Development of an insect model for the in vivo pathogenicity testing of yeasts

Virulence of Enterococcus faecalis dairy strains in an insect model: the role of fsrB and gelE

Despite the existence of various virulence factors in the Enterococcus genus, enterococcal virulence is still a debated issue. A main consideration is the detection of the same virulence genes in strains isolated from nosocomial or community-acquired infections, and from food products. The goal of this study was to evaluate the roles of two well-characterized enterococcal virulence factors, Fsr and gelatinase, in the potential virulence of Enterococcus faecalis food strains. Virulence of unrelated Enterococcus isolates, including dairy strains carrying fsr and gelE operons, was compared in the Galleria mellonella insect model. E. faecalis dairy strains were able to kill larvae and were as virulent as strain OG1RF, one of the most widely used for virulence studies. In contrast, Enterococcus durans and Enterococcus faecium strains were avirulent or poorly virulent for G. mellonella. To evaluate the role of fsrB and gelE in virulence of E. faecalis dairy strains, both genes were deleted independently in two strains. The Delta fsrB and Delta gelE deletion mutants both produced a gelatinase-negative phenotype. Although both mutations significantly attenuated virulence in G. mellonella, the Delta fsrB strains were more strongly attenuated. These results agree with previous findings suggesting the involvement of fsrB in the control of other cell functions relevant to virulence. Our work demonstrates that the presence of functional fsrB, and to a lesser extent gelE, in dairy enterococci should be considered with caution.

Galleria mellonella as a model system to study Acinetobacter baumannii pathogenesis and therapeutics

Nonmammalian model systems of infection such as Galleria mellonella (caterpillars of the greater wax moth) have significant logistical and ethical advantages over mammalian models. In this study, we utilize G. mellonella caterpillars to study host-pathogen interactions with the gram-negative organism Acinetobacter baumannii and determine the utility of this infection model to study antibacterial efficacy. After infecting G. mellonella caterpillars with a reference A. baumannii strain, we observed that the rate of G. mellonella killing was dependent on the infection inoculum and the incubation temperature postinfection, with greater killing at 37 degrees C than at 30 degrees C (P = 0.01). A. baumannii strains caused greater killing than the less-pathogenic species Acinetobacter baylyi and Acinetobacter lwoffii (P < 0.001). Community-acquired A. baumannii caused greater killing than a reference hospital-acquired strain (P < 0.01). Reduced levels of production of the quorum-sensing molecule 3-hydroxy-C(12)-homoserine lactone caused no change in A. baumannii virulence against G. mellonella. Treatment of a lethal A. baumannii infection with antibiotics that had in vitro activity against the infecting A. baumannii strain significantly prolonged the survival of G. mellonella caterpillars compared with treatment with antibiotics to which the bacteria were resistant. G. mellonella is a relatively simple, nonmammalian model system that can be used to facilitate the in vivo study of host-pathogen interactions in A. baumannii and the efficacy of antibacterial agents.

Conidiation color mutants of Aspergillus fumigatus are highly pathogenic to the heterologous insect host Galleria mellonella

The greater wax moth Galleria mellonella has been widely used as a heterologous host for a number of fungal pathogens including Candida albicans and Cryptococcus neoformans. A positive correlation in pathogenicity of these yeasts in this insect model and animal models has been observed. However, very few studies have evaluated the possibility of applying this heterologous insect model to investigate virulence traits of the filamentous fungal pathogen Aspergillus fumigatus, the leading cause of invasive aspergillosis. Here, we have examined the impact of mutations in genes involved in melanin biosynthesis on the pathogenicity of A. fumigatus in the G. mellonella model. Melanization in A. fumigatus confers bluish-grey color to conidia and is a known virulence factor in mammal models. Surprisingly, conidial color mutants in B5233 background that have deletions in the defined six-gene cluster required for DHN-melanin biosynthesis caused enhanced insect mortality compared to the parent strain. To further examine and confirm the relationship between melanization defects and enhanced virulence in the wax moth model, we performed random insertional mutagenesis in the Af293 genetic background to isolate mutants producing altered conidia colors. Strains producing conidia of previously identified colors and of novel colors were isolated. Interestingly, these color mutants displayed a higher level of pathogenicity in the insect model compared to the wild type. Although some of the more virulent color mutants showed increased resistance to hydrogen peroxide, overall phenotypic characterizations including secondary metabolite production, metalloproteinase activity, and germination rate did not reveal a general mechanism accountable for the enhanced virulence of these color mutants observed in the insect model. Our observations indicate instead, that exacerbated immune response of the wax moth induced by increased exposure of PAMPs (pathogen-associated molecular patterns) may cause self-damage that results in increased mortality of larvae infected with the color mutants. The current study underscores the limitations of using this insect model for inferring the pathogenic potential of A. fumigatus strains in mammals, but also points to the importance of understanding the innate immunity of the insect host in providing insights into the pathogenicity level of different fungal strains in this model. Additionally, our observations that melanization defective color mutants demonstrate increased virulence in the insect wax moth, suggest the potential of using melanization defective mutants of native insect fungal pathogens in the biological control of insect populations.

