Distinct pathogenesis and host responses during infection of C. elegans by P. aeruginosa and S. aureus

Early ICU energy deficit is a risk factor for Staphylococcus aureus ventilator-associated pneumonia

BACKGROUND

Caloric insufficiency during the first week of ICU stay has been associated with increased infection rates. The connection between specific pathogens and host nutritional status in the ICU is not well known. This study was undertaken to determine the impact of patients' early in-ICU energy balance on the pathogens responsible for ventilator-associated pneumonia (VAP).

METHODS

In this prospective, observational, cohort study conducted in a teaching hospital ICU, energy balance (energy delivered - calculated resting energy expenditure) was compared according to the microbiologic results of the fiber-optic BAL cultures of 76 consecutive patients receiving acute prolonged (≥ 96 h) mechanical ventilation who developed VAP during their ICU stay.

RESULTS

Among the 76 BAL cultures, 22 contained significant Staphylococcus aureus concentrations. The cumulated energy deficit of patients with S aureus VAP was greater than those with VAP caused by other pathogens (-10,275 ± 4,211 kcal vs -7,376 ± 4,013 kcal from ICU admission to day of BAL, P < .01). ICU admission, nutritional status, and conditions potentially limiting feeding did not differ significantly between the two groups. Patients with S aureus VAP had lower prescribed and delivered energy, causing higher energy deficits. Multivariate analysis identified energy deficit as being independently associated with S aureus VAP. More-severe energy deficit and higher rate of S aureus-positive BAL cultures (P = .01 comparing quartiles) were observed.

CONCLUSIONS

Early ICU energy deficit is an independent determinant for acquiring S aureus VAP in patients on acute prolonged mechanical ventilation.

Protist-type lysozymes of the nematode Caenorhabditis elegans contribute to resistance against pathogenic Bacillus thuringiensis

Pathogens represent a universal threat to other living organisms. Most organisms express antimicrobial proteins and peptides, such as lysozymes, as a protection against these challenges. The nematode Caenorhabditis elegans harbours 15 phylogenetically diverse lysozyme genes, belonging to two distinct types, the protist- or Entamoeba-type (lys genes) and the invertebrate-type (ilys genes) lysozymes. In the present study we characterized the role of several protist-type lysozyme genes in defence against a nematocidal strain of the Gram-positive bacterium Bacillus thuringiensis. Based on microarray and subsequent qRT-PCR gene expression analysis, we identified protist-type lysozyme genes as one of the differentially transcribed gene classes after infection. A functional genetic analysis was performed for three of these genes, each belonging to a distinct evolutionary lineage within the protist-type lysozymes (lys-2, lys-5, and lys-7). Their knock-out led to decreased pathogen resistance in all three cases, while an increase in resistance was observed when two out of three tested genes were overexpressed in transgenic lines (lys-5, lys-7, but not lys-2). We conclude that the lysozyme genes lys-5, lys-7, and possibly lys-2 contribute to resistance against B. thuringiensis, thus highlighting the particular role of lysozymes in the nematode's defence against pathogens.

Changes in Caenorhabditis elegans life span and selective innate immune genes during Staphylococcus aureus infection

Caenorhabditis elegans has been increasingly used to study the innate immunity and for the screening of microbe/host-specific pathogenic factors. Staphylococcus aureus-mediated infections with live C. elegans were performed on solid (full-lawn) and liquid assays. S. aureus required 90 ± 10 h for the complete killing of C. elegans, but the infection was started only after 32 h of exposure with 20% inoculum of S. aureus. The short time exposure studies revealed that, in 20% of inoculum, continuous exposure to the pathogen was required for the killing of nematode. In 100% of inoculum, only 8 h of exposure was sufficient to kill the C. elegans. To evaluate kinetically at the innate immune level, the regulation of representative candidate antimicrobial genes was investigated. Both semi-quantitative reverse transcriptase polymerase chain reaction (PCR) and real-time PCR analyses indicated the regulation of candidate immune regulatory genes of lysozyme (lys-7), cysteine protease (cpr-2), and C-type lectin (clec-60 and clec-87) family members during the course of S. aureus infections, indicating the possible contribution of the above players during the host immune response against S. aureus exposures.

