterial pathogens commonly show remarkable physiological heterogeneity during infection of host cells. Physiological heterogeneity results in distinct ultrastructural morphotypes, but the correlation between bacterial physiological state and ultrastructural appearance remains to be established. In this study, we showed that individual cells of Salmonella enterica serovar Typhimurium are heterogeneous in their ultrastructure. Two morphotypes based on the criterion of cytoplasmic density were discriminated after growth under standard culture conditions, as well as during intracellular lifestyle in mammalian host cells. We identified environmental conditions which affect cytoplasmic densities. Using compounds generating oxygen radicals and defined mutant strains, we were able to link the occurrence of an electron-dense ultrastructural morphotype to exposure to oxidative stress and other stressors. Furthermore, by combining ultrastructural analyses of Salmonella during infection and fluorescence reporter analyses for cell viability, we provided evidence that two characterized ultrastructural morphotypes with electron-lucent or electron-dense cytoplasm represent viable cells. Moreover, the presence of electron-dense types is stress related and can be experimentally induced only when amino acids are available in the medium. Our study proposes ultrastructural morphotypes as marker for physiological states of individual intracellular pathogens providing a new marker for single cell analyses.

Non-Canonical Host Intracellular Niche Links to New Antimicrobial Resistance Mechanism

Globally, infectious diseases are one of the leading causes of death among people of all ages. The development of antimicrobials to treat infectious diseases has been one of the most significant advances in medical history. Alarmingly, antimicrobial resistance is a widespread phenomenon that will, without intervention, make currently treatable infections once again deadly. In an era of widespread antimicrobial resistance, there is a constant and pressing need to develop new antibacterial drugs. Unraveling the underlying resistance mechanisms is critical to fight this crisis. In this review, we summarize some emerging evidence of the non-canonical intracellular life cycle of two priority antimicrobial-resistant bacterial pathogens: Pseudomonas aeruginosa and Staphylococcus aureus. The bacterial factors that modulate this unique intracellular niche and its implications in contributing to resistance are discussed. We then briefly discuss some recent research that focused on the promises of boosting host immunity as a combination therapy with antimicrobials to eradicate these two particular pathogens. Finally, we summarize the importance of various strategies, including surveillance and vaccines, in mitigating the impacts of antimicrobial resistance in general.

Flow Cytometry-Based Single Cell Analyses of Bacterial Adaptation to Intracellular Environments

Since decades, flow cytometry (FC) is a powerful technique to perform single cell analyses with high accuracy and throughput. Moreover, FC is the method of choice to study bacterial cell heterogeneity and complements single-cell imaging techniques. The complex experimental approaches for FC sample preparation and the subsequent FC adjustment and gating strategy demand careful considerations to be successful when analyzing complex microbial populations, especially when liberated populations of intracellular bacterial pathogens, or bacterial pathogens inside intact host cells are analyzed. Here, we provide a set of experimental protocols for FC sample preparation of (1) in vitro cultured bacterial cells, (2) liberated intracellular bacteria from host cells, or (3) preparation of intact infected phagocytic or epithelial cells commonly used as host cells in infection biology. Since sample preparation, cytometer adjustment, and gating strategy are essential for experimental success, we aim to provide our expertise to support application of FC by other researchers.

Murine AML12 hepatocytes allow Salmonella Typhimurium T3SS1-independent invasion and intracellular fate

Numerous studies have demonstrated the key role of the Salmonella Pathogenicity Island 1-encoded type III secretion system (T3SS1) apparatus as well as its associated effectors in the invasion and intracellular fate of Salmonella in the host cell. Several T3SS1 effectors work together to control cytoskeleton networks and induce massive membrane ruffles, allowing pathogen internalization. Salmonella resides in a vacuole whose maturation requires that the activity of T3SS1 subverts early stages of cell signaling. Recently, we identified five cell lines in which Salmonella Typhimurium enters without using its three known invasion factors: T3SS1, Rck and PagN. The present study investigated the intracellular fate of Salmonella Typhimurium in one of these models, the murine hepatocyte cell line AML12. We demonstrated that both wild-type Salmonella and T3SS1-invalidated Salmonella followed a common pathway leading to the formation of a Salmonella containing vacuole (SCV) without classical recruitment of Rho-GTPases. Maturation of the SCV continued through an acidified phase that led to Salmonella multiplication as well as the formation of a tubular network resembling Salmonella induced filaments (SIF). The fact that in the murine AML12 hepatocyte, the T3SS1 mutant induced an intracellular fate resembling to the wild-type strain highlights the fact that Salmonella Typhimurium invasion and intracellular survival can be completely independent of T3SS1.

Single cell analyses reveal distinct adaptation of typhoidal and non-typhoidal Salmonella enterica serovars to intracellular lifestyle

Salmonella enterica is a common foodborne, facultative intracellular enteropathogen. Human-restricted typhoidal S. enterica serovars Typhi (STY) or Paratyphi A (SPA) cause severe typhoid or paratyphoid fever, while many S. enterica serovar Typhimurium (STM) strains have a broad host range and in human hosts usually lead to a self-limiting gastroenteritis. Due to restriction of STY and SPA to primate hosts, experimental systems for studying the pathogenesis of typhoid and paratyphoid fever are limited. Therefore, STM infection of susceptible mice is commonly considered as model system for studying these diseases. The type III secretion system encoded by Salmonella pathogenicity island 2 (SPI2-T3SS) is a key factor for intracellular survival of Salmonella. Inside host cells, the pathogen resides within the Salmonella-containing vacuole (SCV) and induces tubular structures extending from the SCV, termed Salmonella-induced filaments (SIF). This study applies single cell analyses approaches, which are flow cytometry of Salmonella harboring dual fluorescent protein reporters, effector translocation, and correlative light and electron microscopy to investigate the fate and activities of intracellular STY and SPA. The SPI2-T3SS of STY and SPA is functional in translocation of effector proteins, SCV and SIF formation. However, only a low proportion of intracellular STY and SPA are actively deploying SPI2-T3SS and STY and SPA exhibited a rapid decline of protein biosynthesis upon experimental induction. A role of SPI2-T3SS for proliferation of STY and SPA in epithelial cells was observed, but not for survival or proliferation in phagocytic host cells. Our results indicate that reduced intracellular activities are factors of the stealth strategy of STY and SPA and facilitate systemic spread and persistence of the typhoidal Salmonella.

Typhoidal Salmonella enterica serovars Typhi (STY) and Paratyphi A (SPA) cause a major disease burden to the human population. The restriction of these pathogens to human hosts limits experimental analyses of molecular mechanisms of diseases. S. enterica serovar Typhimurium is commonly used as surrogate model for typhoidal Salmonella (TS), and allowed the identification of virulence factors for intracellular lifestyle of S. enterica in mammalian host cells. If virulence factors, such as the Salmonella Pathogenicity Island 2-encoded type III secretion system (SPI2-T3SS) have similar roles for intracellular lifestyle of TS is largely unknown. We analyzed, on single cell level, the intracellular activities of STY and SPA in comparison to STM. STY and SPA deploy SPI2-T3SS to actively manipulate their host cells, but with far lower frequency than STM. Our work supports a model of TS as stealth pathogens that persist in host cells.