mBiosphere

The Black Queen Hypothesis: how microbes lose a necessary function and survive to tell the tale

Pared down genomes are the norm in symbiotic microbes, but how do non-symbionts get away with cutting out functions it would appear that they need? The authors of an Opinion piece this week explain their ideas about the matter. They say microbes that shed necessary functions may well be getting others to do the hard work for them, an adaptation that can encourage microorganisms to live in cooperative communities.

The Black Queen Hypothesis, as they call it, puts forth the idea that eliminating one function and getting another organism to carry out that process instead confers a selective advantage. In these cases, it would make evolutionary sense for a microbe to lose a burdensome gene for a function it doesn't have to perform for itself. The authors, Richard Lenski and J. Jeffrey Morris of Michigan State University, and Erik Zinser of the University of Tennessee, named the hypothesis for the queen of spades in the game Hearts, in which the usual strategy is to avoid taking this card. The organism stuck with the job of carrying out the process for another organism is, according to their theory, holding the queen of spades, a card that is a burden for the holder, but necessary for the cardgame.

"It's a sweeping hypothesis for how free-living microorganisms evolve to become dependent on each other," says Richard Losick of Harvard University, who edited the paper. "The heart of the hypothesis is that many genetic functions provide products that leak in and out of cells and hence become public goods," he says.

To illustrate their hypothesis, the authors apply it to one particular microbial system that has been a source of some confusion: one of the most common plankton species in the open ocean, Prochlorococcus, which has a much smaller genome than you might expect from a free-living bacterium. Scientists have wondered how Prochlorococcus has managed to be extremely successful while shedding important genes, including the gene for catalase-peroxidase, which allows it to neutralize hydrogen peroxide, a compound that can damage or even kill cells. Prochlorococcus relies on the other microorganisms around it to remove hydrogen peroxide from the environment, say the authors, allowing it to fob off its responsibilities on the unlucky card holders around it. This is an instance of how one species can profit from paring down while relying on other members of the community to help out.

Losick says the Black Queen Hypothesis offers a new way of looking at complicated, inter-dependent communities of microorganisms. "I have a special interest in how bacteria form biofilms, complex natural communities that often consist of many different kinds of bacteria. The Black Queen Hypothesis provides a valuable new way to think about how the members of these biofilm communities coevolved.”

Meningitis: A protein in spinal fluid is associated with survival

Bacterial meningitis strikes around 1,000 people in the U.S. every year, according to the CDC, and 10-14% of those cases are fatal. Researchers are drilling down into what happens in the spinal fluid to make a bad case of meningitis and deadly one, but they still know little about the role of one protein involved in the process: a component of the immune system called complement C3. This week, mBio features a study that shows the presence of complement C3 in spinal fluid correlates with survival, a fact that indicates it plays a key role in the pathogenesis of bacterial meningitis.

Looking for proteins that indicate survival, the team used samples of cerebrospinal fluid (CSF) from 80 patients with bacterial meningitis and 10 non-infected controls. The fluid samples were a useful resource for the team: they represented 40 meningitis patients who survived and 40 who did not. They measured the levels of several central nervous system and serum proteins that had been identified as possible indicators in earlier proteomics work. Goonetilleke et al. found that complement C3 was present at significantly higher concentration in the spinal fluid of patients who later pulled through than in patients who later died.

What does this mean? Complement C3 aids innate immunity by coating pathogens with fragments of itself, which stimulates destruction of the pathogen by other components of the immune system or the release of proinflammatory mediators. It's easy to see that if a person's spinal fluid is depleted in C3, this cascade of immune reactions wouldn't be effective and the pathogen might run rampant, endangering the patient's life. Knowing that C3 is present at greater concentrations in patients who survive, it may well serve as a prognostic marker - a sort of oracle that can indicate just how severe a case of bacterial meningitis is bound to become.

Another interesting aspect of the study is what it says about apoptosis, a process that has been the target of experimental therapies for bacterial meningitis. In mice, apoptosis (programmed cell death) is an important mechanism for neuronal cell death, which can cause neurological problems in survivors. If apoptosis were also a factor in human neuronal damage from bacterial meningitis, then treatments that inhibit apoptosis could prevent some of the neurological damage wrought by the pathogen. But in their study, Goonetilleke et al. found that standard apoptosis markers were absent in patients' spinal fluid, suggesting that apoptosis is not a factor in human cases of meningitis and that treatment with citicoline and caspase inhibitors to prevent apoptosis is unlikely to have an effect.

