Bacterial swarms recruit cargo bacteria in toxic environments

Paenibacillus vortex Antibiotic resistance is a major health threat. A new paper in mBio shows a novel mechanism for the spread of antibiotic resistance. This involves interactions between different bacteria: one species provides an enzyme that detoxifies the antibiotic (a cargo bacterium carrying a resistance gene), while the other (Paenibacillus vortex) moves itself and transports the cargo. P. vortex used a bet-hedging strategy, colonizing new environments alone when the cargo added no benefit, but cooperating when the cargo was needed. This work sheds light on fundamental questions such as how environmental antibiotic resistance may lead to clinical resistance and also microbial social organization, as well as the costs, benefits, and risks of dispersal in the environment.


Bacterial swarms recruit cargo bacteria to pave the way in toxic environments. (2015) MBio 12;6(3). pii: e00074-15. doi: 10.1128/mBio.00074-15
Swarming bacteria are challenged by the need to invade hostile environments. Swarms of the flagellated bacterium Paenibacillus vortex can collectively transport other microorganisms. Here we show that P. vortex can invade toxic environments by carrying antibiotic-degrading bacteria; this transport is mediated by a specialized, phenotypic subpopulation utilizing a process not dependent on cargo motility. Swarms of beta-lactam antibiotic (BLA)-sensitive P. vortex used beta-lactamase-producing, resistant, cargo bacteria to detoxify BLAs in their path. In the presence of BLAs, both transporter and cargo bacteria gained from this temporary cooperation; there was a positive correlation between BLA resistance and dispersal. P. vortex transported only the most beneficial antibiotic-resistant cargo (including environmental and clinical isolates) in a sustained way. P. vortex displayed a bet-hedging strategy that promoted the colonization of nontoxic niches by P. vortex alone; when detoxifying cargo bacteria were not needed, they were lost. This work has relevance for the dispersal of antibiotic-resistant microorganisms and for strategies for asymmetric cooperation with agricultural and medical implications.

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Viral membrane fusion

Influenza HA fusion protein A useful short review from Stephen Harrison which approaches viral memberane fusion from a structural biology standpoint.


Viral membrane fusion. Virology. 10 Apr 2015 doi: 10.1016/j.virol.2015.03.043
Membrane fusion is an essential step when enveloped viruses enter cells. Lipid bilayer fusion requires catalysis to overcome a high kinetic barrier; viral fusion proteins are the agents that fulfill this catalytic function. Despite a variety of molecular architectures, these proteins facilitate fusion by essentially the same generic mechanism. Stimulated by a signal associated with arrival at the cell to be infected (e.g., receptor or co-receptor binding, proton binding in an endosome), they undergo a series of conformational changes. A hydrophobic segment (a “fusion loop” or “fusion peptide”) engages the target-cell membrane and collapse of the bridging intermediate thus formed draws the two membranes (virus and cell) together. We know of three structural classes for viral fusion proteins. Structures for both pre- and postfusion conformations of illustrate the beginning and end points of a process that can be probed by single-virion measurements of fusion kinetics.

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Chagas Disease vaccine works in mice

Trypanosoma cruzi Chagas disease, caused by Trypanosoma cruzi and transmitted by insects in Latin America is among the most common tropical diseases, and so far without effective vaccine. A new study published in PLOS Pathogens shows that a candidate vaccine can induce long-lasting immunity against the parasite in mice.

The relatively mild acute phase of Chagas disease is followed by the more dangerous chronic phase, during which parasites take up residence in the host, mostly in the heart and stomach muscles. About a third of infected people (presumably those with higher chronic parasite numbers) develop serious heart disease or digestive tract complications many years after the initial infection. An ideal Chagas vaccine would prevent infection altogether, but one that prevents the complications during the chronic phase by keeping parasite numbers low would eliminate most of the disease burden.

Researchers had previously shown that a vaccine that contains three particular parasite proteins is a good candidate: Mice that were infected with T. cruzi immediately after vaccination were able to keep parasite numbers down during the acute infection and showed none of the inflammation in muscle tissue seen after infection of un-vaccinated mice. In this study, the researchers tested whether the vaccinated mice would be protected in the long run. To do this, they vaccinated mice with a combination of two of the T. cruzi proteins (TcG2 and TcG4), which they had shown to be the most potent in provoking both an antibody and a T-cell immune response.

The vaccination was done in two steps: the first injection contained DNA coding for the TcG2 and TcG4 proteins, and the second, three weeks later, a mix of the two proteins themselves (D/P regimen). Some mice were also given a booster immunization three months later, which consisted again of a mix of the two proteins (D/P/P regimen). Even without the booster shot, the D/P regimen caused long-lasting, T. cruzi-specific, changes in the immune system. The vaccine generated a pool of TH1 CD4+T cells (also called helper T cells) that are necessary for an effective antibody response as well as a stable pool of CD8+T memory cells. Both pools rapidly expanded when mice were infected with T. cruzi four months after vaccination, and the vaccinated mice were able to keep parasite numbers 2-3 fold lower than unvaccinated infected mice.

