The human body is host to a plethora of microorganisms and, for the most part, their presence has no ill effects. Some, particularly intestinal bacteria, even provide benefit. From a microbial perspective, harming the host does not have any obvious survival benefit (unless it enables infection to spread, such as the sneezing induced by the cold virus). So why is it that inoffensive organisms occasionally turn nasty, evolving properties that are damaging or even deadly to us? A study funded by the US Public Health Service and the Wellcome Trust provides one answer to the question.
Since many pathogens interact with their host at mucosal surfaces and have to compete with other microflora, scientists at the University of Pennsylvania School of Medicine and Oxford University used a mouse model of nasal infection to investigate whether competition between microbes promoted virulence. They found that Haemophilus influenzae was able to out-compete Streptococcus pneumoniae by recruiting the host’s immune system. S. pneumoniae is normally harmless and ignored by the immune system, but the immune response stimulated by H. influenzae has unintended consequences. S. pneumoniae strains with polysaccharide capsules that confer resistance to the immune attack are able to survive at the expense of non-resistant strains, resulting in a S. pneumoniae population dominated by the resistant phenotype. Unfortunately, the resistant strains are also more dangerous – if they are able to enter the bloodstream they can multiply unchecked and go on to cause pneumonia, septicaemia and meningitis. So in this battle between S. pneumoniae and H. influenzae, with weapons provided by the host, S. pneumoniae prevails at the expense of the host.
Mucormycosis is a potentially fatal infection of the sinuses, brain, or lungs, which is most commonly caused by the fungus Rhizopus oryzae. Even if the disease is successfully treated with antifungal agents together with surgery to remove necrotic tissue, survivors are typically left with considerable disfigurement. The condition is seen most often in people with diabetic ketoacidosis (DKA) who have elevated serum glucose and iron levels and a team of researchers at the University of California Los Angeles has now discovered why these individuals are more susceptible to infection. In mucormycosis, fungal invasion of blood vessels results in the formation of blood clots and destruction of local tissue and the team set out to identify the endothelial cell receptor that the fungus uses, and also whether iron and glucose play a role in regulating the expression of the receptor.
In human endothelial cells, glucose-regulated protein 78 (GRP78) was found to play a key role in endocytosis of R. oryzae and subsequent damage; enhanced expression of the protein in the presence of high concentrations of glucose, especially when iron levels are also elevated, offers an explanation of the increased susceptibility of individuals with DKA to mucormycosis. Mice with DKA, which have elevated levels of glucose and available iron, and which are also susceptible to mucormycosis, showed increased expression of GRP78 in sinus, lungs, and brain compared with normal mice. In further studies, treatment of DKA mice with GRP78-specific immune serum was shown to protect them from mucormycosis.
The study, which is published in the Journal of Clinical investigation, provides a new understanding of the pathogenesis of R. oryzae and may lead to new treatments for potentially lethal mucormycosis.
Tuberculosis (TB) is an airborne infectious disease caused by Mycobacterium tuberculosis (Mtb). TB is difficult to treat and the most commonly used antibiotics, rifampicin and isoniazid, need to be used for many months to eliminate the infection. The recent resurgence of TB, together with the emergence of drug-resistant strains of the bacterium, underscores the need for new treatments and researchers at Weill Cornell Medical College and the Novartis Institute for Tropical Diseases have identified a metabolic vulnerability in Mtb that could lead to new targets for drug therapy.
In vitro, Mtb is able to grow on a variety of carbon sources but fatty acids are believed to be the major source of carbon and energy for the bacterium during infection of a host. When bacterial metabolism is primarily fuelled by fatty acids, biosynthesis of sugars from intermediates of the tricarboxylic acid cycle is known to be essential for growth but the role of gluconeogenesis in the pathogenesis of Mtb had not been explored. Using genetic analyses and 13C carbon tracing, the team were able to show that phosphoenolpyruvate carboxykinase (PEPCK) – the enzyme that catalyses the first committed step of gluconeogenesis – is essential for the growth of Mtb supported by fatty acids. PEPCK was shown to be needed for growth of Mtb both in isolated macrophages derived from the bone marrow of mice and in infected mice.
Mtb lacking PEPCK failed to replicate in mouse lungs and silencing PEPCK during the chronic phase resulted in clearance of the infection, showing that Mtb relies on gluconeogenesis throughout the course of the infection. The finding that PEPCK plays a pivotal role in the growth and persistence of Mtb during both acute and latent infections in mice – and that PEPCK depletion also attenuates Mtb in IFNγ-deficient mice – suggests that this enzyme is an attractive target for chemotherapy.
The near pandemic of SARS (Severe Acute Respiratory Syndrome) that occurred in 2002/2003 underlined the need for vigilance in understanding and containing the global spread of pathogens. Genomic sequencing is an important tool in characterising novel pathogens but, to best understand the etiology of epidemics, genetic information must be integrated with host biology and geography. To this end, researchers at Ohio State University have developed a web-based application that allows users to visualise genetic and evolutionary information about organisms that cause infectious diseases as they spread across the globe.
The application, Supramap, was initially created to track the avian influenza virus, H5N1, and has more recently been use to monitor the spread of the H1N1 virus. With the H5N1 virus, mapping a mutation (Glu627Lys) in the PB2 protein supported the hypothesis that this mutation allows for increased replication of the virus in mammals but not that viruses with this mutation were moving westwards.
