MicrobiologyBytes

Plasmodium falciparum, a mosquito-borne parasite that causes malaria, kills nearly one million people every year, mostly in sub-Saharan Africa. People become infected with P. falciparum when they are bitten by a mosquito that has acquired the parasite in a blood meal taken from an infected person. P. falciparum malaria, which is characterized by recurring fevers and chills, anemia (loss of red blood cells), and damage to vital organs, can be fatal within hours of symptom onset if untreated. Until recently, treatment in Africa relied on chloroquine and sulfadoxine–pyrimethamine. Unfortunately, parasites resistant to both these antimalarial drugs is now widespread. Consequently, the World Health Organization currently recommends artemisinin combination therapy for the treatment of P. falciparum malaria in Africa and other places where drugresistant malaria is common. In this therapy, artemisinin derivatives (new fast-acting antimalarial agents) are used in combination with another antimalarial to reduce the chances of P. falciparum becoming resistant to either drug.

P. falciparum becomes resistant to antimalarial drugs by acquiring “resistance mutations,” genetic changes that prevent these drugs from killing the parasite. A mutation in the gene encoding a protein called the chloroquine resistance transporter causes resistance to chloroquine, a specific group of mutations in the dihydrofolate reductase gene causes resistance to pyrimethamine, and several mutations in dhps, the gene that encodes dihydropteroate synthase, are associated with resistance to sulfadoxine. Scientists have discovered that the mutations causing chloroquine and pyrimethamine resistance originated in Asia and spread into Africa (probably multiple times) in the late 1970s and mid-1980s, respectively. These Asian-derived mutations are now common throughout Africa and, consequently, it is not possible to determine how they spread across the continent. Information of this sort would, however, help experts design effective measures to control the spread of drug-resistant P. falciparum. Because the mutations in dhps that cause sulfadoxine resistance only began to emerge in the mid- 1990s, they haven’t spread evenly across Africa yet. In this study, therefore, the researchers use genetic methods to characterize the geographical origins and contemporary distribution of dhps resistance mutations in Africa.

The researchers analyzed dhps mutations in P. falciparum DNA from blood samples collected from patients with malaria in various African countries and searched the scientific literature for other similar studies. Together, these data show that five major variant dhps sequences (three of which contain mutations that confer various degrees of resistance to sulphadoxine in laboratory tests) are currently present in Africa, each with a unique geographical distribution. In particular, the data show that P. falciparum parasites in east and west Africa carry different resistance mutations. Next, the researchers looked for microsatellite variants in the DNA flanking the dhps gene. Microsatellites are DNA regions that contain short, repeated sequences of nucleotides. Because the number of repeats can vary and because microsatellites are inherited together with nearby genes, the ancestry of various resistance mutations can be worked out by examining the microsatellites flanking different mutant dhps genes. This analysis revealed five regional clusters in which the same resistance lineage was present at all the sites examined within the region and also showed that the resistance mutations in east and west Africa have a different ancestry.

These findings show that sulfadoxine-resistant P. falciparum has recently emerged independently at multiple sites in Africa and that the molecular basis for sulfadoxine resistance is different in east and west Africa. This latter result may have clinical implications because it suggests that the effectiveness of sulfadoxine as an antimalarial drug may vary across the continent. Finally, although many more samples need to be analyzed to build a complete picture of the spread of antimalarial resistance across Africa, these findings suggest that economic and transport infrastructures may have played a role in governing recent parasite dispersal across this continent by affecting human migration. Thus, coordinated malaria control campaigns across socioeconomically linked areas in Africa may reduce the African malaria burden more effectively than campaigns that are confined to national territories.

