Serratia plymuthica HK9-3 enhances tomato resistance against Phytophthora capsici by modulating antioxidant defense systems and rhizosphere micro-ecological condition.

Tomatoes are frequently challenged by various pathogens, among which Phytophthora capsici (P. capsici) is a destructive soil-borne pathogen that seriously threatens the safe production of tomatoes. Plant growth-promoting rhizobacteria (PGPR) positively induced plant resistance against multiple pathogens. However, little is known about the role and regulatory mechanism of PGPR in tomato resistance to P. capsici. Here, we identified a new strain Serratia plymuthica (S. plymuthica), HK9-3, which has a significant antibacterial effect on P. capsici infection. Meanwhile, stable colonization in roots by HK9-3, even under P. capsici infection, improved tomato growth parameters, root system architecture, photosynthetic capacity, and boosted biomass. Importantly, HK9-3 colonization significantly alleviated the damage caused by P. capsici infection through enhancing ROS scavenger ability and inducing antioxidant defense system and pathogenesis-related (PR) proteins in leaves, as evidenced by elevating the activities of peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), and chitinase, ß-1,3-glucanase, and increasing the transcripts of POD, SOD, CAT, APX1, PAL1, PAL2, PAL5, PPO2, CHI17 and ß-1,3-glucanase genes. Notably, HK9-3 colonization not only effectively improved soil microecology and soil fertility, but also significantly enhanced fruit yield by 44.6% and improved quality. Our study presents HK9-3 as a promising and effective solution for controlling P. capsici infection in tomato cultivation while simultaneously promoting plant growth and increasing yield, which may have implications for P. capsici control in vegetable production.

ShlA toxin of Serratia induces P2Y2- and α5ß1-dependent autophagy and bacterial clearance from host cells.

Serratia marcescens is an opportunistic human pathogen involved in antibiotic-resistant hospital acquired infections. Upon contact with the host epithelial cell and prior to internalization, Serratia induces an early autophagic response that is entirely dependent on the ShlA toxin. Once Serratia invades the eukaryotic cell and multiples inside an intracellular vacuole, ShlA expression also promotes an exocytic event that allows bacterial egress from the host cell without compromising its integrity. Several toxins, including ShlA, were shown to induce ATP efflux from eukaryotic cells. Here, we demonstrate that ShlA triggered a nonlytic release of ATP from Chinese hamster ovary (CHO) cells. Enzymatic removal of accumulated extracellular ATP (eATP) or pharmacological blockage of the eATP-P2Y2 purinergic receptor inhibited the ShlA-promoted autophagic response in CHO cells. Despite the intrinsic ecto-ATPase activity of CHO cells, the effective concentration and kinetic profile of eATP was consistent with the established affinity of the P2Y2 receptor and the known kinetics of autophagy induction. Moreover, eATP removal or P2Y2 receptor inhibition also suppressed the ShlA-induced exocytic expulsion of the bacteria from the host cell. Blocking α5ß1 integrin highly inhibited ShlA-dependent autophagy, a result consistent with α5ß1 transactivation by the P2Y2 receptor. In sum, eATP operates as the key signaling molecule that allows the eukaryotic cell to detect the challenge imposed by the contact with the ShlA toxin. Stimulation of P2Y2-dependent pathways evokes the activation of a defensive response to counteract cell damage and promotes the nonlytic clearance of the pathogen from the infected cell.

Novel associations in antibiosis stemming from an insect pupal cell.