Mice that had received the booster shot at three months (D/P/P regimen) and were infected four months later had an even more potent immune response: their parasite numbers were about 5-fold lower than those of unvaccinated controls. The vaccine-induced immunity waned slightly six months after the booster immunization, but was still sufficient to provide 2-fold control of invading pathogens. This should be sufficient to break the parasite transmission cycle (i.e. biting insects don’t pick up enough parasites to infect the next host) and prevent chronic disease symptoms in the vaccinated host.

The TcG2/TcG4 D/P vaccine provided long-term anti-T. cruzi T cell immunity, and booster immunization would be an effective strategy to maintain or enhance the vaccine-induced protective immunity against T. cruzi infection and Chagas disease. The next steps toward clinical studies in humans include characterizing the quality and quantity of immunity to the vaccine candidates in naïve individuals.

A Two-Component DNA-Prime/Protein-Boost Vaccination Strategy for Eliciting Long-Term, Protective T Cell Immunity against Trypanosoma cruzi. (2015) PLoS Pathog 11(5): e1004828. doi:10.1371/journal.ppat.1004828

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This is not the cure for HIV

CCR5 CCR5 regulates various aspects of the adaptive immune response. A nonfunctional allele resulting from a 32-bp deletion (CCR5-Δ32) causes loss of expression of the functional CCR5 receptor. CCR5 is a co-receptor for HIV and the CCR5-Δ32 allele reduces susceptibility to HIV infection. But there’s a problem. Homozygosity for the CCR5-Δ32 allele is a strong risk factor for symptomatic West Nile virus infection and also correlates with disease severity after tick-borne encephalitis virus infection. And now we know that CCR5 deficiency predisposes to fatal outcomes in influenza virus infection:

CCR5 deficiency predisposes to fatal outcome in influenza virus infection. J Gen Virol. 27 April 2015 doi: 10.1099/vir.0.000165
Influenza epidemics affect all age groups, although children, the elderly, and those with underlying medical conditions are the most severely affected. Whereas co-morbidities are present in 50% of fatal cases, 25-50% of deaths are of apparently healthy individuals. This suggests underlying genetic determinants that govern infection severity. Although some viral factors that contribute to influenza disease are known, the role of host genetic factors remains undetermined. Data for small cohorts of influenza-infected patients are contradictory regarding the potential role of chemokine receptor 5 deficiency (CCR5-Δ32 mutation, a 32-base pair deletion in CCR5) in the outcome of influenza virus infection. We tested 171 respiratory samples from influenza patients (2009 pandemic) for CCR5-Δ32 and evaluated its correlation with patient mortality. CCR5-Δ32 patients (17.4%) showed a higher mortality rate than wild-type individuals (4.7%; p = 0.021), which indicates that CCR5-Δ32 patients are at higher risk than the normal population of fatal outcome in influenza infection.

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Escape of non-enveloped virus from intact cells

Poliovirus In the past it was thought that there was a sharp distinction between lytic and nonlytic viruses. However, some nonenveloped viruses such as picornaviruses seem to be able to spread without lysis. Measurements of cell populations cannot exclude lysis of a few cells but single-cell microscopy has conclusively shown nonlytic cell-to-cell poliovirus spread. Poliovirus, hepatitis A and coxsackievirus B3 can be found in extracellular vesicles. This short review discusses complex exit strategies such as the creation of new compartments whose complex topologies allow the exit of cytoplasm and its contents without violating the integrity of the cell.

Escape of non-enveloped virus from intact cells. Virology. 15 Apr 2015. doi: 10.1016/j.virol.2015.03.044
How do viruses spread from cell to cell? Enveloped viruses acquire their surrounding membranes by budding. If a newly enveloped virus has budded through the plasma membrane, it finds itself outside the cell immediately. If it has budded through the bounding membrane of an internal compartment such as the ER, the virus finds itself in the lumen, from which it can exit the cell via the conventional secretion pathway. Thus, although some enveloped viruses destroy the cells they infect, there is no topological need to do so. On the other hand, naked viruses such as poliovirus lack an external membrane. They are protein-nucleic acid complexes within the cytoplasm or nucleus of the infected cell, like a ribosome, a spliceosome or an aggregate of Huntingtin protein. The simplest way for such a particle to pass through the single lipid bilayer that separates it from the outside of the cell would be to violate the integrity of that bilayer. Thus, it is not surprising that the primary mode of exit for non-enveloped viruses is cell lysis. However, more complex exit strategies are possible, such as the creation of new compartments whose complex topologies allow the exit of cytoplasm and its contents without violating the integrity of the cell. Here we will discuss the non-lytic spread of poliovirus and recent observations of such compartments during viral infection with several different picornaviruses.

