The microbiota conditions a gut milieu that selects for wild-type Salmonella Typhimurium virulence.

Salmonella Typhimurium elicits gut inflammation by the costly expression of HilD-controlled virulence factors. This inflammation alleviates colonization resistance (CR) mediated by the microbiota and thereby promotes pathogen blooms. However, the inflamed gut-milieu can also select for hilD mutants, which cannot elicit or maintain inflammation, therefore causing a loss of the pathogen's virulence. This raises the question of which conditions support the maintenance of virulence in S. Typhimurium. Indeed, it remains unclear why the wild-type hilD allele is dominant among natural isolates. Here, we show that microbiota transfer from uninfected or recovered hosts leads to rapid clearance of hilD mutants that feature attenuated virulence, and thereby contributes to the preservation of the virulent S. Typhimurium genotype. Using mouse models featuring a range of microbiota compositions and antibiotic- or inflammation-inflicted microbiota disruptions, we found that irreversible disruption of the microbiota leads to the accumulation of hilD mutants. In contrast, in models with a transient microbiota disruption, selection for hilD mutants was prevented by the regrowing microbiota community dominated by Lachnospirales and Oscillospirales. Strikingly, even after an irreversible microbiota disruption, microbiota transfer from uninfected donors prevented the rise of hilD mutants. Our results establish that robust S. Typhimurium gut colonization hinges on optimizing its manipulation of the host: A transient and tempered microbiota perturbation is favorable for the pathogen to both flourish in the inflamed gut and also minimize loss of virulence. Moreover, besides conferring CR, the microbiota may have the additional consequence of maintaining costly enteropathogen virulence mechanisms.

Mother-to-infant microbiota transmission and infant microbiota development across multiple body sites.

Early-life microbiota seeding and subsequent development is crucial to future health. Cesarean-section (CS) birth, as opposed to vaginal delivery, affects early mother-to-infant transmission of microbes. Here, we assess mother-to-infant microbiota seeding and early-life microbiota development across six maternal and four infant niches over the first 30 days of life in 120 mother-infant pairs. Across all infants, we estimate that on average 58.5% of the infant microbiota composition can be attributed to any of the maternal source communities. All maternal source communities seed multiple infant niches. We identify shared and niche-specific host/environmental factors shaping the infant microbiota. In CS-born infants, we report reduced seeding of infant fecal microbiota by maternal fecal microbes, whereas colonization with breastmilk microbiota is increased when compared with vaginally born infants. Therefore, our data suggest auxiliary routes of mother-to-infant microbial seeding, which may compensate for one another, ensuring that essential microbes/microbial functions are transferred irrespective of disrupted transmission routes.

The rearing environment persistently modulates mouse phenotypes from the molecular to the behavioural level.

The phenotype of an organism results from its genotype and the influence of the environment throughout development. Even when using animals of the same genotype, independent studies may test animals of different phenotypes, resulting in poor replicability due to genotype-by-environment interactions. Thus, genetically defined strains of mice may respond differently to experimental treatments depending on their rearing environment. However, the extent of such phenotypic plasticity and its implications for the replicability of research findings have remained unknown. Here, we examined the extent to which common environmental differences between animal facilities modulate the phenotype of genetically homogeneous (inbred) mice. We conducted a comprehensive multicentre study, whereby inbred C57BL/6J mice from a single breeding cohort were allocated to and reared in 5 different animal facilities throughout early life and adolescence, before being transported to a single test laboratory. We found persistent effects of the rearing facility on the composition and heterogeneity of the gut microbial community. These effects were paralleled by persistent differences in body weight and in the behavioural phenotype of the mice. Furthermore, we show that environmental variation among animal facilities is strong enough to influence epigenetic patterns in neurons at the level of chromatin organisation. We detected changes in chromatin organisation in the regulatory regions of genes involved in nucleosome assembly, neuronal differentiation, synaptic plasticity, and regulation of behaviour. Our findings demonstrate that common environmental differences between animal facilities may produce facility-specific phenotypes, from the molecular to the behavioural level. Furthermore, they highlight an important limitation of inferences from single-laboratory studies and thus argue that study designs should take environmental background into account to increase the robustness and replicability of findings.

