Viruses are among the weirdest things on the planet. Existing in a Limbo-like world somewhere between the living and the dead, viruses present undergraduate microbiologists with their very first philosophical conundrum: What is life? The question may be forever impossible to answer because even though viruses evolve -- as do other bizarre creatures such as transposons and prions -- they continue to defy our understanding of what it means to be "alive."

Regardless of their existential status, viruses have been wildly successful. It is thought that any given cell can be infected by at least one type of virus, and viruses are the most abundant lifeform on Earth. It goes without saying, therefore, that some of them are rather strange. From giant viruses larger than some bacteria to viruses that infect viruses, evolution has produced some real oddities.

Included among these oddities are the lemon-shaped (a.k.a. spindle-shaped) viruses. Known only to infect archaea (bacteria-like cells that are famous for thriving in extreme environments), little was previously known about the architecture of lemon-shaped viruses. So, a team of virologists set out to change that.

The researchers examined a virus, called His1, that infects the salt-loving microbe Haloarcula hispanica. They used cryo-electron tomography to generate 2D and 3D images of the virus. (See figure.)

As shown, the virus has a "tail" that consists of six spikes. Like other tailed viruses, these spikes probably latch on to the bacterium that the virus infects.

The authors also determined that the virus is highly resistant to destruction. Detergent, however, is lethal. Upon exposure to detergent, the virus expels its genome, and the lemon-shaped virion turns into a hollow tube. (See figure.)

Because the virion shows such tremendous flexibility in terms of its morphology, the authors believe that this may help the virus inject its genome into the cells it infects. (Though the authors do not mention this, perhaps the His1 virus, similar to herpesvirus, maintains a very high internal pressure inside the virion that allows the genome to "explode" into the cell.)

The authors propose a model in which the lemon-shaped virus grabs onto a bacterium, and the injection of its DNA into the victim is aided by the "deflation" of the virus into a tube-like structure. It's an intriguing hypothesis that is essentially (and amazingly) based upon some very good photography.

Source: Chuan Hong, Maija K. Pietilä, Caroline J. Fu, Michael F. Schmid, Dennis H. Bamford, and Wah Chiu. "Lemon-shaped halo archaeal virus His1 with uniform tail but variable capsid structure." PNAS 112 (8): 2449–2454. doi: 10.1073/pnas.1425008112

(Photo: Lemon via Petar43/Wikimedia Commons)