The Candida albicans pescadillo homolog is required for normal hypha-to-yeast morphogenesis and yeast proliferation

A single species, Candida albicans, causes half of all invasive fungal infections in humans. Unlike other fungal pathogens, this organism switches between growth as budding yeast and as pseudohyphal and hyphal filaments in host organs and in vitro. Both cell types play a role in invasive disease: while hyphal and pseudohyphal filaments penetrate host cells and tissues, yeast cells are likely to facilitate dissemination through the bloodstream and establishment of distant foci of infection. Many regulators of the yeast-to-hypha switch have emerged from intensive investigations of this morphogenetic step, but the hypha-to-yeast switch remains poorly understood. Using a forward genetic approach, a novel putative regulator involved in the hypha-to-yeast switch was identified, the C. albicans pescadillo homolog, PES1. In eukaryotes from yeast to human, pescadillo homologs are involved in cell cycle control and ribosome biogenesis, and are essential. We find a pescadillo homolog to act in fungal morphogenesis, specifically in lateral yeast growth on filamentous cells. We also find essentiality of PES1 in C. albicans to be dependent on cell type, because hyphal cells, but not yeast cells, tolerate its loss. PES1 is therefore critical for completion of the C. albicans life cycle, in which the fungus switches between filamentous and yeast growth. Consistent with these in vitro findings, PES1 is required for C. albicans virulence in an in vivo insect model of infection.

Use of Galleria mellonella larvae to evaluate the in vivo anti-fungal activity of [Ag2(mal)(phen)3]

Larvae of the insect Galleria mellonella were employed to assess the in vivo antifungal efficacy of ([Ag(2)(mal)(phen)(3)]), AgNO(3) and 1,10-phenanthroline. Larvae pre-inoculated with these compounds were protected from a subsequent lethal infection by the yeast Candida albicans while larvae inoculated 1 and 4 h post-infection showed significantly increased survival (P < 0.01) compared to control larvae. Administration of these compounds resulted in an increase over 48 h in the density of insect haemocytes (immune cells) but there was no widespread activation of genes for antimicrobial peptides. This work demonstrates that G. mellonella larvae may be employed to ascertain the antifungal efficacy of silver(I) compounds and offers a rapid and effective means of assessing the in vivo activity of inorganic antimicrobial compounds.

Antibacterial activity in vivo and in vitro in the hemolymph of Galleria mellonella infected with Pseudomonas aeruginosa

The antibacterial activity of hemolymph from Galleria mellonella infected with entomopathogenic strain of Pseudomonas aeruginosa and non-pathogenic bacterium Escherichia coli was studied. In vivo, the antimicrobial activity appeared shortly after P. aeruginosa infection, reached the maximum level 18 h postinjection, while 30 h later only trace activity was noted. The activity induced by E. coli sustained on the high level until 48 h after infection. We also noted that the antimicrobial activity level induced by the non-pathogenic bacterium was higher in comparison to that measured in insects infected with the pathogenic strain of P. aeruginosa. The results of our in vitro studies indicated that inducible antimicrobial peptides of G. mellonella larvae were digested by P. aeruginosa elastase B. After 1 h incubation of cell-free hemolymph of immune-challenged larvae with elastase B, no antibacterial activity was observed. It was also shown that elastase B degraded synthetic cecropin B while in the presence of 6 mM EDTA antibacterial activity of cell-free hemolymph as well as cecropin B, was not changed which confirmed that the activity was abolished by the metalloprotease.

