SulA-induced filamentation in Salmonella enterica serovar Typhimurium: effects on SPI-1 expression and epithelial infection

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.

A primary cell-based in vitro model of the human small intestine reveals host olfactomedin 4 induction in response to Salmonella Typhimurium infection

Infection research largely relies on classical cell culture or mouse models. Despite having delivered invaluable insights into host-pathogen interactions, both have limitations in translating mechanistic principles to human pathologies. Alternatives can be derived from modern Tissue Engineering approaches, allowing the reconstruction of functional tissue models in vitro. Here, we combined a biological extracellular matrix with primary tissue-derived enteroids to establish an in vitro model of the human small intestinal epithelium exhibiting in vivo-like characteristics. Using the foodborne pathogen Salmonella enterica serovar Typhimurium, we demonstrated the applicability of our model to enteric infection research in the human context. Infection assays coupled to spatio-temporal readouts recapitulated the established key steps of epithelial infection by this pathogen in our model. Besides, we detected the upregulation of olfactomedin 4 in infected cells, a hitherto unrecognized aspect of the host response to Salmonella infection. Together, this primary human small intestinal tissue model fills the gap between simplistic cell culture and animal models of infection, and shall prove valuable in uncovering human-specific features of host-pathogen interplay.

Filamentous morphology of bacterial pathogens: regulatory factors and control strategies

Several studies have demonstrated that when exposed to physical, chemical, and biological stresses in the environment, many bacteria (Gram-positive and Gram-negative) change their morphology from a normal cell to a filamentous shape. The formation of filamentous morphology is one of the survival strategies against environmental stress and protection against phagocytosis or protist predators. Numerous pathogenic bacteria have shown filamentous morphologies when examined in vivo or in vitro. During infection, certain pathogenic bacteria adopt a filamentous shape inside the cell to avoid phagocytosis by immune cells. Filamentous morphology has also been seen in biofilms formed on biotic or abiotic surfaces by certain bacteria. As a result, in addition to protecting against phagocytosis by immune cells or predators, the filamentous shape aids in biofilm adhesion or colonization to biotic or abiotic surfaces. Furthermore, these filamentous morphologies of bacterial pathogens lead to antimicrobial drug resistance. Clinically, filamentous morphology has become one of the most serious challenges in treating bacterial infection. The current review went into great detail about the various factors involved in the change of filamentous morphology and the underlying mechanisms. In addition, the review discussed a control strategy for suppressing filamentous morphology in order to combat bacterial infections. Understanding the mechanism underlying the filamentous morphology induced by various environmental conditions will aid in drug development and lessen the virulence of bacterial pathogens. KEY POINTS: • The bacterial filamentation morphology is one of the survival mechanisms against several environmental stress conditions and protection from phagocytosis by host cells and protist predators. • The filamentous morphologies in bacterial pathogens contribute to enhanced biofilm formation, which develops resistance properties against antimicrobial drugs. • Filamentous morphology has become one of the major hurdles in treating bacterial infection, hence controlling strategies employed for inhibiting the filamentation morphology from combating bacterial infections.

Salmonella Establishment in Agricultural Soil and Colonization of Crop Plants Depend on Soil Type and Plant Species

Human pathogenic bacteria, such as Salmonella enterica, are able to colonize crop plants. So far, not much is known about biotic and abiotic factors influencing this colonization in field soil. This understanding, however, is imperative for the provision of safe fresh produce to the consumer. In this study, we investigated the effects of soil type, organic fertilization, plant species and the way of Salmonella entry into the plant production system, on the survival of S. enterica in soil as well as the colonization of plants. The selected S. enterica serovar Typhimurium strain 14028s, S. Typhimurium strain LT2 and S. Senftenberg were able to persist in soil for several weeks. Salmonella's persistence in soil was prolonged in loamy, if compared to sandy soil, and when applied together with organic fertilizer. The leaves of lettuce and corn salad were colonized by S. enterica providing evidence for internalization from the soil via the root. Colonization rates were affected by soil type, plant species and S. enterica strain. Overall, S. enterica was detected in leaves of 0.5-0.9% of the plants, while lettuce was more frequently colonized than corn salad. Plants grown in sandy soil were more often colonized than plants grown in loamy soil. After spray inoculation, S. enterica could be detected on and in leaves for several weeks by cultivation-depending methods, confirmed by confocal microscopy using GFP-labeled S. Typhimurium 14028s. On the one hand, transcriptome data from S. Typhimurium 14028s assessed in response to lettuce medium or lettuce root exudates showed an upregulation of genes associated with biofilm formation and virulence. On the other hand, lettuce inoculated with S. Typhimurium 14028s showed a strong upregulation of genes associated with plant immune response and genes related to stress response. In summary, these results showed that organic fertilizers can increase the persistence of Salmonella in soil and that soil type and plant species play a crucial role in the interactions between human pathogens and crop plants. This understanding is therefore a starting point for new strategies to provide safe food for the consumer.