The pupal soil cell of the pecan weevil, Curculio caryae (Coleoptera: Curculionidae), was reported previously to exhibit antibiosis to an entomopathogenic fungus, Beauveria bassiana. The objectives of this study were to examine 1) if the antimicrobial effect occurs in other insects that form pupal cells, 2) whether the effect extends to plant pathogenic fungi, and 3) identify the source of antibiosis in pupal soil cells of C. caryae. Antibiosis of pupal cells against B. bassiana was confirmed in-vitro in three additional curculionids, Diaprepes abbreviatus, Conotrachelus nenuphar, and Pissodes nemorensis, all of which had fewer fungal colonies relative to controls. Pupal soil cells were found to suppress phytopathogenic fungi in-vitro, including suppression of Alternaria solani by D. abbreviatus pupal cell, and that of Monilinia fructicola by C. caryae. The detection of antibiosis of soil cells formed by surface-sterilized insects using sterile soil implies the antimicrobial effect stemmed from inside the insect. Further, a novel biotic mechanism was identified: a bacterium related to Serratia nematodiphila was isolated from C. caryae pupal soil cells and was found to be associated with antibiosis. The bacterial cultures with or without autoclave had similar effects but were not as potent as pupal soil cells for suppressing B. bassiana. Also, autoclaved soil cells and autoclaved bacterial culture suppressed M. fructicola but were not as inhibitory as non-autoclaved soil cells. This indicates that antibiosis may be due to bacterial metabolites, although other factors may also be involved. Our findings suggest potential to develop the antibiotic compounds as novel bio-fungicides to control plant diseases.

Translocated microbiome composition determines immunological outcome in treated HIV infection.

The impact of the microbiome on HIV disease is widely acknowledged although the mechanisms downstream of fluctuations in microbial composition remain speculative. We detected rapid, dynamic changes in translocated microbial constituents during two years after cART initiation. An unbiased systems biology approach revealed two distinct pathways driven by changes in the abundance ratio of Serratia to other bacterial genera. Increased CD4 T cell numbers over the first year were associated with high Serratia abundance, pro-inflammatory innate cytokines, and metabolites that drive Th17 gene expression signatures and restoration of mucosal integrity. Subsequently, decreased Serratia abundance and downregulation of innate cytokines allowed re-establishment of systemic T cell homeostasis promoting restoration of Th1 and Th2 gene expression signatures. Analyses of three other geographically distinct cohorts of treated HIV infection established a more generalized principle that changes in diversity and composition of translocated microbial species influence systemic inflammation and consequently CD4 T cell recovery.

Serratia symbiotica Enhances Fatty Acid Metabolism of Pea Aphid to Promote Host Development.

Bacterial symbionts associated with insects are often involved in host development and ecological adaptation. Serratia symbiotica, a common facultative endosymbiont harbored in pea aphids, improves host fitness and heat tolerance, but studies concerning the nutritional metabolism and impact on the aphid host associated with carrying Serratia are limited. In the current study, we showed that Serratia-infected aphids had a shorter nymphal developmental time and higher body weight than Serratia-free aphids when fed on detached leaves. Genes connecting to fatty acid biosynthesis and elongation were up-regulated in Serratia-infected aphids. Specifically, elevated expression of fatty acid synthase 1 (FASN1) and diacylglycerol-o-acyltransferase 2 (DGAT2) could result in accumulation of myristic acid, palmitic acid, linoleic acid, and arachidic acid in fat bodies. Impairing fatty acid synthesis in Serratia-infected pea aphids either by a pharmacological inhibitor or through silencing FASN1 and DGAT2 expression prolonged the nymphal growth period and decreased the aphid body weight. Conversely, supplementation of myristic acid (C14:0) to these aphids restored their normal development and weight gain. Our results indicated that Serratia promoted development and growth of its aphid host through enhancing fatty acid biosynthesis. Our discovery has shed more light on nutritional effects underlying the symbiosis between aphids and facultative endosymbionts.

A natural symbiotic bacterium drives mosquito refractoriness to Plasmodium infection via secretion of an antimalarial lipase.