Microneedles could be a shot in the arm for public health

Seasonal flu is serious. Every year, flu kills around 40,000 Americans, but only 30 to 40% of the population gets a flu shot. But what if the annual flu shot wasn't a shot at all? If you could be immunized with a scratchy little pad pressed to your skin, would you be more likely to get that vaccine? We are one step closer to that option thanks to the results of a study in mBio this week, which looked at the immunological reaction to immunization with metal microneedle patches coated with influenza antigens.

Metal microneedle patches look kind of like little squares of rough velcro. They're designed to deliver antigen to the surface of the skin and the cells just below the surface to the epidermis. The skin is an important actor in the immune system, so delivering a vaccine to skin cells provides access to macrophages, Langerhans cells, and dermal dendritic cells, all of which are crucial for initiating adaptive immune responses. The conventional mode of vaccination - intramuscular - is (need I remind you?) painful, but it has another drawback as well: when using intramuscular injections, full immunization for influenza is often only achieved with high doses or multiple doses (ouch). The other option, the intranasal flu vaccine, is only approved for people between the ages of 2 and 49, a range that excludes those most vulnerable to flu-related complications: the elderly.

Using mice, Martin et al. looked at the immune response to influenza antigens administered with metal microneedle patches. They showed that immunization elicited an increase in cytokines at the immunization site that are important for recruitment of neutrophils, monocytes and dendritic cells. They also observed dendritic cells loaded with influenza viruses migrating from the place where they were deposited in the skin, which suggests they can travel to the lymph nodes - something desirable in the case of a vaccine.

These results, combined with the group's earlier work, which showed flu vaccine delivered via microneedle patches offers the same or better protection as injections, paints a rosy picture for the future of these little devices. Maybe microneedles can eventually make a dent in the annual number of flu deaths?

Commentaries: how to balance risk of H5N1 escape with benefits of research?

Where can you safely work with avian H5N1 influenza? The debate over H5N1 continues*, and this  week we present two Commentaries (by Imperiale and Hanna and García-Sastre) that can help stimulate an honest discussion about the most appropriate place to carry out research on newly developed strains. As Arturo Casadevall and Tom Shenk point out in an accompanying editorial, the decision about where to house H5N1 research has profound implications for society and poses an explicit trade-off between safety and preparedness for future outbreaks.

The Commentaries offer opposing views of the appropriate level of security for dealing with H5N1 viruses. The authors agree that, with a case fatality rate as high as 50% or more, H5N1 could create a pandemic of disastrous proportions, but they differ in their opinions of how to strike a balance between biosecurity and effective research.

BSL-4 is too much, enhanced BSL-3 is just right

Adolfo García-Sastre of the Mount Sinai School of Medicine argues that the H5N1 viruses in question may well be less pathogenic than they were before passage through ferrets, but they could still be quite dangerous, so preventing human exposure is crucial. However, he says, the ultimate level of biosecurity, BSL-4, is excessive in this case and would stifle the pace of discovery. There are both therapeutics and vaccines available for H5N1, says García-Sastre, so he advocates for conducting the research in enhanced BSL-3 facilities, which he says offer the necessary security measures, including interlocked rooms with negative pressure, Hepa-filtered air circulation, and appropriate decontamination and/or sterilization practices for material leaving the facility.

Bad virus, top security

Michael Imperiale and Michael Hanna, on the other hand, make their case that the H5N1 viruses merit BSL-4 containment. Although H5N1 that cannot transmit from human to human would normally be handled in a BSL-3 facility, researchers changed the virus' biosafety profile when they enhanced its ability to transmit between mammals. According to Imperiale and Hanna, the vaccine for H5N1 is not widely available, and drug resistance and a slow distribution system for antiviral drugs mean a small outbreak could never be contained.

Read the Commentaries and make up your own mind about the issue. Where do you stand?

*In case you've been under a rock these past few months, here's the back story: this fall, the U.S. National Science Advisory Board for Biosecurity recommended that the authors of two avian H5N1 investigations and the scientific journals that planned to publish the studies to withhold crucial details of the research in the interest of biosecurity. The researchers had used ferrets to develop strains of influenza that can transmit from mammal-to-mammal, something the virus did not readily do before. The concern is that the viruses are now honed to kill. If they've maintained their case fatality rate and if humans are anything like ferrets (and they are), these viruses could cause a pandemic of biblical proportions.