Utilization of Bombyx mori larvae as a surrogate animal model for evaluation of the anti-infective potential of oxazolidinones

Silkworm (Bombyx mori) larvae were investigated as an alternative animal model for the efficacy testing of novel oxazolidinones. The minimal lethal dose (MLD) for Staphylococcus aureus was 1-5 x 10(7) CFU per larva, exhibiting more than 90% mortality within 2 to 4 days post-infection. Treatment with vancomycin, linezolid, and novel oxazolidinones (RBx 7644, RBx 8700, and RBx 2006171) showed survival in a dose-dependent manner. The antibacterial potential of RBx 7644 and RBx 8700 was compared with that of linezolid and that of vancomycin and the effective doses (ED)50 obtained in the silkworm model were 37.89, 24.96, 13.38, and 30.72 mg/kg body weight, respectively. The ED50 values showed a similar trend in a murine model of infection. Owing to the small size of the larvae, very small amounts of new chemical entities (NCEs) are required to test their in vivo efficacy in this model. Thus, the silkworm model may be used in the early stage of new discovery research.

Physical stress primes the immune response of Galleria mellonella larvae to infection by Candida albicans

Larvae of the greater wax moth (Galleria mellonella) that had been subjected to physical stress by shaking in cupped hands for 2 min showed reduced susceptibility to infection by Candida albicans when infected 24 h after the stress event. Physically stressed larvae demonstrated an increase in haemocyte density and elevated mRNA levels of galiomicin and an inducible metalloproteinase inhibitor (IMPI) but not transferrin or gallerimycin. In contrast, previous work has demonstrated that microbial priming of larvae resulted in the induction of all four genes. Examination of the expression of proteins in the insect haemolymph using 2D electrophoresis and MALDI TOF analysis revealed an increase in the intensity of a number of peptides showing some similarities with proteins associated with the insect immune response to infection. This study demonstrates that non-lethal physical stress primes the immune response of G. mellonella and this is mediated by elevated haemocyte numbers, increased mRNA levels of genes coding for two antimicrobial peptides and the appearance of novel peptides in the haemolymph. This work demonstrates that physical priming increases the insect immune response but the mechanism of this priming is different to that induced by low level exposure to microbial pathogens.

Development of Galleria mellonella as an alternative infection model for the Burkholderia cepacia complex

Burkholderia is an important bacterial genus with a complex taxonomy that contains species of both ecological and pathogenic importance, including nine closely related species collectively termed the Burkholderia cepacia complex (BCC). In order to more thoroughly investigate the virulence of this bacterial complex of microorganisms, alternative infection models would be useful. To this end, we have adapted and developed the use of the Galleria mellonella wax moth larvae as a host for examining BCC infections. The experimental conditions affecting the BCC killing of the "wax worm" were optimized. BCC virulence levels were determined using 50% lethal doses, and differences were observed between both species and strains of the BCC. The BCC pathogenicity trends obtained compare favorably with results acquired using other published alternative infection models, as well as mammalian infection models. In addition, BCC killing activity was determined by directly measuring relative bacterial loads in three different BCC strains, thus demonstrating innate differences in BCC strain virulence. Finally, genetically mutated BCC strains were compared to a wild-type BCC strain in order to show concomitant reduction of BCC virulence and increased wax worm survival. For experimentation examining the virulent properties of the BCC, the wax worm has proven to be a useful alternative infection model.