Salmonella and Reactive Oxygen Species: A Love-Hate Relationship

Salmonella enterica represents an enterobacterial species including numerous serovars that cause infections at, or initiated at, the intestinal epithelium. Many serovars also act as facultative intracellular pathogens with a tropism for phagocytic cells. These bacteria not only survive in phagocytes but also undergo de facto replication therein. Phagocytes, through the activities of phagocyte NADPH-dependent oxidase and inducible nitric oxide synthase, are very proficient in converting molecular oxygen to reactive oxygen (ROS) and nitrogen species (RNS). These compounds represent highly efficient effectors of the innate immune defense. Salmonella is by no means resistant to these effectors, which may stand in contrast to the host niches chosen. To cope with this paradox, these bacteria rely on an array of detoxification and repair systems. Combination these systems allows for a high enough tolerance to ROS and RNS to enable establishment of infection. In addition, salmonella possesses protein factors that have the potential to dampen the infection-associated inflammation, which evidently results in a reduced exposure to ROS and RNS. This review attempts to summarize the activities and strategies by which salmonella tries to cope with ROS and RNS and how the bacterium can make use of these innate defense factors.

Phosphate and carbohydrate facilitate the formation of filamentous Salmonella enterica during osmotic stress

Salmonella enterica is a human pathogen that can produce filamentous cells in response to environmental stress. The molecular mediators and biosynthetic pathways that contribute to the formation of filamentous cells (>10 µm in length) during osmotic stress are mostly unknown. The comparison of filamentous and non-filamentous cells in this study was aided by the use of a filtration step to separate cell types. Osmotic stress caused an efflux of phosphate from cells, and the addition of phosphate and a carbohydrate to Luria broth with 7 % NaCl (LB-7NaCl) significantly increased the proportion of filamentous cells in the population (58 %). In addition to direct measurements of intracellular and extracellular phosphate concentrations, the relative abundance of the iraP transcript that is induced by phosphate limitation was monitored. Non-filamentous cells had a greater relative abundance of iraP transcript than filamentous cells. IraP also affects the stability of RpoS, which regulates the general stress regulon, and was detected in non-filamentous cells but not filamentous cells. Markers of metabolic pathways for the production of acetyl-CoA (pflB, encoding for pyruvate formate lyase) and fatty acids (fabH) that are essential to membrane biosynthesis were found in greater abundance in filamentous cells than non-filamentous cells. There were no differences in the DNA, protein and biomass levels in filamentous and non-filamentous cells after 48 h of incubation, although the filamentous cells produced significantly (P<0.05) more acetate. This study found that phosphate and carbohydrate enhanced the formation of filamentous cells during osmotic stress, and there were differences in key regulatory elements and markers of metabolic pathways in filamentous and non-filamentous S. enterica.