The stalling global progress in the fight against malaria prompts the urgent need to develop new intervention strategies. Whilst engineered symbiotic bacteria have been shown to confer mosquito resistance to parasite infection, a major challenge for field implementation is to address regulatory concerns. Here, we report the identification of a Plasmodium-blocking symbiotic bacterium, Serratia ureilytica Su_YN1, isolated from the midgut of wild Anopheles sinensis in China that inhibits malaria parasites via secretion of an antimalarial lipase. Analysis of Plasmodium vivax epidemic data indicates that local malaria cases in Tengchong (Yunnan province, China) are significantly lower than imported cases and importantly, that the local vector A. sinensis is more resistant to infection by P. vivax than A. sinensis from other regions. Analysis of the gut symbiotic bacteria of mosquitoes from Yunnan province led to the identification of S. ureilytica Su_YN1. This bacterium renders mosquitoes resistant to infection by the human parasite Plasmodium falciparum or the rodent parasite Plasmodium berghei via secretion of a lipase that selectively kills parasites at various stages. Importantly, Su_YN1 rapidly disseminates through mosquito populations by vertical and horizontal transmission, providing a potential tool for blocking malaria transmission in the field.

Vertical Transmission at the Pathogen-Symbiont Interface: Serratia symbiotica and Aphids.

Many insects possess beneficial bacterial symbionts that occupy specialized host cells and are maternally transmitted. As a consequence of their host-restricted lifestyle, these symbionts often possess reduced genomes and cannot be cultured outside hosts, limiting their study. The bacterial species Serratia symbiotica was originally characterized as noncultured strains that live as mutualistic symbionts of aphids and are vertically transmitted through transovarial endocytosis within the mother's body. More recently, culturable strains of S. symbiotica were discovered that retain a larger set of ancestral Serratia genes, are gut pathogens in aphid hosts, and are principally transmitted via a fecal-oral route. We find that these culturable strains, when injected into pea aphids, replicate in the hemolymph and are pathogenic. Unexpectedly, they are also capable of maternal transmission via transovarial endocytosis: using green fluorescent protein (GFP)-tagged strains, we observe that pathogenic S. symbiotica strains, but not Escherichia coli, are endocytosed into early embryos. Furthermore, pathogenic S. symbiotica strains are compartmentalized into specialized aphid cells in a fashion similar to that of mutualistic S. symbiotica strains during later stages of embryonic development. However, infected embryos do not appear to develop properly, and offspring infected by a transovarial route are not observed. Thus, cultured pathogenic strains of S. symbiotica have the latent capacity to transition to lifestyles as mutualistic symbionts of aphid hosts, but persistent vertical transmission is blocked by their pathogenicity. To transition into stably inherited symbionts, culturable S. symbiotica strains may need to adapt to regulate their titer, limit their pathogenicity, and/or provide benefits to aphids that outweigh their cost.IMPORTANCE Insects have evolved various mechanisms to reliably transmit their beneficial bacterial symbionts to the next generation. Sap-sucking insects, including aphids, transmit symbionts by endocytosis of the symbiont into cells of the early embryo within the mother's body. Experimental studies of this process are hampered by the inability to culture or genetically manipulate host-restricted, symbiotic bacteria. Serratia symbiotica is a bacterial species that includes strains ranging from obligate, heritable symbionts to gut pathogens. We demonstrate that culturable S. symbiotica strains, which are aphid gut pathogens, can be maternally transmitted. Cultured S. symbiotica therefore possesses a latent capacity for evolving a host-restricted lifestyle and can be used to understand the transition from pathogenicity to beneficial symbiosis.

Control of damping-off in tomato seedlings exerted by Serratia spp. strains and identification of inhibitory bacterial volatiles in vitro.