Multiple Origins and Regional Dispersal of Resistant dhps in African Plasmodium falciparum Malaria. PLoS Med 6(4): e1000055
Although the molecular basis of resistance to a number of common antimalarial drugs is well known, a geographic description of the emergence and dispersal of resistance mutations across Africa has not been attempted. To that end we have characterised the evolutionary origins of antifolate resistance mutations in the dihydropteroate synthase (dhps) gene and mapped their contemporary distribution. We used microsatellite polymorphism flanking the dhps gene to determine which resistance alleles shared common ancestry and found five major lineages each of which had a unique geographical distribution. The extent to which allelic lineages were shared among 20 African Plasmodium falciparum populations revealed five major geographical groupings. Resistance lineages were common to all sites within these regions. The most marked differentiation was between east and west African P. falciparum, in which resistance alleles were not only of different ancestry but also carried different resistance mutations. Resistant dhps has emerged independently in multiple sites in Africa during the past 10–20 years. Our data show the molecular basis of resistance differs between east and west Africa, which is likely to translate into differing antifolate sensitivity. We have also demonstrated that the dispersal patterns of resistance lineages give unique insights into recent parasite migration patterns.

Related:

• Comprehensive map of global malaria
• Malaria, mosquitoes and the legacy of Ronald Ross
• New ways to beat malaria
• Researchers characterize potential protein targets for malaria vaccine

Releasing highly pathogenic organisms into an urban population is a act of bioterrorism that could result in a large number of casualties. The first indication that a covert open-air release has occurred is quite likely to be individuals reporting for medical attention. If such an attack is suspected, then public health authorities would attempt to identify those individuals who have been infected in order to provide rapid treatment with the aim of reducing the possibility of disease and potential death. Aiming treatment at too small an area might miss individuals infected further down and/or up wind, whereas issues surrounding both treatment resources and serious side effects may rule out mass treatment campaigns of large sections of the population.

A new paper describes a statistical method that can estimate the origin and time of an aerosolized release of anthrax following detection of the first few cases. The method predicts where the most critically affected areas will be following the release of this highly pathogenic agent, which may enable preventative treatment of individuals at risk and protection from the disease. Previously published methods can estimate the date and scale of anthrax release but not the source location or geographic extent of human exposure. The new method uses information about the first people infected, including when they started to experience symptoms of infection and where they live and work, combined with recent weather information, such as wind direction. Anthrax has the potential to cause a large number of deaths in the event of a covert, open air release. If such a release were to occur, it is critical for public health decision makers to evaluate its extent and the potential impact on the population and then to identify the people most at risk of infection as soon as possible. It is critical to treat people as soon as possible after exposure to anthrax. While forecasts based on small numbers of early cases are less reliable than those obtained later in an outbreak, treating individuals based on early estimates is still likely to save lives overall.

Estimating the Location and Spatial Extent of a Covert Anthrax Release. 2009 PLoS Comput Biol 5(1): e1000356
Rapidly identifying the features of a covert release of an agent such as anthrax could help to inform the planning of public health mitigation strategies. Previous studies have sought to estimate the time and size of a bioterror attack based on the symptomatic onset dates of early cases. We extend the scope of these methods by proposing a method for characterizing the time, strength, and also the location of an aerosolized pathogen release. A back-calculation method is developed allowing the characterization of the release based on the data on the first few observed cases of the subsequent outbreak, meteorological data, population densities, and data on population travel patterns. We evaluate this method on small simulated anthrax outbreaks (about 25–35 cases) and show that it could date and localize a release after a few cases have been observed, although misspecifications of the spore dispersion model, or the within-host dynamics model, on which the method relies can bias the estimates. Our method could also provide an estimate of the outbreak’s geographical extent and, as a consequence, could help to identify populations at risk and, therefore, requiring prophylactic treatment. Our analysis demonstrates that while estimates based on the first ten or 15 observed cases were more accurate and less sensitive to model misspecifications than those based on five cases, overall mortality is minimized by targeting prophylactic treatment early on the basis of estimates made using data on the first five cases. The method we propose could provide early estimates of the time, strength, and location of an aerosolized anthrax release and the geographical extent of the subsequent outbreak. In addition, estimates of release features could be used to parameterize more detailed models allowing the simulation of control strategies and intervention logistics.