Increasing the bactofection capacity of a mammalian expression vector by removal of the f1 ori

Bacterial-mediated cancer therapy has shown great promise in in vivo tumour models with increased survival rates post-bacterial treatment. Improving efficiency of bacterial-mediated tumour regression has focused on controlling and exacerbating bacterial cytotoxicity towards tumours. One mechanism that has been used to carry this out is the process of bactofection where post-invasion, bacteria deliver plasmid-borne mammalian genes into target cells for expression. Here we utilised the cancer-targeting Salmonella Typhimurium strain, SL7207, to carry out bactofection into triple negative breast cancer MDA-MB-231 cells. However, we noted that post-transformation with the commonly used mammalian expression vector pEGFP, S. Typhimurium became filamentous, attenuated and unable to invade target cells efficiently. Filamentation did not occur in Escherichia coli-transformed with the same plasmid. Further investigation identified the region inducing S. Typhimurium filamentation as being the f1 origin of replication (f1 ori), an artefact of historic use of mammalian plasmids for single stranded DNA production. Other f1 ori-containing plasmids also induced the attenuated phenotype, while removal of the f1 ori from pEGFP restored S. Typhimurium virulence and increased the bactofection capacity. This work has implications for interpretation of prior bactofection studies employing f1 ori-containing plasmids in S. Typhimurium, while also indicating that future use of S. Typhimurium in targeting tumours should avoid the use of these plasmids.

Salmonella enterica serovar Typhi siderophore production is elevated and Fur inactivation causes cell filamentation and attenuation in macrophages

Salmonella enterica serovars Typhi and Typhimurium are two closely related bacteria causing different types of infection in humans. Iron acquisition is considered essential for virulence. Siderophores are important iron chelators and production of enterobactin and salmochelins by these serovars was quantified. Overall, Salmonella Typhi produced higher levels of siderophores than Salmonella Typhimurium. The role of the global regulator Fur, involved in iron homeostasis, present and conserved in both these serovars, was then investigated. Deletion of the fur gene led to distinct phenotypes in these serovars. Defective growth in iron-rich and iron-limiting conditions and formation of filamentous cells was only observed in the S. Typhi fur mutant. Furthermore, Fur was required for optimal motility in both serovars, but motility was more reduced for the fur mutant of S. Typhi compared to S. Typhimurium. During interaction with human-cultured macrophages, Fur was more important for S. Typhi, as the fur mutant had severe defects in uptake and survival. Globally, these results demonstrate that Fur differentially affects the physiology and the virulence phenotypes of the two strains and is more critical for S. Typhi growth, morphology, motility and interaction with host cells than it is for S. Typhimurium.

Determinants of Bacterial Morphology: From Fundamentals to Possibilities for Antimicrobial Targeting

Bacterial morphology is extremely diverse. Specific shapes are the consequence of adaptive pressures optimizing bacterial fitness. Shape affects critical biological functions, including nutrient acquisition, motility, dispersion, stress resistance and interactions with other organisms. Although the characteristic shape of a bacterial species remains unchanged for vast numbers of generations, periodical variations occur throughout the cell (division) and life cycles, and these variations can be influenced by environmental conditions. Bacterial morphology is ultimately dictated by the net-like peptidoglycan (PG) sacculus. The species-specific shape of the PG sacculus at any time in the cell cycle is the product of multiple determinants. Some morphological determinants act as a cytoskeleton to guide biosynthetic complexes spatiotemporally, whereas others modify the PG sacculus after biosynthesis. Accumulating evidence supports critical roles of morphogenetic processes in bacteria-host interactions, including pathogenesis. Here, we review the molecular determinants underlying morphology, discuss the evidence linking bacterial morphology to niche adaptation and pathogenesis, and examine the potential of morphological determinants as antimicrobial targets.

Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action

Efforts to reduce the global burden of bacterial disease and contend with escalating bacterial resistance are spurring innovation in antibacterial drug and biocide development and related technologies such as photodynamic therapy and photochemical disinfection. Elucidation of the mechanism of action of these new agents and processes can greatly facilitate their development, but it is a complex endeavour. One strategy that has been popular for many years, and which is garnering increasing interest due to recent technological advances in microscopy and a deeper understanding of the molecular events involved, is the examination of treated bacteria for changes to their morphology and ultrastructure. In this review, we take a critical look at this approach. Variables affecting antibacterial-induced alterations are discussed first. These include characteristics of the test organism (e.g. cell wall structure) and incubation conditions (e.g. growth medium osmolarity). The main body of the review then describes the different alterations that can occur. Micrographs depicting these alterations are presented, together with information on agents that induce the change, and the sequence of molecular events that lead to the change. We close by highlighting those morphological and ultrastructural changes which are consistently induced by agents sharing the same mechanism (e.g. spheroplast formation by peptidoglycan synthesis inhibitors) and explaining how changes that are induced by multiple antibacterial classes (e.g. filamentation by DNA synthesis inhibitors, FtsZ disruptors, and other types of agent) can still yield useful mechanistic information. Lastly, recommendations are made regarding future study design and execution.