Effect of pre-incubation temperature on susceptibility of Galleria mellonella larvae to infection by Candida albicans

The use of insects for evaluating the virulence of microbial pathogens and for determining the efficacy of antimicrobial drugs is increasing. When larvae of the greater wax moth Galleria mellonella were incubated at 4 or 37 degrees C for 24 h. prior to infection, they manifested increased resistance to infection by the yeast Candida albicans compared to larvae that had been pre-incubated for 24 h at 30 degrees C. Incubation at 4 or 37 degrees C led to an increase in haemocyte density and the expression of genes coding for gallerimycin, transferrin, an inducible metalloproteinase inhibitor (IMPI) and galiomicin. Peak expression of these genes was recorded at approximately 24 h after the commencement of the 4 or 37 degrees C incubation. These results indicate that exposure of larvae to mild thermal shock conditions induces a protective cellular and humoral immune response mediated by increased numbers of haemocytes and elevated expression of antimicrobial peptides.

Changes in Galleria mellonella apolipophorin III level during Pseudomonas aeruginosa infection

The level of apoLp-III in fat body, hemocytes and plasma from Galleria mellonella larvae infected with Pseudomonas aeruginosa was studied. It was found that the amount of 18kDa protein present in fat body and hemocytes decreased progressively with time after infection. In the case of plasma, an increase in apoLp-III content was observed during the first 19h after infection and then decreased significantly after prolonged infection time. The decreased level of apoLp-III in plasma 24h after infection was accompanied by the appearance of smaller than 18kDa immunoreactive polypeptides. Four intermediate forms with molecular mass of, respectively, 15, 13.3, 12 and 9.5kDa were detectable. The size of polypeptides detected in experiments performed in vivo is comparable with the degradation products of apoLp-III produced by serine protease IV in vitro. In addition, the total proteolytic activity of plasma increased progressively during infection time. The results of our studies suggest that a significant part of total proteolytical activity in the plasma of infected G. mellonella larvae can be attributed to proteases produced by P. aeruginosa during pathogenesis. We discuss the possibility that protease IV of P. aeruginosa is responsible for apoLp-III degradation in vivo.

Purification and characterization of eight peptides from Galleria mellonella immune hemolymph

Defense peptides play a crucial role in insect innate immunity against invading pathogens. From the hemolymph of immune-challenged greater wax moth, Galleria mellonella (Gm) larvae, eight peptides were isolated and characterized. Purified Gm peptides differ considerably in amino acid sequences, isoelectric point values and antimicrobial activity spectrum. Five of them, Gm proline-rich peptide 2, Gm defensin-like peptide, Gm anionic peptides 1 and 2 and Gm apolipophoricin, were not described earlier in G. mellonella. Three others, Gm proline-rich peptide 1, Gm cecropin D-like peptide and Galleria defensin, were identical with known G. mellonella peptides. Gm proline-rich peptides 1 and 2 and Gm anionic peptide 2, had unique amino acid sequences and no homologs have been found for these peptides. Antimicrobial activity of purified peptides was tested against gram-negative and gram-positive bacteria, yeast and filamentous fungi. The most effective was Gm defensin-like peptide which inhibited fungal and sensitive bacteria growth in a concentration of 2.9 and 1.9 microM, respectively. This is the first report describing at least a part of defense peptide repertoire of G. mellonella immune hemolymph.

Developing insect models for the study of current and emerging human pathogens

The study of human diseases requires the testing of microorganisms in model systems. Although mammals are typically used, we argue the validity of using insects as models in order to examine human diseases, particularly the growing number of opportunistic microorganisms. Insects can be used in large numbers, are easily manipulated, and are not subject to the same ethical concerns as mammalian systems. Insects and mammals have many parallels with respect to microbial pathogenesis, from proteinaceous integuments that require breaching before infection to similarities in their innate immune responses. Reactions of insects to Candida and Pseudomonas spp. infections show good correlation with mouse models, providing precedent-setting examples of the study of human pathogens using insects. Insects as pathogen hosts also warrant study because they may act as reservoirs for emerging human pathogens. Finally, insect models may be used to examine the evolutionary processes involved in the acquisition of virulence factors and host-jumping mechanisms indispensable to emerging pathogens. Insect models may be used in 'niche' investigations where large sample sizes can facilitate rapid, informative screening of opportunistic diseases and provide insights into pathogen evolution, while reducing the cost and ethical concerns associated with mammalian models.

