Pathogenicity and virulence

Invertebrate pathologists have multiple definitions for the terms pathogenicity and virulence, and these definitions vary across disciplines that focus on host-pathogen interactions. We surveyed various literatures, including plant pathology, invertebrate pathology, evolutionary biology, and medicine, and found most define pathogenicity as the broader term, which incorporates virulence. Virulence is seen as the severity of disease manifestation that can only be measured in infected individuals. These definitions readily apply to both lethal and non-lethal diseases. Invertebrate pathologists commonly use dose-response bioassays to estimate LD(50) or LC(50) (dose or concentration needed to kill 50% of hosts exposed). These bioassays measure pathogenicity if the bioassay includes a transmission component, and measure virulence if the bioassay is measured in infected individuals only. Another common bioassay estimate is LT(50) (median time to death of infected hosts), which is a measure of virulence as long as survivors are not included in its calculation.     

Mouse pathogenicity studies of Nocardia asteroides complex species and clinical correlation with human isolates

Abstract Nocardia asteroides complex organisms derived from human specimens between 1979 and 1992 were identified on the species level. Of 117 N. asteroides complex organisms, 34 (29%) were N. farcinica , 28 (24%) were N. nova , and 55 (47%) were N. asteroides sensu stricto . An analysis of the specimen sites from which the organisms were derived showed that isolates derived from blood, brain, or bone marrow were more likely to be N. farcinica than the other two species. A study of the virulence of ten strains of each species was undertaken, using a mouse model with intravenous inoculation. The 50% lethal doses (LD₅₀) for N. farcinica were significantly lower than those of the other two species. LD₅₀ values for N. nova and N. asteroides were not significantly different. The above data confirming the greater virulence of N. farcinica support the identification of species within the N. asteroides complex.     

Possible virulence factors of Staphylococcus sciuri

Staphylococcus sciuri is an opportunistic pathogen of controversial clinical significance. The factors that contribute to colonization and/or infection caused by this bacterium have not been studied intensively so far. The present research was carried out in order to study the presence of potential virulence factors in 121 human and animal isolates of this bacterium. Isolates were examined for biofilm formation, hemagglutination, presence of clumping factor, production of spreading factors and exotoxins, cytotoxicity and capacity to stimulate nitric oxide production. The results showed that S. sciuri is highly capable of biofilm production, that it displays strong proteolytic and DNase activities, produces hemolysins and stimulates nitric oxide production by rat macrophages. Although the present study showed existence of a wide spectrum of possible virulence determinants of S. sciuri, their exact contribution to virulence of this bacterium in vivo remains to be determined.     

Presence of cna, emp and pls genes and pathogenicity of methicillin-resistant Staphylococcus aureus strains

Methicillin-resistant strains of Staphylococcus aureus (MRSA) are important etiological factors responsible for hospital-acquired infections. The aim of this study was to analyze the influence of the presence of emp, pls and cna genes on the pathogenicity of MRSA strains. The presence of these genes was tested by PCR in 302 MRSA strains isolated from hospitalized patients and from carriers. For each tested gene, proportions of positive and negative strains were similar among the infected patients and carriers. We did not find any obvious correlation between the presence of the three tested genes and the infectivity of strains. Our results may also suggest that a lack of emp and presence of pls may correlate with reduced virulence of these strains.     

Identification of a putative pathogenicity island in Shigella flexneri using subtractive hybridisation of the S. flexneri and Escherichia coli genomes

The genetic differences between the human pathogen, Shigella flexneri, and the non-pathogenic Escherichia coli were investigated in an attempt to identify pathogenicity islands (PAIs) in the S. flexneri genome. Genomic subtraction identified a large unique region of DNA which was present in S. flexneri serotype 2a but absent from E. coli K-12. This 42-kb DNA segment was localised to the S. flexneri chromosome and was found to contain a number of elements often associated with PAIs including: insertion sequence elements, bacteriophage genes, and a previously identified Shigella virulence gene (criR). These findings indicate that this region may form a new PAI in the S. flexneri genome.     

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.     

细胞壁缺陷白喉棒状杆菌致病作用的观察及其毒力基因检测

目的比较白喉棒状杆菌亲代细菌型及其稳定L型动物致病性的差异,了解白喉棒状杆菌稳定L型变异的特点,探讨其变异的性质及其与细胞壁缺陷突变的关系。方法用氨苄青霉素在非高渗培养基内人工诱导产毒性白喉棒状杆菌为稳定L型。采用白喉棒状杆菌稳定L型纯培养物及其代谢产物皮内感染家兔,观察局部感染部位皮肤或全身的病理改变。采用微量法提取白喉棒状杆菌稳定L型的染色体DNA,用Tox基因特异性引物进行PCR扩增以检测毒素蛋白结构基因,并进行序列测定和分析。结果白喉棒状杆菌在氨苄青霉素作用下可发生细胞壁缺陷而成为L型,白喉棒状杆菌稳定L型不能引起动物局部或全身发生异常表现,该稳定L型的传代培养物可仍然保留同其亲代细菌型一致的Tox基因及其核苷酸序列。结论提示细胞壁缺失将导致白喉棒状杆菌与产生毒素蛋白有关结构基因在宿主菌细胞内的表达受到抑制,以致使白喉棒状杆菌稳定L型丧失了产生外毒素致病的作用。     