The effect of live attenuated influenza vaccine on pneumococcal colonisation densities among children aged 24-59 months in The Gambia: a phase 4, open label, randomised, controlled trial.

BACKGROUND: Influenza and other respiratory viruses promote Streptococcus pneumoniae proliferation in the upper respiratory tract. We sought to investigate for what we believe is the first time, the effect of intranasal live attenuated influenza vaccine (LAIV) on nasopharyngeal S pneumoniae density in a low-income to middle-income country population with high pneumococcal carriage rates. METHODS: In an open-label, randomised, controlled trial in The Gambia, 330 healthy children aged 24-59 months were randomly assigned 2:1 to receive one trivalent LAIV dose at enrolment (day 0, intervention) or at the end of active follow-up (day 21, control). The investigator team were initially masked to block size and randomisation sequence to avoid allocation bias. Group allocation was later revealed to the investigator team. The primary outcome was PCR-quantified day 7 and 21 pneumococcal density. Asymptomatic respiratory viral infection at baseline and LAIV strain shedding were included as covariates in generalised mixed-effects models, to assess the effect of LAIV and other variables on pneumococcal densities. The study is registered at ClinicalTrials.gov, NCT02972957, and is closed to recruitment. FINDINGS: Between Feb 8 and April 12, 2017, and Jan 15 and March 28, 2018, of 343 children assessed for eligibility, 213 in the intervention group and 108 in the control group completed the study and were included in the final analysis. Although no significant differences were seen in pneumococcal carriage or density at each timepoint when comparing groups, changes from baseline were observed in the LAIV group. The baseline S pneumoniae carriage prevalence was high in both LAIV and control groups (75%) and increased by day 21 in the LAIV group (85%, p=0·0037), but not in the control group (79%, p=0·44). An increase in pneumococcal density from day 0 amounts was seen in the LAIV group at day 7 (+0·207 log10 copies per µL, SE 0·105, p=0·050) and day 21 (+0·280 log10 copies per µL, SE 0·105, p=0·0082), but not in the control group. Older age was associated with lower pneumococcal density (-0·015 log10 copies per µL, SE 0·005, p=0·0030), with the presence of asymptomatic respiratory viruses at baseline (+0·259 log10 copies per µL, SE 0·097, p=0·017), and greater LAIV shedding at day 7 (+0·380 log10 copies per µL, SE 0·167, p=0·024) associated with higher pneumococcal density. A significant increase in rhinorrhoea was reported in the LAIV group compared with the control group children during the first 7 days of the study (103 [48%] of 213, compared with 25 [23%] of 108, p<0·0001), and between day 7 and 21 (108 [51%] of 213, compared with 28 [26%] of 108, p<0·0001). INTERPRETATION: LAIV was associated with a modest increase in nasopharyngeal pneumococcal carriage and density in the 21 days following vaccination, with the increase in density lower in magnitude than previously described in the UK. This increase was accelerated when LAIV was administered in the presence of pre-existing asymptomatic respiratory viruses, suggesting that nasopharyngeal S pneumoniae proliferation is driven by cumulative mixed-viral co-infections. The effect of LAIV on pneumococcal density is probably similar to other respiratory viral infections in children. Our findings provide reassurance for the use of LAIV to expand influenza vaccine programmes in low-income to middle-income country populations with high pneumococcal carriage. FUNDING: Wellcome Trust.

Microbial community networks across body sites are associated with susceptibility to respiratory infections in infants.

Respiratory tract infections are a major cause of morbidity and mortality worldwide in young children. Concepts such as the gut-lung axis have highlighted the impact of microbial communities at distal sites in mediating disease locally. However, little is known about the extent to which microbial communities from multiple body sites are linked, and how this relates to disease susceptibility. Here, we combine 16S-based rRNA sequencing data from 112 healthy, term born infants, spanning three body sites (oral cavity, nasopharynx, gut) and the first six months of life. Using a cross-niche microbial network approach, we show that, already from the first week of life on, there is a strong association between both network structure and species essential to these structures (hub species), and consecutive susceptibility to respiratory tract infections in this cohort. Our findings underline the crucial role of cross-niche microbial connections in respiratory health.