Serratia marcescens can be a plant growth promoting bacteria (PGPB) and an opportunistic human and plant pathogen. We have identified and characterized strains of related species of Serratia and evaluated their biological control of damping-off of tomato seeds caused by Pythium cryptoirregulare. Serratia ureilytica, S. bockelmannii and S. nevei were identified by phylogenetic analysis of partial gyrB gene sequence and average nucleotide identity (ANI). Tomato seeds inoculated with S. ureilytica ILBB 145 showed higher germination percentage and reduced damping-off in greenhouse experiment resembling a commercial operation, and volatiles produced by this strain caused the nearly complete inhibition in vitro of P. cryptoirregulare. Analysis of volatile organic compounds (VOCs) showed that ILBB 145 produced dimethyl disulfide (DMDS), which can partially account for this inhibition. Serratia bockelmannii ILBB 162 performance against damping-off was intermediate and the inhibition of P. cryptoirregulare in vitro was lower and explained by volatile and diffusible metabolites. Both strains augmented DMDS production in the presence of P. cryptoirregulare, suggesting this compound may play a role in the context of interspecific competition. Serratia nevei ILBB 219 showed the lowest inhibition of P. cryptoirregulare in vitro, no DMDS production, and no biocontrol in planta. Draft genomes of the three strains were annotated and individual genes and biosynthesis gene clusters were identified in relation with the observed phenotypes. We report S. ureilytica - a low risk species- with activity as a biological control agent and DMDS produced by this bacterial species putatively involved in seed and seedling protection against P. cryptoirregulare.

The Rcs stress response inversely controls surface and CRISPR-Cas adaptive immunity to discriminate plasmids and phages.

Bacteria harbour multiple innate defences and adaptive CRISPR-Cas systems that provide immunity against bacteriophages and mobile genetic elements. Although some bacteria modulate defences in response to population density, stress and metabolic state, a lack of high-throughput methods to systematically reveal regulators has hampered efforts to understand when and how immune strategies are deployed. We developed a robust approach called SorTn-seq, which combines saturation transposon mutagenesis, fluorescence-activated cell sorting and deep sequencing to characterize regulatory networks controlling CRISPR-Cas immunity in Serratia sp. ATCC 39006. We applied our technology to assess csm gene expression for ~300,000 mutants and uncovered multiple pathways regulating type III-A CRISPR-Cas expression. Mutation of igaA or mdoG activated the Rcs outer-membrane stress response, eliciting cell-surface-based innate immunity against diverse phages via the transcriptional regulators RcsB and RcsA. Activation of this Rcs phosphorelay concomitantly attenuated adaptive immunity by three distinct type I and III CRISPR-Cas systems. Rcs-mediated repression of CRISPR-Cas defence enabled increased acquisition and retention of plasmids. Dual downregulation of cell-surface receptors and adaptive immunity in response to stress by the Rcs pathway enables protection from phage infection without preventing the uptake of plasmids that may harbour beneficial traits.

Serratia sp. CP-13 alleviates Cd toxicity by morpho-physio-biochemical improvements, antioxidative potential and diminished Cd uptake in Zea mays L. cultivars differing in Cd tolerance.

Cadmium (Cd) is highly toxic for plant metabolic processes even in low concentration due to higher retention rates, longer half-life and non-biodegradable nature. The current study was designed to assess the bioremediation potential of Cd tolerant PGPR, Serratia sp. CP-13 together with two differentially Cd tolerant maize cultivars (MMRI-Yellow, Sahiwal-2002) selected amongst ten cultivars after screening. The maize cultivars were grown under different Cd treatments (0, 6, 12, 18, 24, 30 µM) in Petri plates both with and without Serratia sp. CP-13 inoculation. Treated plants were analyzed for their biomass accumulation, chlorophylls, carotenoids, proline, anthocyanin, protein, malondialdehyde (MDA), H2O2 as well as for antioxidants (POD, SOD, CAT) and mineral elements (Ca, Mg, Zn, K, Fe, Na, Cd). The maize cultivar MMRI-Yellow (tolerant) and Sahiwal-2002 (sensitive) exhibited significant reduction in leaf area, nutrient contents, plant biomass, activity of antioxidants, total proteins, photosynthetic pigments as well as flavonoids with increased production of H2O2, proline, MDA and relative membrane permeability (RMP) under Cd stress. However, this reduction was cultivar specific and recorded higher in cv. Sahiwal-2002 as compared to MMRI-Yellow. Application of Serratia sp. CP-13 significantly augmented plant biomass, photosynthetic pigments, antioxidative machinery, as well as flavonoids and proline while diminishing H2O2, RMP MDA production even under Cd stress in studied cultivars. Furthermore, CP-13 inoculation assisted the Cd stressed plants to sustain an optimal level of essential nutrients (Ca, Mg, Zn, K, Fe) except for Na and Cd which responded antagonistically. It was inferred that both inoculated maize cultivars exhibited better health and metabolism but substantial Cd tolerance was acquired by the sensitive cv. Sahiwal-2002 than the tolerant cv. MMRI-Yellow under applied Cd regimes. Furthermore, studied maize cultivars depicted maximum Cd tolerance in order of 30 < 24 < 18 < 12 < 6 < 0 µM Cd treatments under Serratia sp. CP-13 inoculation. Findings of current work highlighted the importance of Serratia sp. CP-13 and its inoculation impact on morpho-physio-biochemical attributes of maize growth under Cd dominant environment, which is likely an addition towards efficient approaches for bacterially-assisted Cd bioremediation and minimal Cd retention in edible plant parts.

