MicrobiologyBytes

Onchocerciasis (“river blindness”), caused by the filarial roundworm Onchocerca volvulus, is transmitted to humans by the bite of an infected black fly. The parasite causes eye damage that can lead to blindness and skin disease. Brought to the Americas from Africa by the slave trade, onchocerciasis is present in six countries in Latin America. The disease is caused by a round worm and is transmitted to humans by the bite of an infected black fly. Once in a human, the adult worms produce larvae that circulate through the body, causing itching or even blindness. Ivermectin, a drug that kills the larvae, is delivered by public health authorities in countries where the disease is present. If the larvae are killed, then the disease cannot be transmitted to more people. People living in the Escuintla-Guatemala focus, a region in Guatemala where the disease was common, have been taking ivermectin for many years. The Ministry of Health of Guatemala believes that onchocerciasis is no longer being transmitted in the area. To prove that there is no more transmission of the disease, researchers examined the eyes of residents of the area to see if they could find any evidence of the worms. They also conducted analyses of blood in school children to see if they had ever been exposed to the worm, and they caught thousands of black flies and tested them to see if they were infected. These evaluations found no evidence of transmission of the disease in the Escuintla-Guatemala focus. As a result, local public health authorities can stop giving ivermectin and invest their human resources in other important diseases.

Escuintla is now the second of four Guatemalan areas to have stopped the transmission of river blindness. To date eight of 13 endemic study areas in Latin America have ended the transmission of the disease, largely through health education and semiannual mass distribution of ivermectin. Ivermectin had been given to 85 percent of the at-risk Escuintla population of 50,000 since 2001 by the Guatemala Ministry of Public Health and Social Assistance (MPHSA). In 2007, the MPHSA, together with the Onchocerciasis Elimination Program for the Americas (OEPA), The Carter Center, the CDC and the Universidad del Valle de Guatemala launched an evaluation in Escuintla to determine whether transmission had been interrupted and if semiannual treatment could be suspended, following guidelines of the World Health Organization (WHO). This evaluation led to the recommendation that treatment could be halted. Escuintla has now begun a three-year surveillance phase to ensure that infection does not reoccur in the absence of ivermectin distribution.

Successful Interruption of Transmission of Onchocerca volvulus in the Escuintla-Guatemala Focus, Guatemala. 2009 PLoS Negl Trop Dis 3(3): e404
Elimination of onchocerciasis (river blindness) through mass administration of ivermectin in the six countries in Latin America where it is endemic is considered feasible due to the relatively small size and geographic isolation of endemic foci. We evaluated whether transmission of onchocerciasis has been interrupted in the endemic focus of Escuintla-Guatemala in Guatemala, based on World Health Organization criteria for the certification of elimination of onchocerciasis. We conducted evaluations of ocular morbidity and past exposure to Onchocerca volvulus in the human population, while potential vectors (Simulium ochraceum) were captured and tested for O. volvulus DNA; all of the evaluations were carried out in potentially endemic communities (PEC; those with a history of actual or suspected transmission or those currently under semiannual mass treatment with ivermectin) within the focus. The prevalence of microfilariae in the anterior segment of the eye in 329 individuals ($7 years old, resident in the PEC for at least 5 years) was 0% (one-sided 95% confidence interval [CI] 0–0.9%). The prevalence of antibodies to a recombinant O. volvulus antigen (Ov-16) in 6,432 school children (aged 6 to 12 years old) was 0% (one-sided 95% IC 0–0.05%). Out of a total of 14,099 S. ochraceum tested for O. volvulus DNA, none was positive (95% CI 0–0.01%). The seasonal transmission potential was, therefore, 0 infective stage larvae per person per season. Based on these evaluations, transmission of onchocerciasis in the Escuintla-Guatemala focus has been successfully interrupted. Although this is the second onchocerciasis focus in Latin America to have demonstrated interruption of transmission, it is the first focus with a well-documented history of intense transmission to have eliminated O. volvulus.

Related:

• River blindness and drug resistance
• Neglected Tropical Diseases in Latin America and the Caribbean
• Elimination of elephantiasis

Spring 2009 Meeting, Harrogate International Centre, 30 March – 2 April 2009

By inserting green flourescent protein into the gag gene of HIV, researchers have been able to observe the way HIV-infected T-cells interact with uninfected ones. When an infected cell comes into contact with another host cell, a bridge is created between them, called a virological synapse.