The pathogenicity of cytomegalovirus

Human cytomegalovirus is ubiquitous, yet causes little illness in immunocompetent individuals. Disease is evident in immunodeficient groups such as neonates, transplant recipients and AIDS patients either following a primary infection or reactivation of a latent infection. Little is known of the mechanisms underlying the pathogenicity of the virus. The recent determination of the nucleotide sequence of both human cytomegalovirus (strain AD169) and murine cytomegalovirus (murine cytomegalovirus strain Smith) has allowed an analysis of the biological importance of several virus genes. Studies with human cytomegalovirus have indicated that many viral genes are non-essential for replication in vitro which are thus assumed to be important in the pathogenesis of the virus. This is being examined in the murine model where the role of the gene and its product in disease can be directly examined in vivo using viral mutants in which the relevant gene has been interrupted or deleted. Current information on the role of cytomegalovirus genes in tissue tropism, immune evasion, latency, reactivation from latency and damage is described.     

A study on pathogenicity and mosquito transmission success in the rodent malaria parasite Plasmodium chabaudi adami

We investigated the parasitology, pathogenicity (virulence) and infectivity to mosquitoes of blood infections in mice, of two strains, DS and DK, of the rodent malaria parasite Plasmodium chabaudi adami. Blood infections of DS were found to be highly pathogenic; the asexual parasites in these infections were fast-growing and showed no evidence of selectivity in their infection of host erythrocytes. In contrast to DS, blood infections of DK were much less pathogenic; the asexual parasites were slower-growing and showed a moderate degree of selectivity to a subset of erythrocytes which were not reticulocytes. In both DS and DK infections, infectivity to mosquitoes was highest before the peak of asexual parasitaemia had occurred; usually this did not coincide with the time when gametocyte numbers in the blood were highest. Infections with the pathogenic DS strain in CBA mice produced fewer gametocytes than did the less pathogenic DK strain. The DS strain infections in both CBA and C57 mice were also significantly much less infective to mosquitoes than the DK strain. Investigations by others on the related rodent malaria parasite subspecies, Plasmodium chabaudi chabaudi, have indicated that the mosquito infectivity of blood infections in mice tended to be higher in the more pathogenic (virulent) and lower in the less pathogenic strains of this parasite subspecies. This is the converse of the finding of the present investigation of blood infections of P. c. adami in mice in which a more pathogenic, or virulent, strain (DS) of these parasites was significantly much less infective to mosquitoes than was a less pathogenic strain (DK).     

Genome analysis and in vivo virulence of porcine extraintestinal pathogenic Escherichia coli strain PCN033

Background

Strains of extraintestinal pathogenic Escherichia coli (ExPEC) can invade and colonize extraintestinal sites and cause a wide range of infections. Genomic analysis of ExPEC has mainly focused on isolates of human and avian origins, with porcine ExPEC isolates yet to be sequenced. To better understand the genomic attributes underlying the pathogenicity of porcine ExPEC, we isolated two E. coli strains PCN033 and PCN061 from pigs, assessed their in vivo virulence, and completed and compared their genomes.

Results

Animal experiments demonstrated that strain PCN033, but not PCN061, was pathogenic in a pig model. The chromosome of PCN033 was 384 kb larger than that of PCN061. Among the PCN033-specific sequences, genes encoding adhesins, unique lipopolysaccharide, unique capsular polysaccharide, iron acquisition and transport systems, and metabolism were identified. Additionally, a large plasmid PCN033p3 harboring many typical ExPEC virulence factors was identified in PCN033. Based on the genetic variation between PCN033 and PCN061, corresponding phenotypic differences in flagellum-dependent swarming motility and metabolism were verified. Furthermore, the comparative genomic analyses showed that the PCN033 genome shared many similarities with genomic sequences of human ExPEC strains. Additionally, comparison of PCN033 genome with other nine characteristic E. coli genomes revealed 425 PCN033-special coding sequences. Genes of this subset included those encoding type I restriction-modification (R-M) system, type VI secretion system (T6SS) and membrane-associated proteins.

Conclusions

The genetic and phenotypic differences between PCN033 and PCN061 could partially explain their differences in virulence, and also provide insight towards the molecular mechanisms of porcine ExPEC infections. Additionally, the similarities between the genomes of PCN033 and human ExPEC strains suggest that some connections between porcine and human ExPEC strains exist. The first completed genomic sequence for porcine ExPEC and the genomic differences identified by comparative analyses provide a baseline understanding of porcine ExPEC genetics and lay the foundation for their further study.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1890-9) contains supplementary material, which is available to authorized users.     