Ambigols from the Cyanobacterium Fischerella ambigua Increase Prodigiosin Production in Serratia spp.

When a library of 573 cyanobacteria extracts was screened for inhibition of the quorum sensing regulated prodigiosin production of Serratia marcescens, an extract of the cyanobacterium Fischerella ambigua (Näg.) Gomont 108b was found to drastically increase prodigiosin production. Bioactivity-guided isolation of the active compounds resulted in the two new natural products ambigol D and E along with the known ambigols A and C. Ambigol C treatment increased prodiginine production of Serratia sp. ATCC 39006 (S39006) by a factor of 10, while ambigols A and D were found to have antibiotic activity against this strain. The RNA-Seq of S39006 treated with ambigol C and subsequent differential gene expression and functional enrichment analyses indicated a significant downregulation of genes associated with the translation machinery and fatty acid biosynthesis in Serratia, as well as increased expression of genes related to the uptake of l-proline. These results suggest that the ambigols increase prodiginine production in S39006 not by activating the SmaIR quorum sensing system but possibly by increasing the precursor supply of l-proline and malonyl-CoA.

Study on the community structure and function of symbiotic bacteria from different growth and developmental stages of Hypsizygus marmoreus.

BACKGROUND: The symbiotic bacteria associated with edible fungi are valuable microbial resources worthy of in-depth exploration. It is important to analyze the community structure and succession of symbiotic bacteria in mushrooms. This can assist in the isolation of growth-promoting strains that have an essential relationship with the cultivation cycle as well as the agronomic traits and yields of fruiting bodies. RESULTS: In all of the samples from cultivation bags of Hypsizygus marmoreus, 34 bacterial phyla were detected. Firmicutes was the most abundant bacterial phylum (78.85%). The genus Serratia showed an exponential increase in abundance in samples collected from the cultivation bags in the mature period, reaching a peak abundance of 55.74% and the dominant symbiotic flora. The most predominant strain was Serratia odorifera HZSO-1, and its abundance increased with the amount of hyphae of H. marmoreus. Serratia odorifera HZSO-1 could reside in the hyphae of H. marmoreus, promote growth and development, shorten the fruiting cycle by 3-4 days, and further increase the fruiting body yield by 12%. CONCLUSIONS: This study is a pioneering demonstration of the community structure of the symbiotic microbiota and bacteria-mushroom interaction in the growth and development of edible fungi. This work lays a theoretical foundation to improve the industrial production of mushrooms with symbiotic bacteria as assisting agents.

Growth stimulatory effect of AHL producing Serratia spp. from potato on homologous and non-homologous host plants.