Susceptibility of larvae of Galleria mellonella to infection by Aspergillus fumigatus is dependent upon stage of conidial germination

The ability of conidia of the human pathogenic fungus Aspergillus fumigatus to kill larvae of the insect Galleria mellonella was investigated. Conidia at different stages of the germination process displayed variations in their virulence as measured using the Galleria infection model. Non-germinating ('resting') conidia were avirulent except when an inoculation density of 1 x 10(7) conidia per insect was used. Conidia that had been induced to commence the germination process by pre-culturing in growth medium for 3 h were capable of killing larvae at densities of 1 x 10(6) and 1 x 10(7) per insect. An inoculation density of 1 x 10(5) conidia per insect remained avirulent. Conidia in the outgrowth phase of germination (characterised as the formation of a germ tube) were the most virulent and were capable of killing 100% of larvae after 5 or 24 h when 1 x 10(7) or 1 x 10(6) conidia, that had been allowed to germinate for 24 h, were used. Examination of the response of insect haemocytes to conidia at different stages of the germination process established that haemocytes could engulf non-germinating conidia and those in the early stages of the germination process but that conidia, which had reached the outgrowth stages of germination were not phagocytosed. The results presented here indicate that haemocytes of G. mellonella are capable of phagocytosing A. fumigatus conidia less than 3.0 microm in diameter but that conidia greater than this are too large to be engulfed. The virulence of A. fumigatus in G. mellonella larvae can be ascertained within 60-90 h if infection densities of 1 x 10(6) or 1 x 10(7) activated conidia (pre-incubated for 2-3 h) per insect are employed.

Pre-exposure to yeast protects larvae of Galleria mellonella from a subsequent lethal infection by Candida albicans and is mediated by the increased expression of antimicrobial peptides

Pre-exposure of the larvae of Galleria mellonella to Candida albicans or Saccharomyces cerevisiae protects against a subsequent infection with 10(6) C. albicans cells. This protection can also be induced by exposing larvae to glucan or laminarin prior to the administration of the potentially lethal inoculum. Analysis of the genes coding for galiomicin, a defensin in G. mellonella, a cysteine-rich antifungal peptide gallerimycin, an iron-binding protein transferrin and an inducible metalloproteinase inhibitor (IMPI) from G. mellonella demonstrated increased expression, which is at its highest after 24 h of the initial inoculum. Examination of the expression of proteins in the insect haemolymph using 2D electrophoresis and MALDI TOF analysis revealed an increased expression of a number of proteins associated with the insect immune response to infection 24 h after the initial exposure. This study demonstrates that the larvae of G. mellonella can withstand a lethal inoculum of C. albicans if pre-exposed to a non-lethal dose of yeast or polysaccharide 24 h previously which is mediated by increased expression of a number of antimicrobial peptides and the appearance of a number of peptides in the challenged larvae.

Using non-mammalian hosts to study fungal virulence and host defense

Non-mammalian hosts have been used to study host-fungal interactions. Hosts such as Drosophila melanogaster, Caenorhabditis elegans, Acathamoeba castellanii, Dictyostelium discoideum, and Galleria mellonella have provided means to examine the physical barriers, cellular mechanisms and molecular elements of the host response. The Drosophila host-response to fungi is mediated through the Toll pathway, whereas in C. elegans the host-response is TIR-1-dependent. Virulence traits that are involved in mammalian infection are important for the interaction of fungi with these hosts. Screening of fungal virulence traits using mutagenized fungi to determine changes in fungal infectivity of non-mammalian hosts has been used to identify novel virulence proteins used to infect C. elegans such as Kin1 (a serine/threonine protein kinase) and Rom2 (a Rho1 guanyl-nucleotide exchange factor) from Cryptococcus neoformans. These heterologous non-mammalian hosts highlight the similarities and differences between different hosts in fungal pathogenesis and they complement studies in mammalian systems and those using other genetic approaches.

