Rearing and injection of Manduca sexta larvae to assess bacterial virulence

KED gene expression in early response to wounding stress in tomato plants

The wounding-responsive KED gene, named for its coding for a lysine (K), glutamic acid (E), and aspartic acid (D)-rich protein, is widely present among land plants. However, little is known about its regulation or function. In this study, we found that transcription of the tomato (Solanum lycopersicum) KED gene, SlKED, was rapidly and transiently elevated by wounding or ethephon treatment. Compared to the wild-type plants, the CRISPR/Cas9-mediated SlKED knockout plants did not exhibit altered expression patterns for genes involved in hormone biosynthesis or stress signaling, suggesting a lack of pleiotropic effect on other stress-responsive genes. Conversely, jasmonic acid did not appear to directly regulate SlKED expression. Wounded leaves of the KED-lacking plants exhibited higher binding of Evans blue dye than the wild-type, indicating a possible role for KED in healing damaged tissues. The SlKED knockout plants showed a similar dietary effect as the wild-type on the larval growth of tobacco hornworm. But a higher frequency of larval mandible (mouth) movement was recorded during the first 2 minutes of feeding on the wounded KED-lacking SlKED knockout plants than on the wounded KED-producing wild-type plants, probably reflecting an initial differential response by the feeding larvae to the SlKED knockout plants. Our findings suggest that SlKED may be an ethylene-mediated early responder to mechanical stress in tomato, acting downstream of the wound stress response pathways. Although its possible involvement in response to other biotic and abiotic stresses is still unclear, we propose that SlKED may play a role in plant's rapid, short-term, early wounding responses, such as in cellular damage healing.

Physiological Tradeoffs of Immune Response Differs by Infection Type in Pieris napi

Understanding the tradeoffs that result from successful infection responses is central to understanding how life histories evolve. Gaining such insights, however, can be challenging, as they may be pathogen specific and confounded with experimental design. Here, we investigated whether infection from gram positive or negative bacteria results in different physiological tradeoffs, and whether these infections impact life history later in life (post-diapause development), in the butterfly Pieris napi. During the first 24 h after infection (3, 6, 12, and 24 h), after removing effects due to injection, larvae infected with Micrococcus luteus showed a strong suppression of all non-immunity related processes while several types of immune responses were upregulated. In contrast, this tradeoff between homeostasis and immune response was much less pronounced in Escherichia coli infections. These differences were also visible long after infection, via weight loss and slower development, as well as an increased mortality at higher infection levels during later stages of development. Individuals infected with M. luteus, compared to E. coli, had a higher mortality rate, and a lower pupal weight, developmental rate and adult weight. Further, males exhibited a more negative impact of infection than females. Thus, immune responses come at a cost even when the initial infection has been overcome, and these costs are likely to affect later life history parameters with fitness consequences.

Animal infection models using non-mammals

The use of non-human animal models for infection experiments is important for investigating the infectious processes of human pathogenic bacteria at the molecular level. Mammals, such as mice and rabbits, are also utilized as animal infection models, but large numbers of animals are needed for these experiments, which is costly, and fraught with ethical issues. Various non-mammalian animal infection models have been used to investigate the molecular mechanisms of various human pathogenic bacteria, including Staphylococcus aureus, Streptococcus pyogenes, and Pseudomonas aeruginosa. This review discusses the desirable characteristics of non-mammalian infection models and describes recent non-mammalian infection models that utilize Caenorhabditis elegans, silkworm, fruit fly, zebrafish, two-spotted cricket, hornworm, and waxworm.

The insect pathogenic bacterium Xenorhabdus innexi has attenuated virulence in multiple insect model hosts yet encodes a potent mosquitocidal toxin

BACKGROUND

Xenorhabdus innexi is a bacterial symbiont of Steinernema scapterisci nematodes, which is a cricket-specialist parasite and together the nematode and bacteria infect and kill crickets. Curiously, X. innexi expresses a potent extracellular mosquitocidal toxin activity in culture supernatants. We sequenced a draft genome of X. innexi and compared it to the genomes of related pathogens to elucidate the nature of specialization.