Quantitative 3D Video Microscopy of HIV Transfer Across T Cell Virological Synapses. 2009 Science 323: 1743-1747
The spread of HIV between immune cells is greatly enhanced by cell-cell adhesions called virological synapses, although the underlying mechanisms have been unclear. With use of an infectious, fluorescent clone of HIV, we tracked the movement of Gag in live CD4 T cells and captured the direct translocation of HIV across the virological synapse. Quantitative, high-speed three-dimensional (3D) video microscopy revealed the rapid formation of micrometer-sized “buttons” containing oligomerized viral Gag protein. Electron microscopy showed that these buttons were packed with budding viral crescents. Viral transfer events were observed to form virus-laden internal compartments within target cells. Continuous time-lapse monitoring showed preferential infection through synapses. Thus, HIV dissemination may be enhanced by virological synapse-mediated cell adhesion coupled to viral endocytosis.

Related:

• Studying the structure of HIV
• HIV is evolving rapidly to escape the immune system
• HIV gene therapy trial promising
• STDs disrupt genetic bottleneck against HIV
• Virus Entry
• Parasitic worm may increase susceptibility to AIDS virus

Polyomaviruses occupy a replicative niche in animals from birds to humans. Two human polyomaviruses, BKV and JCV, were discovered in 1971 and within the last two years, three new polyomaviruses have been found in humans: KI (KIV), WU (WUV), and Merkel Cell (MCV) polyomavirus. MCV was identified in Merkel Cell carcinomas, a rare skin cancer. KIV and WUV were detected in nasal secretions, and may be respiratory viruses. Previously, it had not been determined what percentage of the human population is exposed to KIV, WUV, and MCV, and when initial exposure to these viruses occurs. A new study now suggests that a majority of the human population has been exposed to newly discovered KIV, WUV, and Merkel cell (MCV) human polyomaviruses. The results, based on antibody measurements in serum samples, also suggest that infection with these viruses occurs early in childhood.

In this study, researchers tested over 2220 anonymous donor blood samples (more than 1500 adults and more than 700 young people). They measured antibodies that reacted with specific viral proteins. In addition to KIV, WUV, MCV, BKV, and JCV, two monkey polyomaviruses, SV40 and lymphotropic polyomavirus (LPV), were also studied. Antibodies to LPV were detected in a fraction of people (15%), confirming previous studies suggesting that a relative of this virus may infect humans. The majority of antibodies against SV40 proteins may be attributed to the immune response to BKV. The diseases caused by these viruses remain to be fully described. Future studies will be important to help determine differences in the prevalence of these infections in other geographic areas.

Seroepidemiology of Human Polyomaviruses. 2009 PLoS Pathog 5(3): e1000363
In addition to the previously characterized viruses BK and JC, three new human polyomaviruses (Pys) have been recently identified: KIV, WUV, and Merkel Cell Py (MCV). Using an ELISA employing recombinant VP1 capsid proteins, we have determined the seroprevalence of KIV, WUV, and MCV, along with BKV and JCV, and the monkey viruses SV40 and LPV. Soluble VP1 proteins were used to assess crossreactivity between viruses. We found the seroprevalence (+/- 1%) in healthy adult blood donors (1501) was SV40 (9%), BKV (82%), JCV (39%), LPV (15%), KIV (55%), WUV (69%), MCV strain 350 (25%), and MCV strain 339 (42%). Competition assays detected no sero-crossreactivity between the VP1 proteins of LPV or MCV or between WUV and KIV. There was considerable sero-crossreactivity between SV40 and BKV, and to a lesser extent, between SV40 and JCV VP1 proteins. After correcting for crossreactivity, the SV40 seroprevalence was ~2%. The seroprevalence in children under 21 years of age (n=721) for all Pys was similar to that of the adult population, suggesting that primary exposure to these viruses likely occurs in childhood.

Related:

• Polyomavirus and Human Merkel Cell Carcinoma
• DNA Viruses

Arsenic is the most common toxic substance in the environment, ranking first on the US Superfund list of hazardous substances. It is introduced to the environment primarily from geologic sources and is acted on biologically, creating an arsenic biogeocycle. Geothermal environments are well known for their elevated arsenic content and thus provide an excellent setting in which to study microbe–arsenic interactions. So far, studies aimed at identifying the organisms participating in these and other arsenic transformations have focused almost entirely on microorganisms belonging to the domains Archaea and Bacteria. In contrast, comparatively little attention has been paid to the Eukarya that inhabit these extreme environments, much less their potential contribution to biogeochemical cycles in these extreme habitats. Now it would appear that algae play a significant role in arsenic cycling in the geothermal environment as also found in a range of marine and freshwater environments. These observations indicate that arsenic methylation forms an important component of the global arsenic biogeocycle.