A nonribosomal peptide synthetase (Pes1) confers protection against oxidative stress in Aspergillus fumigatus

Aspergillus fumigatus is an important human fungal pathogen. The Aspergillus fumigatus genome contains 14 nonribosomal peptide synthetase genes, potentially responsible for generating metabolites that contribute to organismal virulence. Differential expression of the nonribosomal peptide synthetase gene, pes1, in four strains of Aspergillus fumigatus was observed. The pattern of pes1 expression differed from that of a putative siderophore synthetase gene, sidD, and so is unlikely to be involved in iron acquisition. The Pes1 protein (expected molecular mass 698 kDa) was partially purified and identified by immunoreactivity, peptide mass fingerprinting (36% sequence coverage) and MALDI LIFT-TOF/TOF MS (four internal peptides sequenced). A pes1 disruption mutant (delta pes1) of Aspergillus fumigatus strain 293.1 was generated and confirmed by Southern and western analysis, in addition to RT-PCR. The delta pes1 mutant also showed significantly reduced virulence in the Galleria mellonella model system (P < 0.001) and increased sensitivity to oxidative stress (P = 0.002) in culture and during neutrophil-mediated phagocytosis. In addition, the mutant exhibited altered conidial surface morphology and hydrophilicity, compared to Aspergillus fumigatus 293.1. It is concluded that pes1 contributes to improved fungal tolerance against oxidative stress, mediated by the conidial phenotype, during the infection process.

Methylcitrate synthase from Aspergillus fumigatus. Propionyl-CoA affects polyketide synthesis, growth and morphology of conidia

Methylcitrate synthase is a key enzyme of the methylcitrate cycle and required for fungal propionate degradation. Propionate not only serves as a carbon source, but also acts as a food preservative (E280-283) and possesses a negative effect on polyketide synthesis. To investigate propionate metabolism from the opportunistic human pathogenic fungus Aspergillus fumigatus, methylcitrate synthase was purified to homogeneity and characterized. The purified enzyme displayed both, citrate and methylcitrate synthase activity and showed similar characteristics to the corresponding enzyme from Aspergillus nidulans. The coding region of the A. fumigatus enzyme was identified and a deletion strain was constructed for phenotypic analysis. The deletion resulted in an inability to grow on propionate as the sole carbon source. A strong reduction of growth rate and spore colour formation on media containing both, glucose and propionate was observed, which was coincident with an accumulation of propionyl-CoA. Similarly, the use of valine, isoleucine and methionine as nitrogen sources, which yield propionyl-CoA upon degradation, inhibited growth and polyketide production. These effects are due to a direct inhibition of the pyruvate dehydrogenase complex and blockage of polyketide synthesis by propionyl-CoA. The surface of conidia was studied by electron scanning microscopy and revealed a correlation between spore colour and ornamentation of the conidial surface. In addition, a methylcitrate synthase deletion led to an attenuation of virulence, when tested in an insect infection model and attenuation was even more pronounced, when whitish conidia from glucose/propionate medium were applied. Therefore, an impact of methylcitrate synthase in the infection process is discussed.

Superoxide production in Galleria mellonella hemocytes: identification of proteins homologous to the NADPH oxidase complex of human neutrophils

The insect immune response has a number of structural and functional similarities to the innate immune response of mammals. The objective of the work presented here was to establish the mechanism by which insect hemocytes produce superoxide and to ascertain whether the proteins involved in superoxide production are similar to those involved in the NADPH oxidase-induced superoxide production in human neutrophils. Hemocytes of the greater wax moth (Galleria mellonella) were shown to be capable of phagocytosing bacterial and fungal cells. The kinetics of phagocytosis and microbial killing were similar in the insect hemocytes and human neutrophils. Superoxide production and microbial killing by both cell types were inhibited in the presence of the NADPH oxidase inhibitor diphenyleneiodonium chloride. Immunoblotting of G. mellonella hemocytes with antibodies raised against human neutrophil phox proteins revealed the presence of proteins homologous to gp91phox, p67phox, p47phox, and the GTP-binding protein rac 2. A protein equivalent to p40phox was not detected in insect hemocytes. Immunofluorescence analysis localized insect 47-kDa and 67-kDa proteins throughout the cytosol and in the perinuclear region. Hemocyte 67-kDa and 47-kDa proteins were immunoprecipitated and analyzed by matrix-assisted laser desorption ionization--time of flight analysis. The results revealed that the hemocyte 67-kDa and 47-kDa proteins contained peptides matching those of p67phox and p47phox of human neutrophils. The results presented here indicate that insect hemocytes phagocytose and kill bacterial and fungal cells by a mechanism similar to the mechanism used by human neutrophils via the production of superoxide. We identified proteins homologous to a number of proteins essential for superoxide production in human neutrophils and demonstrated that significant regions of the 67-kDa and 47-kDa insect proteins are identical to regions of the p67phox and p47phox proteins of neutrophils.