Supplemented nutrition decreases helminth burden and increases drug efficacy in a natural host-helminth system.

Gastrointestinal (GI) helminths are common parasites of humans, wildlife, and livestock, causing chronic infections. In humans and wildlife, poor nutrition or limited resources can compromise an individual's immune response, predisposing them to higher helminth burdens. This relationship has been tested in laboratory models by investigating infection outcomes following reductions of specific nutrients. However, much less is known about how diet supplementation can impact susceptibility to infection, acquisition of immunity, and drug efficacy in natural host-helminth systems. We experimentally supplemented the diet of wood mice (Apodemus sylvaticus) with high-quality nutrition and measured resistance to the common GI nematode Heligmosomoides polygyrus. To test whether diet can enhance immunity to reinfection, we also administered anthelmintic treatment in both natural and captive populations. Supplemented wood mice were more resistant to H. polygyrus infection, cleared worms more efficiently after treatment, avoided a post-treatment infection rebound, produced stronger general and parasite-specific antibody responses, and maintained better body condition. In addition, when applied in conjunction with anthelmintic treatment, supplemented nutrition significantly reduced H. polygyrus transmission potential. These results show the rapid and extensive benefits of a well-balanced diet and have important implications for both disease control and wildlife health under changing environmental conditions.

Nasopharyngeal Microbiota Profiles in Rural Venezuelan Children Are Associated With Respiratory and Gastrointestinal Infections.

BACKGROUND: Recent research suggests that the microbiota affects susceptibility to both respiratory tract infections (RTIs) and gastrointestinal infections (GIIs). In order to optimize global treatment options, it is important to characterize microbiota profiles across different niches and geographic/socioeconomic areas where RTI and GII prevalences are high. METHODS: We performed 16S sequencing of nasopharyngeal swabs from 209 Venezuelan Amerindian children aged 6 weeks-59 months who were participating in a 13-valent pneumococcal conjugate vaccine (PCV13) study. Using random forest models, differential abundance testing, and regression analysis, we determined whether specific bacteria were associated with RTIs or GIIs and variation in PCV13 response. RESULTS: Microbiota compositions differed between children with or without RTIs (P = .018) or GIIs (P = .001). Several species were associated with the absence of infections. Some of these health-associated bacteria are also observed in developed regions, such as Corynebacterium (log2(fold change [FC]) = 3.30 for RTIs and log2(FC) = 1.71 for GIIs), while others are not commonly observed in developed regions, such as Acinetobacter (log2(FC) = 2.82 and log2(FC) = 5.06, respectively). Klebsiella spp. presence was associated with both RTIs (log2(FC) = 5.48) and GIIs (log2(FC) = 7.20). CONCLUSIONS: The nasopharyngeal microbiota of rural Venezuelan children included several bacteria that thrive in tropical humid climates. Interestingly, nasopharyngeal microbiota composition not only differed in children with an RTI but also in those with a GII, which suggests a reciprocal interplay between the 2 environments. Knowledge of region-specific microbiota patterns enables tailoring of preventive and therapeutic approaches.

Infant respiratory syncytial virus prophylaxis and nasopharyngeal microbiota until 6 years of life: a subanalysis of the MAKI randomised controlled trial.