A new DNA test for Human Papillomavirus (HPV) which causes cervical cancer is so much better than current methods that some gynecologists hope it will eventually replace the Pap smear in wealthy countries and other tests in poor ones. Not only could the new test for HPV save lives, scientists say that women over 30 could drop annual Pap smears and instead have the DNA test just once every 3, 5 or even 10 years, depending on which expert is asked. This optimism is based on an eight-year study of 130,000 women in India financed by the Bill and Melinda Gates Foundation and published recently in the New England Journal of Medicine:

HPV Screening for Cervical Cancer in Rural India. 2009 New Engl J. Med. 360: 1385-1394
In October 1999, we began to measure the effect of a single round of screening by testing for human papillomavirus (HPV), cytologic testing, or visual inspection of the cervix with acetic acid (VIA) on the incidence of cervical cancer and the associated rates of death in the Osmanabad district in India. In this cluster-randomized trial, 52 clusters of villages, with a total of 131,746 healthy women between the ages of 30 and 59 years, were randomly assigned to four groups of 13 clusters each. The groups were randomly assigned to undergo screening by HPV testing (34,126 women), cytologic testing (32,058), or VIA (34,074) or to receive standard care (31,488, control group). Women who had positive results on screening underwent colposcopy and directed biopsies, and those with cervical precancerous lesions or cancer received appropriate treatment. In the HPV-testing group, cervical cancer was diagnosed in 127 subjects (of whom 39 had stage II or higher), as compared with 118 subjects (of whom 82 had advanced disease) in the control group (hazard ratio for the detection of advanced cancer in the HPV-testing group, 0.47; 95% confidence interval [CI], 0.32 to 0.69). There were 34 deaths from cancer in the HPV-testing group, as compared with 64 in the control group (hazard ratio, 0.52; 95% CI, 0.33 to 0.83). No significant reductions in the numbers of advanced cancers or deaths were observed in the cytologic-testing group or in the VIA group, as compared with the control group. Mild adverse events were reported in 0.1% of screened women. In a low-resource setting, a single round of HPV testing was associated with a significant reduction in the numbers of advanced cervical cancers and deaths from cervical cancer.

Related:

• From India to the World – A Better Way to Prevent Cervical Cancer. 2009 New Engl J. Med. 360: 1453-1455
• Should we cure cancer?
• HPV vaccines: prospects for eliminating ano-genital cancer
• Dumb and Dumber

In eukaryotes, cell walls are found in many plant, fungal and algal species. The fungal cell wall is located outside the cell membrane and serves a protective function by providing rigidity, mechanical strength and shielding of the protoplast from the environment. Fungal cell walls are composed of a tight, semipermeable fibrous network of polymers such as chitin, glucan polysaccharides and mannoproteins. Most human pathogenic fungi have cell walls, which are important targets for antifungal drug discovery. The importance of the cell wall for fungal cell survival is evident by the fact that antifungal drugs that disrupt cell wall biosynthesis are fungicidal. The ability of the cell wall to protect the cell by limiting access to outside molecules also provides a potential barrier to diffusion of fungal products. Fungi in the environment obtain their food by digesting organic matter in their environment with enzymatic cocktails that produce small molecules that are then absorbed. Consequently, fungal cells must have efficient mechanisms for the transport and export of cellular products required for nutrient acquisition.

Vesicular transport across the fungal cell wall. Trends Microbiol. 2009 17(4): 158-162
Recent findings indicate that fungi use vesicular transport to deliver substances across their cell walls. Fungal vesicles are similar to mammalian exosomes and could originate from cytoplasmic multivesicular bodies. Vesicular transport enables the export of large molecules across the cell wall, and vesicles contain lipids, proteins and polysaccharides, many of which are associated with virulence. Concentration of fungal products in vesicles could increase their efficiency in food acquisition and/or delivering potentially noxious substances to other cells, such as amoebae or phagocytes. The discovery of vesicular transport in fungi opens many new avenues for investigation in basic cell biology and pathogenesis.