Galleria mellonella as a model system to study Cryptococcus neoformans pathogenesis

Evaluation of Cryptococcus neoformans virulence in a number of nonmammalian hosts suggests that C. neoformans is a nonspecific pathogen. We used the killing of Galleria mellonella (the greater wax moth) caterpillar by C. neoformans to develop an invertebrate host model system that can be used to study cryptococcal virulence, host immune responses to infection, and the effects of antifungal compounds. All varieties of C. neoformans killed G. mellonella. After injection into the insect hemocoel, C. neoformans proliferated and, despite successful phagocytosis by host hemocytes, killed caterpillars both at 37 degrees C and 30 degrees C. The rate and extent of killing depended on the cryptococcal strain and the number of fungal cells injected. The sequenced C. neoformans clinical strain H99 was the most virulent of the strains tested and killed caterpillars with inocula as low as 20 CFU/caterpillar. Several C. neoformans genes previously shown to be involved in mammalian virulence (CAP59, GPA1, RAS1, and PKA1) also played a role in G. mellonella killing. Combination antifungal therapy (amphotericin B plus flucytosine) administered before or after inoculation was more effective than monotherapy in prolonging survival and in decreasing the tissue burden of cryptococci in the hemocoel. The G. mellonella-C. neoformans pathogenicity model may be a substitute for mammalian models of infection with C. neoformans and may facilitate the in vivo study of fungal virulence and efficacy of antifungal therapies.

Correlation between gliotoxin production and virulence of Aspergillus fumigatus in Galleria mellonella

Aspergillus fumigatus is a pathogenic fungus capable of causing both allergic lung disease and invasive aspergillosis, a serious, life-threatening condition in neutropenic patients. Aspergilli express an array of mycotoxins and enzymes which may facilitate fungal colonisation of host tissue. In this study we investigated the possibility of using the insect, Galleria mellonella, for in vivo pathogenicity testing of Aspergillus species. Four clinical isolates of Aspergillus fumigatus and a single strain of Aspergillus niger were characterised for catalase and elastase activity and for the production of gliotoxin. Gliotoxin is an immunosuppressive agent previously implicated in assisting tissue penetration. Results illustrated a strain dependent difference in elastase activity but no significant difference in catalase activity. Gliotoxin production was detected in vitro and in vivo by Reversed Phase-High Performance Liquid Chromatography, with highest amounts being produced by A. fumigatus ATCC 26933 (350 ng/mg hyphae). Survival probability plots (Kaplan-Meier) of experimental groups infected with Aspergillus conidia indicate that G. mellonella is more susceptible to fungal infection by A. fumigatus ATCC 26933, implicating a critical role for gliotoxin production rather than growth rate or enzymatic activity in the virulence of A. fumigatus in this model.

Exploiting the potential of insects for in vivo pathogenicity testing of microbial pathogens

Conventional assays for quantifying the virulence of microbial pathogens and mutants have traditionally relied upon the use of a range of mammalian species. A number of workers have demonstrated that insects can be used for evaluating microbial pathogenicity and provide results comparable to those that can be obtained with mammals since one component of the vertebrate immune system, the innate immune response, remains similar to that found in insects. Larvae of the Greater Wax Moth Galleria mellonella have been used to evaluate the virulence of a range of bacterial and fungal pathogens and a correlation with the virulence of these microbes in mice has been established. This review highlights the similarities of the vertebrate and insect innate immune responses to infection and identifies the potential use of insects for the in vivo evaluation of the microbial pathogenicity.