Plant growth promoting rhizobacteria are known to improve plant performance by developing healthy and productive interactions with the host plants. These associations may be symbiotic or asymbiotic depending upon the genetic potential of the resident microbe and promiscuity of the host. Present study describes the potential of two Serratia spp. strains for promotion of plant growth in homologous as well as non-homologous hosts. The strains KPS-10 and KPS-14; native to potato rhizosphere belong to genus Serratia based on 16S rRNA gene sequences (accession no. LN831934 and LN831937 respectively) and contain multiple plant growth promoting properties along-with the production of quorum sensing acyl homoserine lactone (AHL) molecules. Both Serratia spp. strains showed solubilization of inorganic tri-calcium phosphate while KPS-14 also exhibited phytase activity (1.98 10-10 kcat). KPS-10 showed higher P-solubilization activity (128.5 µg/mL), IAA production (8.84 µg/mL), antifungal activity and also showed the production of two organic acids i.e., gluconic acid and lactic acid. Both strains produced three common AHLs: C6-HSL, 3oxo-C10-HSL, 3oxo-C12-HSL while some strain-specific AHLs (3OH-C5-HSL, 3OH-C6-HSL, C10-HSL specific to KPS-10 and 3OH-C6-HSL, C8-HSL, 3oxo-C9-HSL, 3OH-C9-HSL specific to KPS-14). Strains showed roots and rhizosphere colonization of potato and other non-homologous hosts up to one month. In planta AHLs-detection confirmed a likely role of AHLs during seedling growth and development where both extracted AHLs or bacteria inoculated roots showed extensive root hair. A significant increase in root/shoot lengths, root/ shoot fresh weights, root/shoot dry weights was observed by inoculation in different hosts. PGP-characteristics along with the AHLs-production signify the potential of both strains as candidate for the development of bio-inoculum for potato crop in specific and other crops in general. This inoculum will not only reduce the input of chemical fertilizer to the environment but also improve soil quality and plant growth.

An aphid facultative symbiont suppresses plant defence by manipulating aphid gene expression in salivary glands.

Aphids often carry facultative symbionts to achieve diverse advantages. Serratia symbiotica, one of facultative endosymbionts, increases aphid tolerance to heat. However, whether it benefits aphid colonization on host plants is yet to be determined. In the current study, we found that Acyrthosiphon pisum harbouring S. symbiotica had longer feeding duration on Medicago truncatula than Serratia-free aphids. Contrastingly, Serratia-free aphids triggered higher accumulation of reactive oxygen species (ROS), jasmonic acid and salicylic acid responsive genes and cytosolic Ca2+ elevations than Serratia-infected aphids. Transcriptomic analysis of salivary glands indicated that a histidine-rich Ca2+ -binding protein-like gene (ApHRC) was expressed more highly in the salivary gland of Serratia-infected aphids than that of Serratia-free aphids. Once ApHRC was silenced, Serratia-infected aphids also displayed shorter phloem-feeding duration and caused Ca2+ elevation and ROS accumulation in plants. Our results suggest that ApHRC, a potential effector up-regulated by S. symbiotica in the salivary glands, impairs plant defence response by suppressing Ca2+ elevation and ROS accumulation, allowing colonization of aphids. This study has provided an insight into how facultative symbionts facilitate aphid colonization and adaptation to host plants.

Parallel Evolution in the Integration of a Co-obligate Aphid Symbiosis.

Insects evolve dependence-often extreme-on microbes for nutrition. This includes cases in which insects harbor multiple endosymbionts that function collectively as a metabolic unit [1-5]. How do these dependences originate [6], and is there a predictable sequence of events leading to the integration of new symbionts? While co-obligate symbioses, in which hosts rely on multiple nutrient-provisioning symbionts, have evolved numerous times across sap-feeding insects, there is only one known case in aphids, involving Buchnera aphidicola and Serratia symbiotica in the Lachninae subfamily [7-9]. Here, we identify three additional independent transitions to the same co-obligate symbiosis in different aphids. Comparing recent and ancient associations allow us to investigate intermediate stages of metabolic and anatomical integration of Serratia. We find that these uniquely replicated evolutionary events support the idea that co-obligate associations initiate in a predictable manner-through parallel evolutionary processes. Specifically, we show how the repeated losses of the riboflavin and peptidoglycan pathways in Buchnera lead to dependence on Serratia. We then provide evidence of a stepwise process of symbiont integration, whereby dependence evolves first. Then, essential amino acid pathways are lost (at ∼30-60 mya), which coincides with the increased anatomical integration of the companion symbiont. Finally, we demonstrate that dependence can evolve ahead of specialized structures (e.g., bacteriocytes), and in one case with no direct nutritional basis. More generally, our results suggest the energetic costs of synthesizing nutrients may provide a unified explanation for the sequence of gene losses that occur during the evolution of co-obligate symbiosis.