RESULTS

Using green fluorescent protein-expressing X. innexi we confirm previous reports using culture-dependent techniques that X. innexi colonizes its nematode host at low levels (~3-8 cells per nematode), relative to other Xenorhabdus-Steinernema associations. We found that compared to the well-characterized entomopathogenic nematode symbiont X. nematophila, X. innexi fails to suppress the insect phenoloxidase immune pathway and is attenuated for virulence and reproduction in the Lepidoptera Galleria mellonella and Manduca sexta, as well as the dipteran Drosophila melanogaster. To assess if, compared to other Xenorhabdus spp., X. innexi has a reduced capacity to synthesize virulence determinants, we obtained and analyzed a draft genome sequence. We found no evidence for several hallmarks of Xenorhabdus spp. toxicity, including Tc and Mcf toxins. Similar to other Xenorhabdus genomes, we found numerous loci predicted to encode non-ribosomal peptide/polyketide synthetases. Anti-SMASH predictions of these loci revealed one, related to the fcl locus that encodes fabclavines and zmn locus that encodes zeamines, as a likely candidate to encode the X. innexi mosquitocidal toxin biosynthetic machinery, which we designated Xlt. In support of this hypothesis, two mutants each with an insertion in an Xlt biosynthesis gene cluster lacked the mosquitocidal compound based on HPLC/MS analysis and neither produced toxin to the levels of the wild type parent.

CONCLUSIONS

The X. innexi genome will be a valuable resource in identifying loci encoding new metabolites of interest, but also in future comparative studies of nematode-bacterial symbiosis and niche partitioning among bacterial pathogens.

High Levels of the Xenorhabdus nematophila Transcription Factor Lrp Promote Mutualism with the Steinernema carpocapsae Nematode Host

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.

The Global Transcription Factor Lrp Controls Virulence Modulation in Xenorhabdus nematophila

The bacterium Xenorhabdus nematophila engages in phenotypic variation with respect to pathogenicity against insect larvae, yielding both virulent and attenuated subpopulations of cells from an isogenic culture. The global regulatory protein Lrp is necessary for X. nematophila virulence and immunosuppression in insects, as well as colonization of the mutualistic host nematode Steinernema carpocapsae, and mediates expression of numerous genes implicated in each of these phenotypes. Given the central role of Lrp in X. nematophila host associations, as well as its involvement in regulating phenotypic variation pathways in other bacteria, we assessed its function in virulence modulation. We discovered that expression of lrp varies within an isogenic population, in a manner that correlates with modulation of virulence. Unexpectedly, although Lrp is necessary for optimal virulence and immunosuppression, cells expressing high levels of lrp were attenuated in these processes relative to those with low to intermediate lrp expression. Furthermore, fixed expression of lrp at high and low levels resulted in attenuated and normal virulence and immunosuppression, respectively, and eliminated population variability of these phenotypes. These data suggest that fluctuating lrp expression levels are sufficient to drive phenotypic variation in X. nematophila.

IMPORTANCE

Many bacteria use cell-to-cell phenotypic variation, characterized by distinct phenotypic subpopulations within an isogenic population, to cope with environmental change. Pathogenic bacteria utilize this strategy to vary antigen or virulence factor expression. Our work establishes that the global transcription factor Lrp regulates phenotypic variation in the insect pathogen Xenorhabdus nematophila, leading to attenuation of virulence and immunosuppression in insect hosts. Unexpectedly, we found an inverse correlation between Lrp expression levels and virulence: high levels of expression of Lrp-dependent putative virulence genes are detrimental for virulence but may have an adaptive advantage in other aspects of the life cycle. Investigation of X. nematophila phenotypic variation facilitates dissection of this phenomenon in the context of a naturally occurring symbiosis.