Related:

• Ancient fungus farmers
• Drug-resistant fungi
• Metal-fungus hybrids make powerful catalysts

Persistent viruses such as hepatitis C virus (HCV) or HIV can defeat the body’s defense system and cause devastating epidemics worldwide. Recent attempts at vaccinating against HIV have relied on the induction of specific antiviral killer T lymphocytes but have failed to confer protection on the host. Better knowledge about how a successful defense should operate is therefore essential for developing and refining new vaccines. Researchers have recently used a mouse model to investigate basic defense mechanisms required to eliminate persisting viruses. Experiments in several genetically engineered mouse models show that contrary to common belief, not only antiviral killer T cells, but also antibodies (produced by B cells), are needed to prevent a virus from persisting in its host. These findings suggest that induction of antibodies, along with antiviral killer T lymphocytes, should be envisaged when devising new strategies for vaccinating against HIV or HCV.

Impaired antibody response causes persistence of prototypic T cell–contained virus. 2009 PLoS Biol 7(4): e1000080
CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor c chain or Fc c receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell–controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.

Related:

• Natural Regulatory T Cells and Persistent Virus Infection
• Presence of Antibodies Signals Healthier Teeth and Gums
• New Insights Could Lead to a Better Pneumococcal Vaccine

Killing only older mosquitoes could be a more sustainable way of controlling malaria, and has the potential to lead to evolution-proof insecticides that never become obsolete, according to a new article. Each year, malaria – spread through mosquito bites – kills around a million people, and many of the chemicals used to kill the insects become ineffective as the mosquito’s resistance to them evolves. New theoretical work predicts that simple changes to the way insecticides are used could prevent the evolution of resistance and reduce the burden of malaria.

The authors argue that insecticides – chemical or biological – which kill only older mosquitoes are a more sustainable way to fight the deadly disease. The development of biological or chemical insecticides that target older, malaria-infected mosquitoes could save millions dollars that would otherwise be spent endlessly looking for new insecticides to replace ones that have become ineffective. Done right, a one-off investment could create a single insecticide that would solve the problem of mosquito resistance forever. Insecticides sprayed on house walls or bed nets are some of the most successful ways of controlling malaria, but they work by killing the insects or denying them the human blood they use to make eggs. This imposes an enormous selection in favor of insecticide-resistant mosquitoes. However, once malaria parasites infect a mosquito, they need at least 10 to 14 days – or two to six cycles of egg production – to mature and migrate to the insect’s salivary glands. From there they can pass into humans when a mosquito bites. Most mosquitoes do not live long enough to transmit the disease. To stop malaria, we only need to kill the old mosquitoes.

To study the impact of late-acting insecticides on mosquito populations, the researchers constructed a mathematical model of malaria transmission using factors such as the egg laying cycle of the mosquito and the development of parasites within the insect. Analyses of the model using data on mosquito lifespan and malaria development from hotspots in Africa and Papua New Guinea reveal that insecticides killing only mosquitoes that have completed at least four cycles of egg production reduce the number of infectious bites by about 95 percent. Critically, the researchers also found that resistance to late-acting insecticides spreads much more slowly among mosquitoes, compared to conventional insecticides, and that in many cases, it never spreads at all. Aging mosquitoes are easier to kill with insecticides like DDT but new generation pesticides could do it too. Since most mosquitoes die before they become dangerous, late-acting insecticides will not have much impact on breeding, so there is much less pressure for the mosquitoes to evolve resistance. This means that late-life insecticides will be useful for much, much longer – maybe forever – than conventional insecticides. Insects usually have to pay a price for resistance, and if only a few older mosquitoes gain the benefits, evolutionary economics can stop resistance from ever spreading.

The researchers are working on fungal pesticide – a form of biological control – that kills mosquitoes late in life. This could be sprayed onto walls or onto treated materials such as bed nets, from where the mosquito would get infected by the fungal spores. The fungi take 10 to 12 days to kill the insects. This achieves the benefit of killing the old, dangerous mosquitoes, while dramatically reducing the selection for the evolution of resistance. The next step is to test the approach in the field. The main challenge to overcome might be human perception. Young mosquitoes aren’t dangerous, though they are a nuisance. Getting rid of all mosquitoes comes at a high price. Insecticides that kill indiscriminately impose maximal selection for mosquitoes that render those insecticides useless. Late-life acting insecticides would avoid that fate.