Biotransformation of arsenic by a Yellowstone thermoacidophilic eukaryotic alga. PNAS USA March 10, 2009
Arsenic is the most common toxic substance in the environment, ranking first on the Superfund list of hazardous substances. It is introduced primarily from geochemical sources and is acted on biologically, creating an arsenic biogeocycle. Geothermal environments are known for their elevated arsenic content and thus provide an excellent setting in which to study microbial redox transformations of arsenic. To date, most studies of microbial communities in geothermal environments have focused on Bacteria and Archaea, with little attention to eukaryotic microorganisms. Here, we show the potential of an extremophilic eukaryotic alga of the order Cyanidiales to influence arsenic cycling at elevated temperatures. Cyanidioschyzon sp. isolate 5508 oxidized arsenite [As(III)] to arsenate [As(V)], reduced As(V) to As(III), and methylated As(III) to form trimethylarsine oxide (TMAO) and dimethylarsenate [DMAs(V)]. Two arsenic methyltransferase genes, CmarsM7 and CmarsM8, were cloned from this organism and demonstrated to confer resistance to As(III) in an arsenite hypersensitive strain of Escherichia coli. The two recombinant CmArsMs were purified and shown to transform As(III) into monomethylarsenite, DMAs(V), TMAO, and trimethylarsine gas, with a Topt of 60–70°C. These studies illustrate the importance of eukaryotic microorganisms to the biogeochemical cycling of arsenic in geothermal systems, offer a molecular explanation for how these algae tolerate arsenic in their environment, and provide the characterization of algal methyltransferases.

Related:

• Molecular approaches to bioremediation
• Saturday Cinema: Bioremediation
• Alpine microorganisms and low-temperature bioremediation
• Microbial survival in challenging environments

One of my sons asked me this question a few days ago. I said no, but I’m happy that others agree with me:

Ten reasons to exclude viruses from the tree of life. 2009 Nature Reviews Microbiology 7: 306-311
When viruses were discovered, they were accepted as missing links between the inert world and living organisms. However, this idea was soon abandoned as information about their molecular parasitic nature accumulated. Recently, the notion that viruses are living organisms that have had a role in the evolution of some essential features of cells has experienced a renaissance owing to the discovery of unusually large and complex viruses that possess typical cellular genes. Here, we contend that there is strong evidence against the notion that viruses are alive and represent ancient lineages of the tree of life.

Viruses do not reproduce by division, but are replicated by the self-assembly of preformed components. This, not size, differentiates them from cellular living organisms such as bacteria. A virus-infected cell is more like a car factory than a womb.
Also, unlike living organisms, no virus has the means of generating its own energy – they are all energy pirates.

Related:

• Virology Blog: Viruses and the tree of life
• The end of the world? Dr Franken-Venter? Nope

Malaria is one of the most common infectious diseases in the world and one of the greatest global public health problems. The Plasmodium falciparum parasite causes approximately 500 million cases each year and over one million deaths in sub-Saharan Africa. More than 40% of the world’s population is at risk of malaria. The parasite is transmitted to people through the bites of infected mosquitoes. These insects inject a life stage of the parasite called sporozoites, which invade human liver cells where they reproduce briefly. The liver cells then release merozoites (another life stage of the parasite), which invade red blood cells. Here, they multiply again before bursting out and infecting more red blood cells, causing fever and damaging vital organs. The infected red blood cells also release gametocytes, which infect mosquitoes when they take a blood meal. In the mosquito, the gametocytes multiply and develop into sporozoites, thus completing the parasite’s life cycle. Malaria can be prevented by controlling the mosquitoes that spread the parasite and by avoiding mosquito bites by sleeping under insecticide-treated bed nets. Effective treatment with antimalarial drugs also helps to decrease malaria transmission.