Loss of intestinal nuclei and intestinal integrity in aging C. elegans

The roundworm C. elegans is widely used as an aging model, with hundreds of genes identified that modulate aging (Kaeberlein et al., 2002. Mech. Ageing Dev.123, 1115-1119). The development and bodyplan of the 959 cells comprising the adult have been well described and established for more than 25 years (Sulston & Horvitz, 1977. Dev. Biol.56, 110-156; Sulston et al., 1983. Dev. Biol.100, 64-119.). However, morphological changes with age in this optically transparent animal are less well understood, with only a handful of studies investigating the pathobiology of aging. Age-related changes in muscle (Herndon et al., 2002. Nature419, 808-814), neurons (Herndon et al., 2002), intestine and yolk granules (Garigan et al., 2002. Genetics161, 1101-1112; Herndon et al., 2002), nuclear architecture (Haithcock et al., 2005. Proc. Natl Acad. Sci. USA102, 16690-16695), tail nuclei (Golden et al., 2007. Aging Cell6, 179-188), and the germline (Golden et al., 2007) have been observed via a variety of traditional relatively low-throughput methods. We report here a number of novel approaches to study the pathobiology of aging C. elegans. We combined histological staining of serial-sectioned tissues, transmission electron microscopy, and confocal microscopy with 3D volumetric reconstructions and characterized age-related morphological changes in multiple wild-type individuals at different ages. This enabled us to identify several novel pathologies with age in the C. elegans intestine, including the loss of critical nuclei, the degradation of intestinal microvilli, changes in the size, shape, and cytoplasmic contents of the intestine, and altered morphologies caused by ingested bacteria. The three-dimensional models we have created of tissues and cellular components from multiple individuals of different ages represent a unique resource to demonstrate global heterogeneity of a multicellular organism.

Candida albicans infection of Caenorhabditis elegans induces antifungal immune defenses

Candida albicans yeast cells are found in the intestine of most humans, yet this opportunist can invade host tissues and cause life-threatening infections in susceptible individuals. To better understand the host factors that underlie susceptibility to candidiasis, we developed a new model to study antifungal innate immunity. We demonstrate that the yeast form of C. albicans establishes an intestinal infection in Caenorhabditis elegans, whereas heat-killed yeast are avirulent. Genome-wide, transcription-profiling analysis of C. elegans infected with C. albicans yeast showed that exposure to C. albicans stimulated a rapid host response involving 313 genes (124 upregulated and 189 downregulated, ∼1.6% of the genome) many of which encode antimicrobial, secreted or detoxification proteins. Interestingly, the host genes affected by C. albicans exposure overlapped only to a small extent with the distinct transcriptional responses to the pathogenic bacteria Pseudomonas aeruginosa or Staphylococcus aureus, indicating that there is a high degree of immune specificity toward different bacterial species and C. albicans. Furthermore, genes induced by P. aeruginosa and S. aureus were strongly over-represented among the genes downregulated during C. albicans infection, suggesting that in response to fungal pathogens, nematodes selectively repress the transcription of antibacterial immune effectors. A similar phenomenon is well known in the plant immune response, but has not been described previously in metazoans. Finally, 56% of the genes induced by live C. albicans were also upregulated by heat-killed yeast. These data suggest that a large part of the transcriptional response to C. albicans is mediated through “pattern recognition,” an ancient immune surveillance mechanism able to detect conserved microbial molecules (so-called pathogen-associated molecular patterns or PAMPs). This study provides new information on the evolution and regulation of the innate immune response to divergent pathogens and demonstrates that nematodes selectively mount specific antifungal defenses at the expense of antibacterial responses.

Despite being a part of the normal flora of healthy individuals, Candida albicans is the most common fungal pathogen of humans and can cause infections that are associated with staggeringly high mortality rates. Here we devise a model for the study of the host immune response to C. albicans infection using the nematode C. elegans. We found that infection with the yeast form of C. albicans induces rapid and robust transcriptional changes in C. elegans. Analyses of these differentially regulated genes indicate that the nematode mounts antifungal defenses that are remarkably distinct from the host responses to pathogenic bacteria and that the nematode recognizes components possessed by heat-killed C. albicans to initiate this response. Interestingly, during infection with a pathogenic fungus, the nematode downregulates antibacterial immune response genes, which may reflect an evolutionary tradeoff between bacterial and fungal defense.

