BIOLOGY & MEDICINE_Viruses Tiny giants Viruses are usually incredibly small, but some deviate from the norm and reach sizes greater than that of a bacterial cell. Matthias Fischer from the Max Planck Institute for Medical Research in Heidelberg is one of a small number of scientists working on giant viruses of this kind. TEXT STEFANIE REINBERGER Photo: Wolfram Scheible 58 MaxPlanckResearch 3 | 19 n the laboratory of Matthias Fischer Although they look like nothing more As giant viruses are about at the Max Planck Institute in Hei- than vials of water to the naked eye, the the same size as bacteria, delberg, vials containing water samples are actually teeming with it is almost impossible to purify them by filtration samples are lined up against one life, which only becomes visible when only. However, as viruses another, each containing a whole viewed through a microscope: countless and bacteria have different I world of aquatic single-celled organ- tiny dots are scurrying back and forth. densities, they form layers isms and viruses. The labels reveal the “The smaller ones are bacteria, which when spun in an ultracen- trifuge. Scientists can then origins of the samples: Guenzburg, are devoured by larger cells that have a extract the viral band using Kiel, but also more exotic locations nucleus. These so-called protists are the a syringe and needle. such as Tallinn or the British Virgin reason we created the collection in the Islands. “The collection is the result of first place,” Fischer explains. Indeed, many years of work,” the microbiolo- these protists are susceptible to attack Photo: Wolfram Scheible gist explains. by giant viruses, which are Fischer‘s 3 | 19 MaxPlanckResearch 59 BIOLOGY & MEDICINE_Viruses main interest. Together with his col- blur the previously firmly established leagues, he therefore screens the waters boundary between cells and viruses. of the Danube, the Baltic Sea or the In- The unwritten rule among virus re- dian Ocean for single-celled organisms searchers was that viruses had a maxi- that can act as hosts for giant viruses. mum diameter of 200 to 300 nanome- ters. Virus experiments were therefore A VIRUS DISGUISED carried out using small-pore filters that AS A BACTERIUM only allowed representatives meeting the common size definition to pass. As These viruses may be tiny by human a result, giant viruses were simply fil- standards, but they are the giants of the tered out of the sample material along virus world. Indeed, they are so much with bacteria and other single-celled bigger than other viruses that scientists organisms – and therefore quite simply initially mistook them for bacteria. overlooked. “Ultimately, the fact that The first example of these viral giants they remained undiscovered for so was discovered by researchers in 1992 long is caused by our own preconcep- in an amoeba found in the water cir- tions. One of the lessons giant viruses cuit of an industrial cooling tower in teach us is therefore to always question Bradford (UK). Assuming the virus to be our assumptions if we genuinely want a bacterium, the researchers initially to discover something new,” says Mat- named it Bradfordcoccus. thias Fischer. It was not until 2003 that researchers at Aix-Marseille University realized A GROWING ARMY that they were actually dealing with a OF GIANT VIRUSES virus – one that was even bigger than some bacteria, with a diameter of 750 Although giant viruses have received nanometers. Moreover, for a virus, it greater attention in recent years, there Global microcosms: each vial teems with also had a huge genome: with 1.2 mil- are still only a few scientists worldwide single- celled organisms and viruses isolated lion base pairs, it was twice as big as the who study them. This may be due to the from different environments. Wheat grains in the vials stimulate the growth of largest known virus genome to date and exotic nature not only of the viruses bacteria, which in turn serve as a food encoded some 1,000 genes. By compar- themselves, but also of their host organ- source for protists. ison, influenza viruses and HIV carry isms. Protists play an important role in about a dozen genes. the world‘s food webs, but only few The French researchers gave the species are of medical or economic sig- giant a new, more suitable name: mimi- nificance, such as the pathogens re- virus, from mimicking microbe – a sponsible for malaria, sleeping sickness virus that pretends to be a bacterium. and toxoplasmosis. Many other giant viruses have since So far, hardly any research has been been discovered in oceans, salt lakes, conducted into the majority of protists, sewage treatment plants, tree barks, for- many of which are also hosts of giant est soils and permafrost. Although the viruses. The circle of colleagues with virus particles differ widely in terms of whom Matthias Fischer can discuss their genomes, structures and shapes, ideas and exchange knowledge and they have one