In 1998, the World Health Organization and several other international agencies launched Roll Back Malaria, a global partnership that aims to reduce the human and socioeconomic costs of malaria. Targets have been continually raised since this time and have culminated in the Roll Back Malaria Global Malaria Action Plan of 2008, where universal coverage of locally appropriate interventions is called for by 2010 and the long-term goal of malaria eradication again tabled for the international community. For malaria control and elimination initiatives to be effective, financial resources must be concentrated in regions where they will have the most impact, so it is essential to have up-to-date and accurate maps to guide effort and expenditure. In 2008, researchers of the Malaria Atlas Project constructed a map that stratified the world into three levels of malaria risk: no risk, unstable transmission risk (occasional focal outbreaks), and stable transmission risk (endemic areas where the disease is always present). Now, researchers extend this work by describing a new evidence-based method for generating continuous maps of P. falciparum endemicity within the area of stable malaria risk over the entire world’s surface. They then use this method to produce a P. falciparum endemicity map for 2007. Endemicity is important as it is a guide to the level of morbidity and mortality a population will suffer, as well as the intensity of the interventions that that will be required to bring the disease under control or additionally to interrupt transmission.

The researchers identified nearly 8,000 surveys of P. falciparum parasite rates (Pf PR; the percentage of a population with parasites detectable in their blood) completed since 1985 that met predefined criteria for inclusion into a global database of PfPR data. They then used ‘‘model-based geostatistics’’ to build a world map of P. falciparum endemicity for 2007 that took into account where and, importantly, when and all these surveys were done. Predictions were comprehensive (for every area of stable transmission globally) and continuous (predicted as a endemicity value between 0% and 100%). The population at risk of three levels of malaria endemicity were identified to help summarize these findings: low endemicity, where PfPR is below 5% and where it should be technically feasible to eliminate malaria; intermediate endemicity where PfPR is between 5% and 40% and it should be theoretically possible to interrupt transmission with the universal coverage of bed nets; high endemicity is where PfPR is above 40% and suites of locally appropriate intervention will be needed to bring malaria under control. The global level of malaria endemicity is much reduced when compared with historical maps. Nevertheless, the resulting map indicates that in 2007 almost 60% of the 2.4 billion people at malaria risk were living in areas with a stable risk of P. falciparum transmission – 0.69 billion people in Central and South East Asia (CSE Asia), 0.66 billion in Africa, Yemen, and Saudi Arabia (Africaþ), and 0.04 billion in the Americas. The people of the Americas were all in the low endemicity class. Although most people exposed to stable risk in CSE Asia were also in the low endemicity class (88%), 11% were in the intermediate class, and 1% were in the high endemicity class. By contrast, high endemicity was most common and widespread in the Africaþ region (53%), but with significant numbers in the intermediate (30%), and low (17%) endemicity classes.

The accuracy of this new world map of P. falciparum endemicity depends on the assumptions made in its construction and critically on the accuracy of the data fed into it, but because of the statistical methods used to construct this map, it is possible to quantify the uncertainty in the results for all users. Thus, this map (which, together with the data used in its construction, will be freely available) represents an important new resource that clearly indicates areas where malaria control can be improved (for example, Africa) and other areas where malaria elimination may be technically possible. In addition, planned annual updates of the global P. falciparum endemicity map and the PfPR database by the Malaria Atlas Project will help public health experts to monitor the progress of the malaria control community towards international control and elimination targets.

A world malaria map: Plasmodium falciparum endemicity in 2007. 2009 PLoS Med 6(3): e1000048
Efficient allocation of resources to intervene against malaria requires a detailed understanding of the contemporary spatial distribution of malaria risk. It is exactly 40 y since the last global map of malaria endemicity was published. This paper describes the generation of a new world map of Plasmodium falciparum malaria endemicity for the year 2007. A total of 8,938 P. falciparum parasite rate (PfPR) surveys were identified using a variety of exhaustive search strategies. Of these, 7,953 passed strict data fidelity tests for inclusion into a global database of PfPR data, age-standardized to 2–10 y for endemicity mapping. A model based geostatistical procedure was used to create a continuous surface of malaria endemicity within previously defined stable spatial limits of P. falciparum transmission. These procedures were implemented within a Bayesian statistical framework so that the uncertainty of these predictions could be evaluated robustly. The uncertainty was expressed as the probability of predicting correctly one of three endemicity classes; previously stratified to be an informative guide for malaria control. Population at risk estimates, adjusted for the transmission modifying effects of urbanization in Africa, were then derived with reference to human population surfaces in 2007. Of the 1.38 billion people at risk of stable P. falciparum malaria, 0.69 billion were found in Central and South East Asia (CSE Asia), 0.66 billion in Africa, Yemen, and Saudi Arabia (Africaþ), and 0.04 billion in the Americas. All those exposed to stable risk in the Americas were in the lowest endemicity class. The vast majority (88%) of those living under stable risk in CSE Asia were also in this low endemicity class; a small remainder(11%) were in the intermediate endemicity class; and the remaining fraction (1%) in high endemicity areas. High endemicity was widespread in the Africaþ region, where 0.35 billion people are at this level of risk. Most of the rest live at intermediate risk (0.20 billion), with a smaller number (0.11 billion) at low stable risk. High levels of P. falciparum malaria endemicity are common in Africa. Uniformly low endemic levels are found in the Americas. Low endemicity is also widespread in CSE Asia, but pockets of intermediate and very rarely high transmission remain. There are therefore significant opportunities for malaria control in Africa and for malaria elimination elsewhere. This 2007 global P. falciparum malaria endemicity map is the first of a series with which it will be possible to monitor and evaluate the progress of this intervention process.