DamX Controls Reversible Cell Morphology Switching in Uropathogenic Escherichia coli

The ability to change cell morphology is an advantageous characteristic adopted by multiple pathogenic bacteria in order to evade host immune detection and assault during infection. Uropathogenic Escherichia coli (UPEC) exhibits such cellular dynamics and has been shown to transition through a series of distinct morphological phenotypes during a urinary tract infection. Here, we report the first systematic spatio-temporal gene expression analysis of the UPEC transition through these phenotypes by using a flow chamber-based in vitro infection model that simulates conditions in the bladder. This analysis revealed a novel association between the cell division gene damX and reversible UPEC filamentation. We demonstrate a lack of reversible bacterial filamentation in a damX deletion mutant in vitro and absence of a filamentous response by this mutant in a murine model of cystitis. While deletion of damX abrogated UPEC filamentation and secondary surface colonization in tissue culture and in mouse infections, transient overexpression of damX resulted in reversible UPEC filamentation. In this study, we identify a hitherto-unknown damX-mediated mechanism underlying UPEC morphotypical switching. Murine infection studies showed that DamX is essential for establishment of a robust urinary tract infection, thus emphasizing its role as a mediator of virulence. Our study demonstrates the value of an in vitro methodology, in which uroepithelium infection is closely simulated, when undertaking targeted investigations that are challenging to perform in animal infection models.

IMPORTANCE

Urinary tract infections (UTIs) are most often caused by uropathogenic Escherichia coli (UPEC) and account for a considerable health care burden. UPEC exhibits a dynamic lifestyle in the course of infection, in which the bacterium transiently adopts alternative morphologies ranging from rod shaped to coccoid and filamentous, rendering it better at immune evasion and host epithelium adhesion. This penchant for morphotype switching might in large measure account for UPEC's success as a pathogen. In aiming to uncover genes underlying the phenomenon of UPEC morphotype switching, this study identifies damX, a cell division gene, as a mediator of reversible filamentation during UTI. DamX-mediated filamentation represents an additional pathway for bacterial cell shape control, an alternative to SulA-mediated FtsZ sequestration during E. coli uropathogenesis, and hence represents a potential target for combating UTI.

Identification of Fic-1 as an enzyme that inhibits bacterial DNA replication by AMPylating GyrB, promoting filament formation

The morphology of bacterial cells is important for virulence, evasion of the host immune system, and coping with environmental stresses. The widely distributed Fic proteins (filamentation induced by cAMP) are annotated as proteins involved in cell division because of the presence of the HPFx[D/E]GN[G/K]R motif. We showed that the presence of Fic-1 from Pseudomonas fluorescens significantly reduced the yield of plasmid DNA when expressed in Escherichia coli or P. fluorescens. Fic-1 interacted with GyrB, a subunit of DNA gyrase, which is essential for bacterial DNA replication. Fic-1 catalyzed the AMPylation of GyrB at Tyr(109), a residue critical for binding ATP, and exhibited auto-AMPylation activity. Mutation of the Fic-1 auto-AMPylated site greatly reduced AMPylation activity toward itself and toward GyrB. Fic-1-dependent AMPylation of GyrB triggered the SOS response, indicative of DNA replication stress or DNA damage. Fic-1 also promoted the formation of elongated cells when the SOS response was blocked. We identified an α-inhibitor protein that we named anti-Fic-1 (AntF), encoded by a gene immediately upstream of Fic-1. AntF interacted with Fic-1, inhibited the AMPylation activity of Fic-1 for GyrB in vitro, and blocked Fic-1-mediated inhibition of DNA replication in bacteria, suggesting that Fic-1 and AntF comprise a toxin-antitoxin module. Our work establishes Fic-1 as an AMPylating enzyme that targets GyrB to inhibit DNA replication and may target other proteins to regulate bacterial morphology.