Differentiating between scavengers and entomopathogenic nematodes: Which is Oscheius chongmingensis?

Entomopathogenic nematodes (EPNs) continue to be explored for their potential usefulness in biological control and pest management programs. As more insect-associated species of nematodes are discovered and described, it is possible that scavengers and kleptoparasites may be mischaracterized as EPNs. If a nematode species is truly an entomopathogen it should display similar infectivity, as well as behaviors and preferences, to those of established EPN species, such as Steinernema carpocapsae. In this study we evaluated dauers of the putative EPN species Oscheius chongmingensis. We examined virulence, odor preferences as a measure of host-seeking behavior, and features of its bacterial symbiont Serratia nematodiphila. We determined that O. chongmingensis behaves more like a scavenger than an EPN. Not only did O. chongmingensis exhibit very poor pathogenicity in Galleria mellonella (wax moth larvae), it also displayed odor (host-seeking) preferences that are contrary to the well-known EPN S. carpocapsae. We also found that the bacterial symbiont of O. chongmingensis was antagonistic to S. carpocapsae; S. carpocapsae IJs were unable to develop when S. nematodiphila was a primary food source. We conclude that there is insufficient evidence to support the characterization of O. chongmingensis as an EPN; and based on the attributes of its preferences for already-infected or deceased hosts, suggest that this nematode is a scavenger, which may be on an evolutionary trajectory leading to an entomopathogenic lifestyle.

The nematicide Serratia plymuthica M24T3 colonizes Arabidopsis thaliana, stimulates plant growth, and presents plant beneficial potential.

Nine bacterial strains were previously isolated in association with pinewood nematode (PWN) from wilted pine trees. They proved to be nematicidal in vitro, and one of the highest activities, with potential to control PWN, was showed by Serratia sp. M24T3. Its ecology in association with plants remains unclear. This study aimed to evaluate the ability of strain M24T3 to colonize the internal tissues of the model plant Arabidopsis thaliana using confocal microscopy. Plant growth-promoting bacteria (PGPB) functional traits were tested and retrieved in the genome of strain M24T3. In greenhouse conditions, the bacterial effects of all nematicidal strains were also evaluated, co-inoculated or not with Bradyrhizobium sp. 3267, on Vigna unguiculata fitness. Inoculation of strain M24T3 increased the number of A. thaliana lateral roots and the confocal analysis confirmed effective bacterial colonization in the plant. Strain M24T3 showed cellulolytic activity, siderophores production, phosphate and zinc solubilization ability, and indole acetic acid production independent of supplementation with L-tryptophan. In the genome of strain M24T3, genes involved in the interaction with the plants such as 1-aminocyclopropane-1-carboxylate (ACC) deaminase, chitinolytic activity, and quorum sensing were also detected. The genomic organization showed ACC deaminase and its leucine-responsive transcriptional regulator, and the activity of ACC deaminase was 594.6 nmol α-ketobutyrate µg protein-1 µl-1. Strain M24T3 in co-inoculation with Bradyrhizobium sp. 3267 promoted the growth of V. unguiculata. In conclusion, this study demonstrated the ability of strain M24T3 to colonize other plants besides pine trees as an endophyte and displays PGPB traits that probably increased plant tolerance to stresses.