BACKGROUND: Respiratory syncytial virus (RSV) infection during infancy is suggested to cause long-term wheeze. In turn, wheeze has been associated with bacterial dysbiosis of the respiratory tract. We investigated the effects of RSV prophylaxis with palivizumab in otherwise healthy preterm infants on respiratory microbiota composition at 1 year and 6 years of age. METHODS: In a multicentre, single-blind, randomised, placebo-controlled trial (the MAKI trial), infants born between 32-35 weeks of gestation, in one university and in 15 regional hospitals in the the Netherlands, were randomly assigned (1:1) to receive palivizumab or placebo during the RSV season of their first year of life. Intramuscular injections of palivizumab 15 mg/kg or placebo were given during one RSV season: either from Oct 1, or from discharge from the neonatal unit until March 10 (minimun of 2 and maximum of 5 injections were given). Children were 6 months old or younger at the start of the RSV season; exclusion criteria included congenital heart disease, bronchopulmonary dysplasia, Down's syndrome, or other serious congenital disorders, use of mechanical ventilation at birth, treatment with surfactant, or physician-diagnosed wheeze before the start of the RSV season. Children were followed up for clinical symptoms until 6 years of age. For this subanalysis, we obtained nasopharyngeal swabs from children aged 1 year and 6 years and analysed them using 16S-rRNA sequencing. At 6 years we also measured reversible airway obstruction. The primary outcome was the effect of palivizumab during infancy on the respiratory microbiota composition at age 1 year and 6 years (intention-to-treat analysis). The trial is registered in the ISRCTN registry, number ISRCTN73641710. FINDINGS: From April 1, 2008, to Dec 31, 2010, 429 infants were enrolled in the MAKI trial (n=214 to the palivizumab group; n=215 to the placebo group). At 1 year, we collected swabs and sequenced DNA from 170 (40%) of 429 children, of which 145 (85%) samples had high-quality DNA. The overall microbiota composition was significantly different (R2 1·3%; p=0·0185) between the palivizumab group and the placebo group at 1 year of life; children in the palivizumab group had a significantly lower abundance of the Staphylococcus-dominated cluster (odds ratio 0·28 [95% CI 0·11-0·68]; p=0·00394), an increased abundance of biomarker species, such as Klebsiella, and a more diverse set of oral taxa, including Streptococcus spp, compared with children in the placebo group. At 6 years, we collected swabs and sequenced DNA from 349 (88%) of 395 children who completed follow-up, of which 342 (98%) samples had high-quality DNA. The overall microbiota composition was not significantly different between groups at 6 years (R2 0·6%; p=0·0575); however, children in the palivizumab group had a significantly increased abundance of Haemophilus spp and lower abundance of Moraxella and Neisseriaceae spp compared with children in the placebo group. Absence of PCR-confirmed RSV infection at 1 year was significantly associated with a higher abundance of Haemophilus spp at age 6 years and a significantly lower abundance of Moraxella and Neisseriaceae than children with RSV infection at 1 year. Reversible airway obstruction at 6 years was also positively associated with Haemophilus abundance and negatively associated with the abundance of health-associated taxa, such as Moraxella, Corynebacterium, Dolosigranulum, and Staphylococcus, even after correction for RSV immunoprophylaxis (all: p<0·05). Additionally, reversible airway instruction was associated with significantly higher Streptococcus pneumoniae abundance. INTERPRETATION: Palivizumab in infancy in otherwise healthy preterm infants is associated with persistent effects on the abundance of specific, potentially pathogenic, microbial taxa in the respiratory tract. Several of the palivizumab-associated biomarker species were associated with reversible airway obstruction at age 6 years. These results warrant further studies to establish the long-term ecological effects and health consequences of palivizumab in infancy. FUNDING: MedImmune.

Variation in Local and Systemic Pro-Inflammatory Immune Markers of Wild Wood Mice after Anthelmintic Treatment.