Characterization and virulence of entomopathogenic fungi from sunn pests in Turkey

The sunn pests (Eurygaster spp.) which are important pests of wheat and barley, can lead to 100% yield loss in the case of an epidemic. This study was conducted to characterize the entomopathogenic fungi, obtained from the sunn pests collected from different overwintering areas of Turkey and determine their pathogenicity. Seventeen Beauveria pseudobassiana, 12 Beauveria bassiana, 16 Cordyceps farinosa, 1 Purpureocillium lilacinum, and 2 Clonostachys rosea species were identified. Pathogenicity tests were performed with three B. pseudobassiana, two B. bassiana, four C. farinosa, and one P. lilacinum isolates. B. pseudobassiana isolates BB34 and AF4 caused 100% death on the 9th day, and another B. pseudobassiana isolate E512 caused 100% death on the 12th day. C. farinosa isolate KOKA2 caused 95.8% death on the 15th day. Ultimately, B. pseudobassiana was determined to be the most prevalent and virulent species on sunn pests.     

Genetic approaches to study Legionella pneumophila pathogenicity

Abstract: Legionella pneumophila is an intracellular pathogen replicating in human macrophages during the course of infection of the lungs, infection by legionellae often leads to severe pneumonia, termed Legionnaires' disease. Genetic approaches to identify the factors responsible for L. pneumophila pathogenicity started with the construction of genomic libraries in Escherichia coli. Various L. pneumophila -specific genes were cloned in E. coli K-12 by identifaction using functional assays, antibody screening and hybridization ('reverse genetics'). By disrupting the genes via allelic exchange, mutants have been created to assess the influence of the factors on pathogenicity. Among the cloned genes, only for the gene product of the mip gene, encoding a 24-kDa surface-associated protein (macrophage infectivity potentiator) unequivocal evidence for its contribution to pathogenicity could be provided. Two hemolytic factors that have been cloned do not seem to play a role in L. pneumophila pathogenicity. Genetic systems for transposon mutagenesis of the L. pneumophila genome (Tn5, Tn903dlIlacZ, MudphoA), including TnphoA shuttle mutagenesis, have been established and specifically adapted to identify mutants which displayed an impaired capability to multiply inside macrophages and with a reduced in vivo virulence. Furthermore, by complementation of avirulent mutants, genetic loci could be identified which restored the virulence.     

Iron depletion and virulence in Staphylococcus aureus

Abstract Staphylococcus aureus is able to grow in the presence of extremely low iron concentrations (0.04 μM). In iron-limiting conditions, this species develops alternative metabolic strategies such as highly efficient iron-uptake mechanisms which are only partially shared with S. epidermidis . Here we summarize the mechanisms induced by iron starvation in S. aureus in order to elucidate the virulence characteristics of this bacterium.     

Gene expression profiling of candidate virulence factors in the laminated root rot pathogen Phellinus sulphurascens

Background

Phellinus sulphurascens is a fungal pathogen that causes laminar root rot in conifers, one of the most damaging root diseases in western North America. Despite its importance as a forest pathogen, this fungus is still poorly studied at the genomic level. An understanding of the molecular events involved in establishment of the disease should help to develop new methods for control of this disease.

Results

We generated over 4600 expressed sequence tags from two cDNA libraries constructed using either mycelia grown on cellophane sheets and exposed to Douglas-fir roots or tissues from P. sulphurascens-infected Douglas-fir roots. A total of 890 unique genes were identified from the two libraries, and functional classification of 636 of these genes was possible using the Functional Catalogue (FunCat) annotation scheme. cDNAs were identified that encoded 79 potential virulence factors, including numerous genes implicated in virulence in a variety of phytopathogenic fungi. Many of these putative virulence factors were also among 82 genes identified as encoding putatively secreted proteins. The expression patterns of 86 selected fungal genes over 7 days of infection of Douglas-fir were examined using real-time PCR, and those significantly up-regulated included rhamnogalacturonan acetylesterase, 1,4-benzoquinone reductase, a cyclophilin, a glucoamylase, 3 hydrophobins, a lipase, a serine carboxypeptidase, a putative Ran-binding protein, and two unknown putatively secreted proteins called 1 J04 and 2 J12. Significantly down-regulated genes included a manganese-superoxide dismutase, two metalloproteases, and an unknown putatively secreted protein called Ps0058.

Conclusions

This first collection of Phellinus sulphurascens EST sequences and its annotation provide an important resource for future research aimed at understanding key virulence factors of this forest pathogen. We examined the expression patterns of numerous fungal genes with potential roles in virulence, and found a collection of functionally diverse genes that are significantly up- or down-regulated during infection of Douglas-fir seedling roots by P. sulphurascens.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-603) contains supplementary material, which is available to authorized users.     

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.