How to make evolution-proof insecticides for malaria control. 2009 PLoS Biol 7(4): e1000058
Insecticides are one of the cheapest, most effective, and best proven methods of controlling malaria, but mosquitoes can rapidly evolve resistance. Such evolution, first seen in the 1950s in areas of widespread DDT use, is a major challenge because attempts to comprehensively control and even eliminate malaria rely heavily on indoor house spraying and insecticide-treated bed nets. Current strategies for dealing with resistance evolution are expensive and open ended, and their sustainability has yet to be demonstrated. Here we show that if insecticides targeted old mosquitoes, and ideally old malaria-infected mosquitoes, they could provide effective malaria control while only weakly selecting for resistance. This alone would greatly enhance the useful life span of an insecticide. However, such weak selection for resistance can easily be overwhelmed if resistance is associated with fitness costs. In that case, late-life–acting insecticides would never be undermined by mosquito evolution. We discuss a number of practical ways to achieve this, including different use of existing chemical insecticides, biopesticides, and novel chemistry. Done right, a one-off investment in a single insecticide would solve the problem of mosquito resistance forever.

Related:

• Tough Choices – DDT or Malaria?
• Comprehensive map of global malaria
• Malaria: now you see me, now you don’t
• Malaria, mosquitoes and the legacy of Ronald Ross
• Nature Collections – Malaria

Stanley Prusiner was awarded the first ever SGM Prize Medal (to a microbiologist of international standing whose work has had a far-reaching impact beyond microbiology) at the SGM Spring meeting at Harrogate on 1st April 2009. MicrobiologyBytes was there and this is a summary of his Prize lecture.

Prions are infectious proteins which multiply by binding to a host cell protein and converting it into insolubile fibrils (“amyloid“). Prions are associated with infectious, inherited and sporadic diseases – a feature unique to these entities. Tikvah Alper was the first person to identify prions in the 1960s, but when Prusiner started working on them in 1974, at first he didn’t believe the protein-only hypothesis. After eight years of failing to be able to identify any nucleic acid associated with them, in 1982 he changed his mind and invented the name prion (“pree-on”).

In prion diseases, the cellular form of the protein, PrP^c, is converted into a disease-associated form, PrP^Sc. If prions really are infectious proteins, PrP^Sc produced in bacteria should be able to cause disease – and it does. It is also possible to produce synthetic amyloids with different biological properties – essentially strains of the protein.

Subscribe to podcasts (free):
[iTunes] Enhanced podcasts & videos
[RSS] mp3 podcasts (audio only)
Play this episode: Enhanced version | Audio only

Quinacrine cures cultured cells of prions. In mice, the drug increases survival time of infected animals by up to 20%, but recently concluded clinical trials in humans have shown little effect. Prusiner’s group have found that quinacrine does work in stationary phase cells – such as those in the brain. Future trials of anti-prion (or amyloid) drugs need to be carried out in stationary cells. The latest assay uses genetically-modified mice which express luciferase when glial cells are disturbed. The resulting luminescence can be detected in the brains of live mice, and signs of disease can be recorded even before any neurological symptoms appear. This is up to eight times faster than waiting for the mice to die and examining their brains, and only requires one tenth of the animals. Prusiner hopes to use this approach to study Alzheimer’s and Parkinson’s disease, which also involve brain injury and amyloid deposits.

Stanley Prusiner’s take home message to all the students present was: it’s important to be lucky! But as Robin Weiss, SGM President, pointed out, Pasteur said: Fortune favours the prepared mind!

Related:

• Alzheimer’s Disease and Prions
• The Origin of BSE
• What the heck are prions for?
• Drugs for treatment of prion infections
• What drove the cow mad? Lessons from a fish

There’s a great collection of freely available resources On the Nature website under Nature Collections – Malaria (maybe NPG is finally getting the message about open access – hey Nature, it’s good to share :-)

The world is on the verge of making major inroads against malaria – a deadly disease that still claims the lives of more than 1 million people annually, mostly children less than 5 years of age. Over the past decade, scientists, large pharmaceutical companies and small biotechnology firms, governments and philanthropic organizations have come together to mount a full frontal attack on malaria, and there is now even talk of the ‘E word’ – that is, eradication. This collection highlights advances in the deployment of existing tools, and in the basic science of malaria – particularly those flowing from sequencing of the malaria parasite genomes – that will underpin the next generation of malaria-control tools, which will be needed if the scourge of malaria is to be eradicated.