Fluctuations in haemocyte density and microbial load may be used as indicators of fungal pathogenicity in larvae of Galleria mellonella

A positive correlation exists between the pathogenicity of bacteria and fungi when evaluated in the insect Galleria mellonella and mice. This work sought to determine whether fluctuations in the number of haemocytes and the proliferation of yeast cells in infected larvae could be used to determine the relative pathogenicity of a range of yeast isolates. Larvae were inoculated with 1 x 10(6) stationary-phase yeast cells and incubated in the dark at 30 degrees C for 48 h. The results indicated that larvae inoculated with the most pathogenic isolates (i.e. those capable of killing >80% of infected larvae) showed a significant reduction in haemocyte density. Larvae inoculated with isolates of low pathogenicity (i.e. capable of killing <20% of infected larvae) demonstrated only a small fluctuation in haemocyte numbers. The most pathogenic yeast isolates proliferated in the larvae, whereas the isolates of low pathogenicity did not. These results demonstrate a relationship between the ability of yeast isolates to kill larvae and changes in haemocyte density and yeast cell density in infected larvae. These end points may extend the applicability of the G. mellonella system for use with a wider range of microbial isolates.

Virulence ofCandida albicansmutants toward larvalGalleria mellonella(Insecta, Lepidoptera, Galleridae)

Culture medium affected the virulence of a strain of Candida albicans toward Galleria mellonella larvae, but the yeast growth rates in yeast extract – peptone – dextrose broth and synthetic Galleria serum were not correlated with yeast virulence. Virulent C. albicans grew rapidly in larval serum, whereas, it limited nodulation and continued development in vivo, producing toxins that damaged the hemocytes and fat body. Nonpathogenic yeast-phase cells grew slowly in larval serum but induced extensively melanized nodules in vivo and developed no further. There was no discernible relationship in 14 exo-enzymes between the virulent and avirulent yeast strains and virulence. The avirulent myosin-I-defective yeast cells were rapidly removed from the hemolymph in vivo because of lysozyme-mediated yeast agglutination and the possible binding of the yeast cells by lysozyme and apolipophorin-III. Both lysozyme and apolipophorin-III are proteins that bind β-1,3-glucan. Finally, insects with nonpathogenic C. albicans exhibited induced immunity and were more resistant to candidiasis from the wild-type yeast cells than were noninduced insects.Key words: Candida, virulence, insect, nodule, melanization, apolipophorin-III.

Infection of chick chorioallantoic membrane (CAM) as a model for invasive hyphal growth and pathogenesis of Candida albicans

We report the development of a simple model for assessing the ability of the fungal pathogen Candida albicans to invade the chorioallantoic membrane (CAM) of fertilized hens' eggs. Wild-type and mutant strains of C. albicans were inoculated onto CAM surfaces either as a liquid suspension or on a sterile filter disc. Invasion of the membrane led to death of the embryo due to damage of the CAM, which could be examined histologically to show cell distribution and morphology, and by RT-PCR for assessment of patterns of fungal gene expression in vivo. Prophylactic or co-administration of fluconazole with the inoculum protected the embryo from infection. Secretory aspartyl protease (Sap) mutant strains with reported attenuation of virulence were virulent in the CAM model. However, a C. albicans strain with mutations in two transcription factors Efg1 and Cph1 was unable to form hyphae on the CAM or to penetrate it. The chick CAM, therefore, represents an experimentally tractable and inexpensive alternative to rodent or tissue culture-based invasion models, and can be used to investigate fungal pathogenesis and the genetic regulation of infection and membrane penetration of C. albicans.

Correlation between virulence of Candida albicans mutants in mice and Galleria mellonella larvae

Candida albicans is a dimorphic human pathogen in which the yeast to hyphal switch may be an important factor in virulence in mammals. This pathogen has recently been shown to also kill insects such as the Greater Wax Moth Galleria mellonella when injected into the haemocoel of the insect larvae. We have investigated the effect of previously characterised C. albicans mutations that influence the yeast to hyphal transition on virulence in G. mellonella larvae. There is a good correlation between the virulence of these mutants in the insect host and the virulence measured through systemic infection of mice. Although the predominant cellular species detected in G. mellonella infections is the yeast form of C. albicans, mutations that influence the hyphal transition also reduce pathogenicity in the insect. The correlation with virulence measured in the mouse infection system suggests that Galleria may provide a convenient and inexpensive model for the in vivo screening of mutants of C. albicans.