Modeling the Potential Role of Engineered Symbiotic Bacteria in Malaria Control.

Recent experimental study suggests that the engineered symbiotic bacteria Serratia AS1 may provide a novel, effective and sustainable biocontrol of malaria. These recombinant bacteria have been shown to be able to rapidly disseminate throughout mosquito populations and to efficiently inhibit development of malaria parasites in mosquitoes in controlled laboratory experiments. In this paper, we develop a climate-based malaria model which involves both vertical and horizontal transmissions of the engineered Serratia AS1 bacteria in mosquito population. We show that the dynamics of the model system is totally determined by the vector reproduction ratio [Formula: see text], and the basic reproduction ratio [Formula: see text]. If [Formula: see text], then the mosquito-free state is globally attractive. If [Formula: see text] and [Formula: see text], then the disease-free periodic solution is globally attractive. If [Formula: see text] and [Formula: see text], then the positive periodic solution is globally attractive. Numerically, we verify the obtained analytic result and evaluate the effects of releasing the engineered Serratia AS1 bacteria in field by conducting a case study for Douala, Cameroon. We find that ideally, by using Serratia AS1 alone, it takes at least 25 years to eliminate malaria from Douala. This implies that continued long-term investment is needed in the fight against malaria and confirms the necessity of integrating multiple control measures.

Serratia microhaemolytica sp. nov., isolated from an artificial lake in Southern China.

A Gram-stain negative, facultatively anaerobic, rod-shaped, non-motile and non-spore forming bacterium, designated ZS-11T, was isolated from an artificial freshwater lake in Guangzhou city, Guangdong province, China. Growth of strain ZS-11T was observed at the temperature 18-42 °C (optimum 32-37 °C), pH 6.0-8.0 (optimum 7.0) and 0.5-3.0% (w/v) NaCl (optimum 0.5%, w/v), and also found to be enhanced in the presence of CO2. Pairwise comparison of 16S rRNA gene sequences showed that the strain shared high similarities with Serratia entomophila DSM 12358T (96.1%), Serratia ficaria DSM 4569T (96.0%), Serratia plymuthica DSM 4540T (96.0%), Rahnella victoriana FRB 225T (95.9%) and Rouxiella badensis DSM 100043T (95.8%). The phylogenomic dendrograms showed that strain ZS-11T formed a distinct cluster within the clade of the genus Serratia. The major fatty acids (> 20%) present in the cells were C16:0, C16:1ω7c/C16:1ω6c and C18:1ω7c/C18:1ω6c, which were consistent with those of S. entomophila CCUG 55496T and Serratia liquefaciens CCUG 9285T. The DNA G + C content for the genome was 49.3%. Based on these phenotypic and genotypic data, strain ZS-11T is considered to represent a new species of the genus Serratia, for which the name Serratia microhaemolytica sp. nov. is proposed. The type strain is ZS-11T (= CCTCC AB 2018040T = KCTC 62413T).

Environmental potassium regulates bacterial flotation, antibiotic production and turgor pressure in Serratia through the TrkH transporter.

Serratia sp. strain ATCC 39006 (S39006) can float in aqueous environments due to natural production of gas vesicles (GVs). Expression of genes for GV morphogenesis is stimulated in low oxygen conditions, thereby enabling migration to the air-liquid interface. Quorum sensing (via SmaI and SmaR) and transcriptional and post-transcriptional regulators, including RbsR and RsmA, respectively, connect the control of cell buoyancy, motility and secondary metabolism. Here, we define a new pleiotropic regulator found in screens of GV mutants. A mutation in the gene trkH, encoding a potassium transporter, caused upregulation of GV formation, flotation, and the prodigiosin antibiotic, and downregulation of flagellar motility. Pressure nephelometry revealed that the mutation in trkH affected cell turgor pressure. Our results show that osmotic change is an important physiological parameter modulating cell buoyancy and antimicrobial production in S39006, in response to environmental potassium levels.