The immune system represents a host's main defense against infection to parasites and pathogens. In the wild, a host's response to immune challenges can vary due to physiological condition, demography (age, sex), and coinfection by other parasites or pathogens. These sources of variation, which are intrinsic to natural populations, can significantly impact the strength and type of immune responses elicited after parasite exposure and infection. Importantly, but often neglected, a host's immune response can also vary within the individual, across tissues and between local and systemic scales. Consequently, how a host responds at each scale may impact its susceptibility to concurrent and subsequent infections. Here we analyzed how characteristics of hosts and their parasite infections drive variation in the pro-inflammatory immune response in wild wood mice (Apodemus sylvaticus) at both the local and systemic scale by experimentally manipulating within-host parasite communities through anthelmintic drug treatment. We measured concentrations of the pro-inflammatory cytokine tumor necrosis factor alpha (TNF-α) produced in vitro in response to a panel of toll-like receptor agonists at the local (mesenteric lymph nodes [MLNs]) and systemic (spleen) scales of individuals naturally infected with two gastrointestinal parasites, the nematode Heligmosomoides polygyrus and the protozoan Eimeria hungaryensis. Anthelmintic-treated mice had a 20-fold lower worm burden compared to control mice, as well as a four-fold higher intensity of the non-drug targeted parasite E. hungaryensis. Anthelmintic treatment differentially impacted levels of TNF-α expression in males and females at the systemic and local scales, with treated males producing higher, and treated females lower, levels of TNF-α, compared to control mice. Also, TNF-α was affected by host age, at the local scale, with MLN cells of young, treated mice producing higher levels of TNF-α than those of old, treated mice. Using complementary, but distinct, measures of inflammation measured across within-host scales allowed us to better assess the wood mouse immune response to changes in parasite infection dynamics after anthelmintic treatment. This same approach could be used to understand helminth infections and responses to parasite control measures in other systems in order to gain a broader view of how variation impacts the immune response.

Age affects antibody levels and anthelmintic treatment efficacy in a wild rodent.

The role of the host immune system in determining parasite burdens and mediating within-host parasite interactions has traditionally been studied in highly controlled laboratory conditions. This does, however, not reflect the diversity of individuals living in nature, which is often characterised by significant variation in host demography, such as host age, sex, and infection history. Whilst studies using wild hosts and parasites are beginning to give insights into the complex relationships between immunity, parasites and host demography, the cause-and-effect relationships often remain unknown due to a lack of high resolution, longitudinal data. We investigated the infection dynamics of two interacting gastrointestinal parasites of wild wood mice (Apodemus sylvaticus), the nematode Heligmosomoides polygyrus and the coccidian Eimeria hungaryensis, in order to assess the links between infection, coinfection, and the immunological dynamics of two antibodies (IgG1 and IgA). In an anthelmintic treatment experiment, mice were given a single oral dose of an anthelmintic treatment, or control dose, and then subsequently followed longitudinally over a period of 7-15 days to measure parasite burdens and antibody levels. Anthelmintic treatment successfully reduced burdens of H. polygyrus, but had no significant impact on E. hungaryensis. Treatment efficacy was driven by host age, with adult mice showing stronger reductions in burdens compared to younger mice. We also found that the relationship between H. polygyrus-specific IgG1 and nematode burden changed from positive in young mice to negative in adult mice. Our results highlight that a key host demographic factor like age could account for large parts of the variation in nematode burden and nematode-specific antibody levels observed in a naturally infected host population, possibly due to different immune responses in young vs. old animals. Given the variable success in community-wide de-worming programmes in animals and humans, accounting for the age-structure of a population could increase overall efficacy.

Parasitic nematodes simultaneously suppress and benefit from coccidian coinfection in their natural mouse host.

Within-host interactions among coinfecting parasites are common and have important consequences for host health and disease dynamics. However, these within-host interactions have traditionally been studied in laboratory mouse models, which often exclude important variation and use unnatural host-parasite combinations. Conversely, the few wild studies of within-host interactions often lack knowledge of parasite exposure and infection history. Here we exposed laboratory-reared wood mice (Apodemus sylvaticus) that were derived from wild-caught animals to two naturally-occurring parasites (nematode: Heligmosomoides polygyrus, coccidia: Eimeria hungaryensis) to investigate the impact of coinfection on parasite infection dynamics, and to determine if the host immune response mediates this interaction. Coinfection led to delayed worm expulsion and prolonged egg shedding in H. polygyrus infections and lower peak E. hungaryensis oocyst burdens. By comparing antibody levels between wild and colony-housed mice, we also found that wild mice had elevated H. polygyrus-IgG1 titres even if currently uninfected with H. polygyrus. Using this unique wild-laboratory system, we demonstrate, for the first time, clear evidence for a reciprocal interaction between these intestinal parasites, and that there is a great discrepancy between antibody levels measured in the wild vs those measured under controlled laboratory conditions in relation to parasite infection and coinfection.

