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Microbial population dynamics in the hemolymph of Manduca sexta infected with Xenorhabdus nematophila and the entomopathogenic nematode Steinernema carpocapsae.

ABSTRACT: Xenorhabdus nematophila engages in a mutualistic association with the nematode Steinernema carpocapsae. The nematode invades and traverses the gut of susceptible insects. X. nematophila is released in the insect blood (hemolymph), where it suppresses host immune responses and functions as a pathogen. X. nematophila produces diverse antimicrobials in laboratory cultures. The natural competitors that X. nematophila encounters in the hemolymph and the role of antimicrobials in interspecies competition in the host are poorly understood. We show that gut microbes translocate into the hemolymph when the nematode penetrates the insect intestine. During natural infection, Staphylococcus saprophyticus was initially present and subsequently disappeared from the hemolymph, while Enterococcus faecalis proliferated. S. saprophyticus was sensitive to X. nematophila antibiotics and was eliminated from the hemolymph when coinjected with X. nematophila. In contrast, E. faecalis was relatively resistant to X. nematophila antibiotics. When injected by itself, E. faecalis persisted (~10(3) CFU/ml), but when coinjected with X. nematophila, it proliferated to ~10(9) CFU/ml. Injection of E. faecalis into the insect caused the upregulation of an insect antimicrobial peptide, while the transcript levels were suppressed when E. faecalis was coinjected with X. nematophila. Its relative antibiotic resistance together with suppression of the host immune system by X. nematophila may account for the growth of E. faecalis. At higher injected levels (10(6) CFU/insect), E. faecalis could kill insects, suggesting that it may contribute to virulence in an X. nematophila infection. These findings provide new insights into the competitive events that occur early in infection after S. carpocapsae invades the host hemocoel.

SUBMITTER: Singh S

PROVIDER: S-EPMC4068695 | biostudies-literature | 2014 Jul

REPOSITORIES: biostudies-literature

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Microbial population dynamics in the hemolymph of Manduca sexta infected with Xenorhabdus nematophila and the entomopathogenic nematode Steinernema carpocapsae.

Singh Swati S Reese Jordan M JM Casanova-Torres Angel M AM Goodrich-Blair Heidi H Forst Steven S

Applied and environmental microbiology 20140509 14

Xenorhabdus nematophila engages in a mutualistic association with the nematode Steinernema carpocapsae. The nematode invades and traverses the gut of susceptible insects. X. nematophila is released in the insect blood (hemolymph), where it suppresses host immune responses and functions as a pathogen. X. nematophila produces diverse antimicrobials in laboratory cultures. The natural competitors that X. nematophila encounters in the hemolymph and the role of antimicrobials in interspecies competit ...[more]

PMID: 24814780

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Project description:Xenorhabdus nematophila bacteria are mutualistic symbionts of Steinernema carpocapsae nematodes and pathogens of insects. The X. nematophila global regulator Lrp controls the expression of many genes involved in both mutualism and pathogenic activities, suggesting a role in the transition between the two host organisms. We previously reported that natural populations of X. nematophila exhibit various levels of Lrp expression and that cells expressing relatively low levels of Lrp are optimized for virulence in the insect Manduca sexta The adaptive advantage of the high-Lrp-expressing state was not established. Here we used strains engineered to express constitutively high or low levels of Lrp to test the model in which high-Lrp-expressing cells are adapted for mutualistic activities with the nematode host. We demonstrate that high-Lrp cells form more robust biofilms in laboratory media than do low-Lrp cells, which may reflect adherence to host tissues. Also, our data showed that nematodes cultivated with high-Lrp strains are more frequently colonized than are those associated with low-Lrp strains. Taken together, these data support the idea that high-Lrp cells have an advantage in tissue adherence and colonization initiation. Furthermore, our data show that high-Lrp-expressing strains better support nematode reproduction than do their low-Lrp counterparts under both in vitro and in vivo conditions. Our data indicate that heterogeneity of Lrp expression in X. nematophila populations provides diverse cell populations adapted to both pathogenic (low-Lrp) and mutualistic (high-Lrp) states.IMPORTANCE Host-associated bacteria experience fluctuating conditions during both residence within an individual host and transmission between hosts. For bacteria that engage in evolutionarily stable, long-term relationships with particular hosts, these fluctuations provide selective pressure for the emergence of adaptive regulatory mechanisms. Here we present evidence that the bacterium Xenorhabdus nematophila uses various levels of the transcription factor Lrp to optimize its association with its two animal hosts, nematodes and insects, with which it behaves as a mutualist and a pathogen, respectively. Building on our previous finding that relatively low cellular levels of Lrp are optimal for pathogenesis, we demonstrate that, conversely, high levels of Lrp promote mutualistic activities with the Steinernema carpocapsae nematode host. These data suggest that X. nematophila has evolved to utilize phenotypic variation between high- and low-Lrp-expression states to optimize its alternating behaviors as a mutualist and a pathogen.

Project description:Entomopathogenic nematodes (EPNs) employ a sophisticated chemosensory apparatus to detect potential hosts. Understanding the molecular basis of relevant host-finding behaviours could facilitate improved EPN biocontrol approaches, and could lend insight to similar behaviours in economically important mammalian parasites. FMRFamide-like peptides are enriched and conserved across the Phylum Nematoda, and have been linked with motor and sensory function, including dispersal and aggregating behaviours in the free living nematode Caenorhabditis elegans. The RNA interference (RNAi) pathway of Steinernema carpocapsae was characterised in silico, and employed to knockdown the expression of the FMRFamide-like peptide 21 (GLGPRPLRFamide) gene (flp-21) in S. carpocapsae infective juveniles; a first instance of RNAi in this genus, and a first in an infective juvenile of any EPN species. Our data show that 5 mg/ml dsRNA and 50 mM serotonin triggers statistically significant flp-21 knockdown (-84%***) over a 48 h timecourse, which inhibits host-finding (chemosensory), dispersal, hyperactive nictation and jumping behaviours. However, whilst 1 mg/ml dsRNA and 50 mM serotonin also triggers statistically significant flp-21 knockdown (-51%**) over a 48 h timecourse, it does not trigger the null sensory phenotypes; statistically significant target knockdown can still lead to false negative results, necessitating appropriate experimental design. SPME GC-MS volatile profiles of two EPN hosts, Galleria mellonella and Tenebrio molitor reveal an array of shared and unique compounds; these differences had no impact on null flp-21 RNAi phenotypes for the behaviours assayed. Localisation of flp-21 / FLP-21 to paired anterior neurons by whole mount in situ hybridisation and immunocytochemistry corroborates the RNAi data, further suggesting a role in sensory modulation. These data can underpin efforts to study these behaviours in other economically important parasites, and could facilitate molecular approaches to EPN strain improvement for biocontrol.

Dataset Information

Microbial population dynamics in the hemolymph of Manduca sexta infected with Xenorhabdus nematophila and the entomopathogenic nematode Steinernema carpocapsae.

Publications

Microbial population dynamics in the hemolymph of Manduca sexta infected with Xenorhabdus nematophila and the entomopathogenic nematode Steinernema carpocapsae.

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