thing in common: they methods is thus rather small. Often, the Photo: Wolfram Scheible 60 MaxPlanckResearch 3 | 19 Matthias Fischer uses pulsed-field gel electrophoresis to separate the genomes of giant viruses for further analysis. standard laboratory techniques don’t work for giant viruses or their hosts. In that case, Fischer has to spend a great deal of time finding alternatives – or new techniques altogether. “On the up- side, however, there’s a good chance we’ll stumble across something new and unexpected in each of our projects,” he says. You could describe the 42-year-old microbiologist as a modern-day discov- erer. He has a sparkle in his eye as he talks about the unsolved mysteries of giant viruses, and you can sense his eagerness to crack as many of them as possible. And there is no shortage of riddles surrounding these giants. One question is how many different types of giant viruses remain to be discov- ered, another one is their unresolved evolutionary origin and why they are so big in the first place. For some time, the question as to their origin has divided the small com- munity of researchers into two camps. Some believe the giant viruses emerged from cells that gradually lost most of their genetic material. With the remain- ing genes, the diminished cells were no longer able to multiply on their own and became dependent on the machin- ery of other cells. Giant viruses may therefore be the remnants of an other- wise extinct domain of life, alongside bacteria, archaea and eukaryotes (cells with a nucleus). “However, our re- search findings – and those of col- leagues – suggest that giant viruses Photo: Wolfram Scheible Photo: Wolfram Scheible rather evolved from smaller viruses, 3 | 19 MaxPlanckResearch 61 CroV infection Exposure to Mavirus Co-infection with CroV and Mavirus Mavirus integrates into the host genome Mavirus replicates at the expense of CroV CroV infection CroV reactivates Mavirus Cell lysis releases CroV particles Flagellate Cafeteria roenbergensis Cell lysis releases Mavirus particles Cell lysis releases particles of CroV and Mavirus Giant virus CroV Virophage Mavirus One host, two viruses: when the giant virus continually incorporating new genes then, I had no idea that every liter of CroV infects its host, the flagellate Cafeteria from other organisms while also dupli- seawater contains several billion virus roenbergensis, it multiples inside the cell and cating their own,” says Fischer. particles,” Fischer recalls. He then con- kills it at the end of the infection cycle (left). If the flagellate is simultaneously infected tacted the author of the article, Curtis with the virophage Mavirus, the giant virus NEW TERRITORY Suttle, in Vancouver. “I applied for a is unable to produce new infectious particles FOR RESEARCHERS doctoral position, booked a flight and and instead, Mavirus replicates. Although stayed for six years.” the infected cell ultimately dies, other host cells are protected from CroV infection by Fischer’s personal interest in viruses This work set the course for Fischer’s the action of Mavirus (center). was sparked towards the end of his de- further scientific career. The first years Mavirus alone cannot multiply in the gree in biochemistry in Bayreuth, when in Vancouver demanded a great deal of flagellate, but it incorporates itself into the he came across a scientific article on perseverance. The young doctoral stu- host’s genetic material, where it can persist for many generations until CroV infection the diversity of viruses in the ocean. “I dent from Germany set out to study a reactivates the virophage (right). was absolutely fascinated because, until new virus, Cafeteria roenbergensis virus Graphic: M. Fischer/MPI for Medical Research 62 MaxPlanckResearch 3 | 19 BIOLOGY & MEDICINE_Viruses The small virus operates like a defense system, protecting the » flagellates from the giant virus. Not all viruses are therefore parasites of their host organisms; rather, some viruses exist in a mutually beneficial symbiosis with their hosts. (CroV), which later turned out to be- it prevents the production of new giant long to the same family as the mimi- viruses and thereby helps its host C. viruses. CroV infects a single-celled roenbergensis to survive albeit indirect- organism by the name of Cafeteria roen- ly. This single-celled organism typically bergensis, a widespread species of marine dies upon infection with CroV, which zooplankton that feeds on bacteria and is a lytic virus: once it has multiplied in is one of the most numerous predators sufficient quantities, the host cell bursts on the planet. and the viral offspring are released. However, when CroV is parasitized by TOXIC DNA the smaller Mavirus, the cell releases new Maviruses particles instead of At the time, researchers had known CroV particles. Although this doesn’t about the giant virus for a number of make a difference for the infected cell, years but hadn’t investigated it further.