Contents:

• Malaria: The end of the beginning – After decades of work, a pioneering malaria vaccine may soon reach the final phase of clinical trials. A vaccine that is far from perfect – but which may provide new direction and save thousands of lives.
• Malaria vaccine gets shot in the arm from tests – Promising results pave the way for a vaccine candidate to undergo full-blown trials across Africa.
• Malaria: The big push – Zambia, with help from partners around the world, is stepping up its battle against malaria.
• The billion-dollar malaria moment – For years the global malaria effort has been asking for more resources. Now the field needs to figure out a systematic strategy for spending the money effectively.
• Review: Malaria research in the post-genomic era

Articles:

• Comparative genomics of the neglected human malaria parasite Plasmodium vivax
• Genome sequence of the human malaria parasite Plasmodium falciparum
• Genome sequence and comparative analysis of the model rodent malaria parasite Plasmodium yoelii yoelii
• The genome of the simian and human malaria parasite Plasmodium knowlesi

Related:

• MicrobiologyBytes: Malaria

Today’s post is from guest blogger Sarah Scrafford, who regularly writes on the topic of ultrasound technician salary. She invites your questions, comments and freelancing job inquiries at her email address: sarah.scrafford25@gmail.com.

MicrobiologyBytes welcomes guest bloggers who would like to contribute occasional posts which conform to the style and content of this site. If you would like to be a guest blogger here, please email your post with a completed copyright release form to me at: alan.cann@gmail.com

It’s a common enough practice to pop antibiotics the moment you’re down with something as harmless as a common cold, but that’s exactly what you’re not supposed to do. An antibiotic will not help relieve your cold or its symptoms because it is caused by a virus, not bacteria. And antibiotics are not effective in fighting viral infections. There’s so much we don’t know about these drugs that can both save lives and threaten them at the same time. So here’s some detailed information about antibiotics, information we must all be familiar with and aware of:

• Antibiotics don’t work against viruses: They are only effective against some bacterial, parasitic and fungal infections. So don’t just pop them in your mouth because they’re lying around the house; instead, consult your medical practitioner and take only the medication that he/she prescribes.
• They contribute to the formation of germs that are resistant to antibiotics: When we misuse antibiotics, especially for illnesses that are not caused by bacteria, it leads to the formation of new strains of bacteria that are immune to the antibiotic you’ve just had. So a new antibiotic has to be invented to take care of this bacteria and the infections it could cause, and when this is misused too, the cycle keeps repeating itself. When more and more antibiotics are used, it leads to the development of many more bacteria that are antibiotic-resistant. As a result, illnesses last longer and you have to spend more on visits to the doctor and newer forms of treatment and medicines.
• You must take them as prescribed: Just because your symptoms have eased or your infection cured, it doesn’t mean you can stop taking your prescribed antibiotics. You need to take them exactly as prescribed, sticking to the times of day and the number of days religiously. A whole course is necessary to kill off all the bacteria that are responsible for causing your illness.
• Don’t use antibiotics without a prescription: First of all, they may not be effective in treating your illness. And worse, when you take antibiotics that are not right for the infection you have, you’re leaving yourself prone to developing new strains of bacteria that become resistant to this particular antibiotic and similar others. So when you become ill with a disease that is curable with the help of this antibiotic, it’s going to be an absolutely useless course of treatment because of the resistance developed by the bacteria. Besides, they also kill the good bacteria that are in your stomach and are needed for a variety of bodily functions like digestion and others.

Antibiotics are not a magical cure to any illness that you have. It’s always good to remember that prevention is better than cure; so maintain good hygiene by keeping yourself and your surroundings clean and by washing your hands regularly to keep illness at bay.

Related:

• MicrobiologyBytes: Antibiotics
• Antibiotics, your gut, and you
• Resistance to Widely-Used Antibiotics among Inhabitants of Remote South American Villages