A comprehensive analysis of gene expression changes provoked by bacterial and fungal infection in C. elegans

While Caenorhabditis elegans specifically responds to infection by the up-regulation of certain genes, distinct pathogens trigger the expression of a common set of genes. We applied new methods to conduct a comprehensive and comparative study of the transcriptional response of C. elegans to bacterial and fungal infection. Using tiling arrays and/or RNA-sequencing, we have characterized the genome-wide transcriptional changes that underlie the host's response to infection by three bacterial (Serratia marcescens, Enterococcus faecalis and otorhabdus luminescens) and two fungal pathogens (Drechmeria coniospora and Harposporium sp.). We developed a flexible tool, the WormBase Converter (available at http://wormbasemanager.sourceforge.net/), to allow cross-study comparisons. The new data sets provided more extensive lists of differentially regulated genes than previous studies. Annotation analysis confirmed that genes commonly up-regulated by bacterial infections are related to stress responses. We found substantial overlaps between the genes regulated upon intestinal infection by the bacterial pathogens and Harposporium, and between those regulated by Harposporium and D. coniospora, which infects the epidermis. Among the fungus-regulated genes, there was a significant bias towards genes that are evolving rapidly and potentially encode small proteins. The results obtained using new methods reveal that the response to infection in C. elegans is determined by the nature of the pathogen, the site of infection and the physiological imbalance provoked by infection. They form the basis for future functional dissection of innate immune signaling. Finally, we also propose alternative methods to identify differentially regulated genes that take into account the greater variability in lowly expressed genes.

Mode of bacterial pathogenesis determines phenotype in elt-2 and elt-7 RNAi Caenorhabditis elegans

Caenorhabditis elegans has become a useful model for studying innate immunity. ELT-2, which is homologous to human GATA-4, -5 and -6, is considered the primary GATA transcription factor controlling intestinal immunity in C. elegans. In this study, we characterize the timeline of intestinal distension in nematodes where ELT-2 and another intestinal GATA transcription factor, ELT-7, are abrogated by RNAi using two different models: colonization and toxin-based infections by Pseudomonas aeruginosa. We show that both ELT-2 and ELT-7 are important for survival of C. elegans exposed to P. aeruginosa. Intestinal distension is accelerated in elt-2 RNAi nematodes, and is observed in colonization but not toxin-based Pseudomonas infection. Upon onset of intestinal distension, nematodes die within 24 h, regardless of experimental treatment. These data provide new insight into the role of ELT-2 and ELT-7 in protecting C. elegans against P. aeruginosa infection.

Discovery of Salmonella virulence factors translocated via outer membrane vesicles to murine macrophages

Salmonella enterica serovar Typhimurium, an intracellular pathogen and leading cause of food-borne illness, encodes a plethora of virulence effectors. Salmonella virulence factors are translocated into host cells and manipulate host cellular activities, providing a more hospitable environment for bacterial proliferation. In this study, we report a new set of virulence factors that is translocated into the host cytoplasm via bacterial outer membrane vesicles (OMV). PagK (or PagK1), PagJ, and STM2585A (or PagK2) are small proteins composed of ∼70 amino acids and have high sequence homology to each other (>85% identity). Salmonella lacking all three homologues was attenuated for virulence in a mouse infection model, suggesting at least partial functional redundancy among the homologues. While each homologue was translocated into the macrophage cytoplasm, their translocation was independent of all three Salmonella gene-encoded type III secretion systems (T3SSs)-Salmonella pathogenicity island 1 (SPI-1) T3SS, SPI-2 T3SS, and the flagellar system. Selected methods, including direct microscopy, demonstrated that the PagK-homologous proteins were secreted through OMV, which were enriched with lipopolysaccharide (LPS) and outer membrane proteins. Vesicles produced by intracellular bacteria also contained lysosome-associated membrane protein 1 (LAMP1), suggesting the possibility of OMV convergence with host cellular components during intracellular trafficking. This study identified novel Salmonella virulence factors secreted via OMV and demonstrated that OMV can function as a vehicle to transfer virulence determinants to the cytoplasm of the infected host cell.

Selective sorting of cargo proteins into bacterial membrane vesicles

In contrast to the well established multiple cellular roles of membrane vesicles in eukaryotic cell biology, outer membrane vesicles (OMV) produced via blebbing of prokaryotic membranes have frequently been regarded as cell debris or microscopy artifacts. Increasingly, however, bacterial membrane vesicles are thought to play a role in microbial virulence, although it remains to be determined whether OMV result from a directed process or from passive disintegration of the outer membrane. Here we establish that the human oral pathogen Porphyromonas gingivalis has a mechanism to selectively sort proteins into OMV, resulting in the preferential packaging of virulence factors into OMV and the exclusion of abundant outer membrane proteins from the protein cargo. Furthermore, we show a critical role for lipopolysaccharide in directing this sorting mechanism. The existence of a process to package specific virulence factors into OMV may significantly alter our current understanding of host-pathogen interactions.