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Is a small artificial virus fragment the key to a Chikungunya vaccine?

Chikungunya virus Chikungunya virus (CHIKV) is transmitted by Aedes mosquitoes and causes Chikungunya fever. CHIKV occurs in the tropical and subtropical parts of the world. Regions where it has already caused epidemics include Africa, territories around the Indian Ocean, Southeast Asia, and meanwhile also the Caribbean, Central America, and South America. Around 1.2 million people are estimated to be infected so far during an epidemic in America. Since the Aedes albopictus mosquito, also known as Asian tiger mosquito, has now reached southern Europe and the USA, we are faced with further spreading of the virus.

Chikungunya fever is characterized by fever and severe joint pain, hence its name, which means “that which bends up”. In 30 to 40 percent of the cases, these joint pains can last several months or even up to several years. Attempts at developing suitable vaccines have up to now been unsuccessful. To develop an effective vaccine, it is nessecary to identify a suitable antigen structure of the virus which will create an effective immune response in humans. Previous approaches have used the entire E2 surface protein as a basis for the vaccine, partly in combination with other virus proteins. These proteins, however, have a relatively large structure, which would make commercial vaccine production difficult.

A new paper investigates whether smaller more specific and less complex parts of E2 would suffice for generating a protective immune response. Based on the three-dimensional structure of the protein, researchers selected different areas exposed on the surface to joined them together, creating several artificial protein fragments. After production in E. coli and purification, mice were immunized with these protein fragments, and their blood was examined for neutralizing antibodies later on. In this experiment, one fragment, described as sAB+, proved to be the most effective one to induce neutralizing antibodies. It was used to immunize mice which were then infected by the wild-type Chikungunya virus. Compared with non-vaccinated animals, the mice treated showed significantly less virus RNA in the blood – a sign of partial immune protection.

A Small Antigenic Determinant of the Chikungunya Virus E2 Protein Is Sufficient to Induce Neutralizing Antibodies which Are Partially Protective in Mice. (2015) PLoS Negl Trop Dis 9(4): e0003684. doi:10.1371/journal.pntd.0003684

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How Salmonella Survives the Macrophage’s Acid Attack

Intracellular Salmonella Macrophages destroy bacteria by engulfing them in intracellular compartments, which they then acidify to kill or neutralize the bacteria. However, some pathogenic bacteria, such as Salmonella enterica, have evolved to exist and even grow while within these acidified compartments. How Salmonella responds to the acidic environment and how that environment affects the virulence of this pathogen are unclear.

A new paper in in PLOS Biology demonstrate thats, instead of combating the acidification of the Salmonella-containing vacuole, Salmonella acidifies its own cytoplasm in response to the extracellular low pH (A FRET-Based DNA Biosensor Tracks OmpR-Dependent Acidification of Salmonella during Macrophage Infection. doi: 10.1371/journal.pbio.1002116). The acidic cytoplasm then acts as a signal to stimulate the secretion of a particular class of Salmonella virulence proteins. These virulence proteins, or effectors, are released into the host cell, where they are able to perturb the immune response.

The findings of this paper contradict other previous reports that suggest that a neutralization step is required for secretion of the virulence proteins. The authors show that Salmonella has adapted what was once an antibacterial response by the macrophage into a signal for when it is in the correct time and place to secret its virulence proteins and establish an infection.

How Salmonella Survives the Macrophage’s Acid Attack. (2015) PLoS Biol 13(4): e1002117. doi:10.1371/journal.pbio.1002117

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Molecular biology of hepatitis B virus infection

HBV replication cycle

Human hepatitis B virus (HBV) is the prototype of a family of small DNA viruses that productively infect hepatocytes, the major cell of the liver, and replicate by reverse transcription of a terminally redundant viral RNA, the pregenome. Upon infection, the circular, partially double-stranded virion DNA is converted in the nucleus to a covalently closed circular DNA (cccDNA) that assembles into a minichromosome, the template for viral mRNA synthesis. Infection of hepatocytes is non-cytopathic. Infection of the liver may be either transient (<6 months) or chronic and lifelong, depending on the ability of the host immune response to clear the infection. Chronic infections can cause immune-mediated liver damage progressing to cirrhosis and hepatocellular carcinoma (HCC). The mechanisms of carcinogenesis are unclear. Antiviral therapies with nucleoside analog inhibitors of viral DNA synthesis delay sequelae, but cannot cure HBV infections due to the persistence of cccDNA in hepatocytes.

Molecular biology of hepatitis B virus infection. Virology. 07 March 2015. doi: 10.1016/j.virol.2015.02.031

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