Loss of Microbial Topography between Oral and Nasopharyngeal Microbiota and Development of Respiratory Infections Early in Life.

Rationale: The respiratory microbiota is increasingly being appreciated as an important mediator in the susceptibility to childhood respiratory tract infections (RTIs). Pathogens are presumed to originate from the nasopharyngeal ecosystem.Objectives: To investigate the association between early life respiratory microbiota and development of childhood RTIs.Methods: In a prospective birth cohort (Microbiome Utrecht Infant Study: MUIS), we characterized the oral microbiota longitudinally from birth until 6 months of age of 112 infants (nine regular samples/subject) and compared them with nasopharyngeal microbiota using 16S-rRNA-based sequencing. We also characterized oral and nasopharynx samples during RTI episodes in the first half year of life.Measurements and Main Results: Oral microbiota were driven mostly by feeding type, followed by age, mode of delivery, and season of sampling. In contrast to our previously published associations between nasopharyngeal microbiota development and susceptibility to RTIs, oral microbiota development was not directly associated with susceptibility to RTI development. However, we did observe an influx of oral taxa, such as Neisseria lactamica, Streptococcus, Prevotella nanceiensis, Fusobacterium, and Janthinobacterium lividum, in the nasopharyngeal microbiota before and during RTIs, which was accompanied by reduced presence and abundance of Corynebacterium, Dolosigranulum, and Moraxella spp. Moreover, this phenomenon was accompanied by reduced niche differentiation indicating loss of ecological topography preceding confirmed RTIs. This loss of ecological topography was further augmented by start of daycare, and linked to consecutive development of symptomatic infections.Conclusions: Together, our results link the loss of topography to subsequent development of RTI episodes. This may lead to new insights for prevention of RTIs and antibiotic use in childhood.

Antibodies and coinfection drive variation in nematode burdens in wild mice.

Coinfections with parasitic helminths and microparasites are highly common in nature and can lead to complex within-host interactions between parasite species which can cause negative health outcomes for humans, and domestic and wild animals. Many of these negative health effects worsen with increasing parasite burdens. However, even though many studies have identified several key factors that determine worm burdens across various host systems, less is known about how the immune response interacts with these factors and what the consequences are for the outcome of within-host parasite interactions. We investigated two interacting gastrointestinal parasites of wild wood mice, Heligmosomoides polygyrus (nematode) and Eimeria spp. (coccidia), in order to investigate how host demographic factors, coinfection and the host's immune response affected parasite burdens and infection probability, and to determine what factors predict parasite-specific and total antibody levels. We found that antibody levels were the only factors that significantly influenced variation in both H. polygyrus burden and infection probability, and Eimeria spp. infection probability. Total faecal IgA was negatively associated with H. polygyrus burden and Eimeria spp. infection, whereas H. polygyrus-specific IgG1 was positively associated with H. polygyrus infection. We further found that the presence of Eimeria spp. had a negative effect on both faecal IgA and H. polygyrus-specific IgG1. Our results show that even in the context of natural demographic and immunological variation amongst individuals, we were able to decipher a role for the host humoral immune response in shaping the within-host interaction between H. polygyrus and Eimeria spp.

The Immune and Non-Immune Pathways That Drive Chronic Gastrointestinal Helminth Burdens in the Wild.