Related:

• MicrobiologyBytes: Malaria
• Malaria, mosquitoes and the legacy of Ronald Ross
• New ways to beat malaria
• Researchers characterize potential protein targets for malaria vaccine
• Researchers Find Essential Proteins for Critical Stage of Malaria Transmission

We all live in an information rich, highly interconnected world, and the success of evolution can be measured in terms of how living organisms make sense of and respond to information. Past posts on quorum sensing are some of the most popular out of all the subjects I have covered on MicrobiologyBytes. Quorum sensing is the use of small molecules by bacteria to coordinate behavior by groups of individual cells and carry out decision-making processes.

Bacteria have evolved a number of communication systems which can be broadly described as contact-independent and contact-dependent signaling mechanisms. Quorum sensing is a contact-independent process since it involves transfer of secreted molecules called autoinducers. As autoinducer levels increase throughout a growing bacterial population, changes in gene transcription are triggered resulting in altered growth rates and group dynamics. There is an energy cost in producing these compounds and throwing them out of the cell, and in some conditions, the secretion of autoinducers may attract unwanted attention from competitors (Bacterial landlines: contact-dependent signaling in bacterial populations. Curr Opin Microbiol. Feb 24 2009). Contact-dependent signaling methods allow bacteria to carry out more direct, and possibly less costly, communication between cells – it’s the landline alternative to expensive cellphone bills.

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Known methods of contact-dependent signaling include C-signaling in Myxococcus xanthus which allows groups of cells to coordinate motile behavior. The process is mediated by a non-diffusable 17 kDa surface protein encoded by the csgA gene. Contact between neighboring myxobacteria initiates a p17-dependent signaling cascade resulting in expression of genes required for control of motility or for sporulation.

The Gram-positive soil bacterium Bacillus subtilis also undergoes a contact-dependent differentiation process as a means to produce dormant spores when faced with starvation. Under nutrient poor conditions, vegetative B. subtilis cells divide asymmetrically, forming a large mother cell and a smaller daughter cell called a forespore. Despite their intimate association, the mother cell and the forespore remain separated by two membranes and maintain distinct gene expression profiles. Endospore formation is an energy intensive process that is coordinated by multiple signaling pathways. Contact-dependent signaling plays an important role in allowing the cells to coordinate this process.

Contact-dependent inhibition also occurs in E. coli, where a single E. coli cell in the logarithmic phase of growth can use a CDI system to inhibit the growth of hundreds of susceptible target cells in mixed cultures, forcing them to enter a viable but non-replicating state. However, one of the first recognized instances of contact-dependent communication between bacteria was, arguably, conjugation mediated by sex (F) pili. Bacteria encode a large variety of other pilus types and adhesive molecules, many of which have been studied primarily with respect to their abilities to modulate bacteria–host cell interactions. However, it is feasible that some of these organelles also function in inter-bacterial communication. For example, recent studies indicate that several types of soil bacteria can express complex networks of electrically conductive pili known as nanowires.

Although quorum sensing has been getting all the attention recently, we have known about contact-dependent communication mechanisms in bacteria for far longer. Perhaps only now are we realizing how these complimentary systems might fit together and how they could shed light on the development of true multicellularity during evolution.

Related:

• Quorum Sensing in Bacteria: We Two Are One
• Pneumococcus: the quorum-sensing killer
• Quorum sensing in Serratia
• Cracking the cholera code

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