Mouse models for assessing the cross-protective efficacy of oral non-typhoidal Salmonella vaccine candidates harbouring in-frame deletions of the ATP-dependent protease lon and other genes

In BALB/c mouse models of Salmonella enterica serovar Typhimurium infection, a single oral immunization with a mutant strain with an insertion of the chloramphenicol resistance gene into the ATP-dependent protease clpP or lon gene decreased the number of salmonellae in each tissue sample 5 days after oral challenge with virulent S. Typhimurium at weeks 26 and 54 post-immunization. These data suggested that an oral immunization with the ClpP- or Lon-disrupted S. Typhimurium strain could provide long-term protection against oral challenge with virulent S. Typhimurium. Accordingly, recombinant oral non-typhoidal Salmonella (NTS) vaccines were constructed by incorporating mutants of both S. Typhimurium and S. enterica serovar Enteritidis harbouring stable in-frame markerless deletions of the clpP-lon-sulA (suppressor of lon), lon-sulA or lon-msbB (acyltransferase) genes. Amongst these orally administered vaccine candidates, those with the lon-sulA gene deletion mutants of S. Typhimurium and S. Enteritidis protected BALB/c and C57BL/6J mice against oral challenge with both virulent S. Typhimurium and virulent S. Enteritidis. Therefore, the in-frame markerless lon-sulA gene deletion mutant of S. Typhimurium or S. Enteritidis could be a promising cross-protective NTS live vaccine candidate for practical use in humans.

Quantification of filamentation by uropathogenic Escherichia coli during experimental bladder cell infection by using semi-automated image analysis

Several rod-shaped pathogens including Escherichia coli, Salmonella spp. and Klebsiella pneumonia are capable of adopting highly filamentous cell shapes under certain circumstances. This phenomenon occurs as a result of continued cell elongation during growth without the usual septation into single rod-shaped cells. Evidence has emerged over the past decade suggesting that this morphological transformation is controlled and reversible and provides selective advantages under certain growth conditions, such as during infection in humans. In order to identify the factors which induce filamentation of bacterial pathogens and study the advantages of bacterial morphological plasticity, methods are needed to accurately quantify changes in bacterial cell shape. In this study, we present a method for quantification of bacterial filamentation based on automatic detection and measurement of bacterial units in focus-stacked microscopy images. Used in combination with a flow-chamber based in vitro cystitis model, we study the factors involved in filament formation by uropathogenic E. coli (UPEC) during infection. The influence of substratum surface, intracellular proliferation and flow media on UPEC filamentation is evaluated. We show that reversible UPEC filamentation during cystitis is not dependent on intracellular infection, which previous studies have suggested. Instead, we find that filamentation can be induced by contact with surfaces, both biological and artificial. Lastly our data indicate that UPEC filamentation is induced by trace-amounts of specific components in urine, rather than being a generic stress-response to high urine salt concentrations. The study shows that the combined methodology is generally useful for investigation of bacterial morphological transitions during cell infection.

Applications of microscopy in Salmonella research

Salmonella enterica is a Gram-negative enteropathogen that can cause localized infections, typically resulting in gastroenteritis, or systemic infection, e.g., typhoid fever, in humans and many other animals. Understanding the mechanisms by which Salmonella induces disease has been the focus of intensive research. This has revealed that Salmonella invasion requires dynamic cross-talk between the microbe and host cells, in which bacterial adherence rapidly leads to a complex sequence of cellular responses initiated by proteins translocated into the host cell by a type 3 secretion system. Once these Salmonella-induced responses have resulted in bacterial invasion, proteins translocated by a second type 3 secretion system initiate further modulation of cellular activities to enable survival and replication of the invading pathogen. Elucidation of the complex and highly dynamic pathogen-host interactions ultimately requires analysis at the level of single cells and single infection events. To achieve this goal, researchers have applied a diverse range of microscopy techniques to analyze Salmonella infection in models ranging from whole animal to isolated cells and simple eukaryotic organisms. For example, electron microscopy and high-resolution light microscopy techniques such as confocal microscopy can reveal the precise location of Salmonella and its relationship to cellular components. Widefield light microscopy is a simpler approach with which to study the interaction of bacteria with host cells and often has advantages for live cell imaging, enabling detailed analysis of the dynamics of infection and cellular responses. Here we review the use of imaging techniques in Salmonella research and compare the capabilities of different classes of microscope to address specific types of research question. We also provide protocols and notes on some microscopy techniques used routinely in our own research.