The application is able to calculate and project evolutionary trees for pathogens onto geographic information systems such as Google Earth. The resultant ‘weather maps for disease’ will allow scientists and public health officials to see when and where pathogens spread, jump between host species, and evolve to become drug-resistant. The development team hope to receive genomic and geographical data about pathogens from around the world and provide regularly updated maps that could determine global hotspots for the emergence of dangerous pathogens and indicate, for example, where and when particular antiviral drugs would be effective. The application is easy to use and researchers can submit raw sequence data to create a phylogenic tree that can be projected onto a map of the globe and viewed with Google Earth. As with all models, the more data that can be collected, the more sophisticated and accurate the analysis will be.
More details about the Supramap project are published in the journal Cladistics.
Onchoceriasis – also known as river blindness – is the world’s second leading infectious cause of blindness. The disease is caused by the nematode, Onchocerca volvulus, and is transmitted to humans through the bite of a blackfly. Once inside the body, the female worm produces thousands of larval worms (microfilariae) which migrate to the skin and eyes. When the microfilariae die, they cause intense itching and a strong immune response that can destroy nearby tissue, leading eventually to blindness and disfiguring skin lesions. Control programmes have involved the use of larvicides to reduce blackfly populations and the use of ivermectin to treat infected people and limit the spread of disease. Ivermectin is most effective against the larval stage of the worm and is believed to kill the parasites by activating glutamate-gated chloride channels which are specific to invertebrates.
A team led by researchers at the Scripps Institute has now focused on a new way to kill the parasite. The protective outer cuticle of the worms is made of chitin and two classes of enzymes – chitin synthases and chitinases – are known to be critical for chitin formation and remodelling. One chitinase, OvCHT1, is expressed only in the infective third-stage larvae and is believed to be involved in development and host transmission. The team screened a small library of compounds for activity against OvCHT1 and found that closantel was able to inhibit the enzyme. When closantel was tested on cultured third-stage larvae, the compound prevented the larvae from moulting and developing into adult worms. Since the mechanism of action of closantel is completely different to that of ivermectin, it – or other chitinase inhibitors – could potentially be used to treat ivermectin-resistant worms. Closantel is a broad-spectrum anti-parasitic agent currently used in some countries in veterinary medicine.
Source: European Commission Prion diseases comprise the transmissible spongiform encephalopathies, including scrapie in sheep, bovine spongiform encephalopathy (BSE, “Mad Cow” disease) in cattle and Creutzfeldt-Jakob disease in humans. Central to these diseases is the conversion of normal cellular prion protein (PrPc) into the abnormally folded, pathogenic species (PrPSc) in the brain. The misfolding results in prion protein with distinct biochemical properties compared to the normal protein, such as reduced solubility and decreased susceptibility to proteases. Aggregates of PrPSc accumulate in association with neurons in affected brain areas, which is thought to lead to the synapse degeneration and neuronal death observed in infected hosts.
Researchers in the UK and Italy have now shown that glimepiride, a sulfonyl urea approved for the treatment of non insulin dependent diabetes mellitus (NIDDM), is able to reduce PrPSc formation in cell culture. The rationale for the study was based on the knowledge that generation of PrPSc is dependent on the presence of PrPc and that this appeared to require PrPc expressed at the cell surface. PrPc is linked to the membrane by a glycosylphosphatidylinositol (GPI) anchor and can be released from the surface of cells by treatment with phosphatidylinositol-phospholipase C (PI-PLC). Consistent with the hypothesis that cell-surface PrPc is required, treatment of prion-infected neuronal cells with PI-PLC reduced PrPSc formation.
Since glimepiride has been shown to stimulate the release of some GPI-anchored proteins in adipocytes (via stimulation of an endogenous GPI-PLC), the team explored the effects of the drug on PrPc/PrPSc in neuronal cell culture. Similarly to PI-PLC, glimepiride reduced the amount of cell-surface PrPc in primary cortical neurons and neuronal cell lines. In addition, glimepiride reduced formation of PrPSc in three prion-infected neuronal cell lines.
The study, published in PLoSone, also demonstrated that glimepiride treated neurons were resistant to the toxicity of a PrP-derived peptide, PrP82-146.
The team note that modulation of cell-surface PrPc may also have application in Alzheimer’s disease since it is a receptor for β-amyloid oligomers. Whether glimepiride is sufficiently CNS-penetrant to be effective remains to be seen.
Image: Wikipedia - Heironymous Rowe Mosquitoes and other blood-feeding insects are attracted by exhaled carbon dioxide and volatile organic compounds produced by their hosts. Scientists at the University of California, Davis have now identified a compound produced by both humans and birds that acts as an attractant for the Culex mosquitoes which transmit West Nile virus. Birds are the principal hosts for West Nile virus and act as amplifying hosts from which the virus can be transmitted to humans and other mammals by mosquito bites.
The discovery of a common semiochemical produced by birds and humans may explain the observed shifts in Culex feeding from birds to humans and consequent transmission of West Nile virus to human populations. Four compounds were found to dominate odorant profiles of humans from different ethnic backgrounds and one of these – nonanal – was also the predominant compound in profiles of both chicken and pigeon. Olfactory receptor neurones in the antennae of Culex quinquefasciatus (Southern house mosquitoes) were shown to be exquisitely sensitive to nonanal, and field populations were attracted to traps baited with the odorant. When nonanal and carbon dioxide were combined, more insects were trapped than when using either bait alone. West Nile virus is now the dominant vector-borne disease in North America and the presence of a common semiochemical in birds and humans may explain the shift in feeding behaviour of Culex mosquitoes that occur in late summer when migrating birds disperse.