Parasitic helminths are extremely resilient in their ability to maintain chronic infection burdens despite (or maybe because of) their hosts' immune response. Explaining how parasites maintain these lifelong infections, identifying the protective immune mechanisms that regulate helminth infection burdens, and designing prophylactics and therapeutics that combat helminth infection, while preserving host health requires a far better understanding of how the immune system functions in natural habitats than we have at present. It is, therefore, necessary to complement mechanistic laboratory-based studies with studies on wild populations and their natural parasite communities. Unfortunately, the relative paucity of immunological tools for non-model species has held these types of studies back. Thankfully, recent progress in high-throughput 'omics platforms provide powerful and increasingly practical means for immunologists to move beyond traditional lab-based model organisms. Yet, assigning both metabolic and immune function to genes, transcripts, and proteins in novel species and assessing how they interact with other physiological and environmental factors requires identifying quantitative relationships between their expression and infection. Here, we used supervised machine learning to identify gene networks robustly associated with burdens of the gastrointestinal nematode Heligmosomoides polygyrus in its natural host, the wild wood mice Apodemus sylvaticus. Across 34 mice spanning two wild populations and across two different seasons, we found 17,639 transcripts that clustered in 131 weighted gene networks. These clusters robustly predicted H. polygyrus burden and included well-known effector and regulatory immune genes, but also revealed a number of genes associated with the maintenance of tissue homeostasis and hematopoiesis that have so far received little attention. We then tested the effect of experimentally reducing helminth burdens through drug treatment on those putatively protective immune factors. Despite the near elimination of H. polygyrus worms, the treatment had surprisingly little effect on gene expression. Taken together, these results suggest that hosts balance tissue homeostasis and protective immunity, resulting in relatively stable immune and, consequently, parasitological profiles. In the future, applying our approach to larger numbers of samples from additional populations will help further increase our ability to detect the immune pathways that determine chronic gastrointestinal helminth burdens in the wild.

Lipopolysaccharide from Crypt-Specific Core Microbiota Modulates the Colonic Epithelial Proliferation-to-Differentiation Balance.

We identified a crypt-specific core microbiota (CSCM) dominated by strictly aerobic, nonfermentative bacteria in murine cecal and proximal colonic (PC) crypts and hypothesized that, among its possible functions, it may affect epithelial regeneration. In the present work, we isolated representative CSCM strains using selective media based upon our initial 16S rRNA-based molecular identification (i.e., Acinetobacter, Delftia, and Stenotrophomonas). Their tropism for the crypt was confirmed, and their influence on epithelial regeneration was demonstrated in vivo by monocolonization of germfree mice. We also showed that lipopolysaccharide (LPS), through its endotoxin activity, was the dominant bacterial agonist controlling proliferation. The relevant molecular mechanisms were analyzed using colonic crypt-derived organoids exposed to bacterial sonicates or highly purified LPS as agonists. We identified a Toll-like receptor 4 (TLR4)-dependent program affecting crypts at different stages of epithelial differentiation. LPS played a dual role: it repressed cell proliferation through RIPK3-mediated necroptosis of stem cells and cells of the transit-amplifying compartment and concurrently enhanced cell differentiation, particularly the goblet cell lineage.IMPORTANCE The LPS from crypt-specific core microbiota controls intestinal epithelium proliferation through necroptosis of stem cells and enhances cell differentiation, mainly the goblet cell lineage.

Expression of parasite genetic variation changes over the course of infection: implications of within-host dynamics for the evolution of virulence.

How infectious disease agents interact with their host changes during the course of infection and can alter the expression of disease-related traits. Yet by measuring parasite life-history traits at one or few moments during infection, studies have overlooked the impact of variable parasite growth trajectories on disease evolution. Here we show that infection-age-specific estimates of host and parasite fitness components can reveal new insight into the evolution of parasites. We do so by characterizing the within-host dynamics over an entire infection period for five genotypes of the castrating bacterial parasite Pasteuria ramosa infecting the crustacean Daphnia magna. Our results reveal that genetic variation for parasite-induced gigantism, host castration and parasite spore loads increases with the age of infection. Driving these patterns appears to be variation in how well the parasite maintains control of host reproduction late in the infection process. We discuss the evolutionary consequences of this finding with regard to natural selection acting on different ages of infection and the mechanism underlying the maintenance of castration efficiency. Our results highlight how elucidating within-host dynamics can shed light on the selective forces that shape infection strategies and the evolution of virulence.