HU-induced polymorphous filamentation in fish pathogen Edwardsiella tarda leading to reduced invasion and virulence in zebrafish

Edwardsiella tarda is a rod-shaped Gram-negative pathogenic bacterium that causes hemorrhagic septicemia in fish. Nucleoid-associated protein HU is a basic DNA-binding protein with structural specificity in regulating genes expression. In wild-type E. tarda EIB202, HU is composed of two subunits HUα (hupA) and HUβ (hupB), and exists in homodimer or heterodimer forms. Different from the wild-type and ΔhupB mutant, ΔhupA mutant was found to be defective in cell growth, H2S production, acid adaptation, and exhibited abnormal cell division resulting in a filamentous phenotype in log phase bacteria. The qRT-PCR result showed that deletion of hupA significantly up-regulated the transcription levels of recA and sulA, which in turn stimulated RecA-dependent pathway to prevent cell division, resulting in filamentous morphology in E. tarda. Furthermore, the elongated ΔhupA cells showed a striking defect in EPC cell invasion, and the adhesion and internalization rates were reduced to 25% and 27% of the wild-type in log phase cultures. Confocal laser scanning microscopy revealed that filamentous bacteria failed to adhere to and could not be internalized into EPC. When some of the bacteria regained the rod-shape morphology in stationary cultures, the ΔhupA mutants showed increased adhesion and internalization rates into EPC. Moreover, ΔhupA mutant exhibited delayed mortalities (for two days) in zebrafish but the LD50 increased 17 folds. Immunohistochemical analysis showed that ΔhupA mutant reduced proliferation abilities in the muscle, liver and intestine of zebrafish. This study indicates that HU protein and strains morphology play essential roles in the virulence network of E. tarda.

Survival and virulence of Salmonella enterica serovar enteritidis filaments induced by reduced water activity

Salmonella enterica serovar Enteritidis strain E40 filaments were developed under conditions of a reduced water activity (a(w)) of 0.95 in tryptic soy broth (TSB) or tryptic soy agar (TSA) supplemented with 8% or 7% NaCl, respectively. Filament formation was accompanied by an increase of biomass without an increase in CFU and was affected by incubation temperature and the physical milieu. The greatest amount of filaments was recovered from TSA with 7% NaCl and incubation at 30°C. Within 2 h of transfer to fresh TSB, filaments started to septate into normal-sized cells, resulting in a rapid increase in CFU. S. Enteritidis E40 filaments were not more tolerant of low- or high-temperature stresses than nonfilamented control cells. However, there was greater survival of filaments in 10% bile salts after 24 to 48 h of incubation, during pH 2.0 acid challenge for 10 min, and under desiccation on stainless steel surfaces at 25°C and 75.5% relative humidity for 7 days. S. Enteritidis E40 filaments invaded and multiplied within Caco-2 human intestinal epithelial cells to a similar degree as control cells when a comparable CFU of filaments and control cells was used. S. Enteritidis E40 filaments established a successful infection in mice via intragastric inoculation. The filaments colonized the gastrointestinal tract and disseminated to the spleen and liver at levels comparable to those attained by control cells, even when animals were inoculated with 10- to 100-fold fewer CFU. To our knowledge this is the first demonstration of virulence of stress-induced Salmonella filaments in vitro and in vivo. Formation of filaments by Salmonella in food products and food processing environments is significant to food safety, because detection and quantitation of the pathogen may be compromised. The finding that these filaments are virulent further enhances their potential public health impact.