The influenza virus is a contagious respiratory illness that affects the nose, throat, and sometimes the lungs. It is responsible for seasonal epidemics that cause a wide range of symptoms, from fever and body aches to more severe complications. Influenza viruses spread through the air when an infected person coughs, sneezes, or talks, making them highly transmissible. Each year, these viruses cause significant public health challenges globally, affecting millions of people. While most healthy individuals recover within a couple of weeks, it can lead to serious outcomes for young children, the elderly, and those with underlying health conditions.
The Challenge of Visualizing Viruses
Viruses present a challenge for visualization due to their small size. An influenza virus measures between 80 and 120 nanometers in diameter. For perspective, a single human red blood cell is about 60 to 80 times wider than an influenza virion. Common bacteria are giants in comparison, often thousands of nanometers long.
This smallness is the primary reason the influenza virus is invisible to standard light microscopes. Optical microscopes work by passing visible light through a specimen, but the wavelength of visible light is much larger than the virus itself. Because the virus is smaller than the waves of light used to see it, the light waves pass by without interacting, making it impossible to form an image. Visualizing something so small requires technology that operates on a sub-light-wave scale.
Viewing Influenza with Electron Microscopes
To overcome the limitations of light, scientists use electron microscopes, which use beams of electrons instead of light waves to create an image. The wavelength of an electron is significantly shorter than that of visible light, allowing for much higher resolution to see objects as small as viruses. This technology first allowed researchers to see the influenza virus and understand its physical form.
Two main types of electron microscopes are used to study viruses like influenza. A Transmission Electron Microscope (TEM) works by passing a beam of electrons directly through an ultrathin slice of the sample. As electrons pass through, some are scattered while others reach a detector, creating a flat, two-dimensional image that reveals the internal structure of the virus.
Another tool is the Scanning Electron Microscope (SEM). Instead of passing electrons through the sample, an SEM scans a beam of electrons across the virus’s surface. The electrons interact with the surface atoms, and detectors capture the resulting secondary electrons. This process generates a detailed three-dimensional image of the virus’s exterior, showing its topography and shape.
Anatomy of the Influenza Virus Revealed
Electron microscopy has documented the detailed anatomy of the influenza virus. The images reveal the virus is often spherical, though some can be more elongated or filamentous. The particle is enclosed by a lipid membrane known as the viral envelope, which is taken from the host cell the virus previously infected. This outer layer is part of the virus’s structure and function.
Protruding from the viral envelope are hundreds of spike-like structures, which are two different proteins: hemagglutinin (HA) and neuraminidase (NA). Hemagglutinin allows the virus to latch onto and enter a host cell, initiating infection. Neuraminidase helps newly replicated viruses cut free from the host cell so they can infect others.
The colorful images of the influenza virus seen in textbooks and online are not what the virus actually looks like. Electron microscope images are inherently black and white, as the process does not involve light and has no color information. Scientists add color to these images in a process called false-coloring or colorization. This is done to highlight specific structures, like the HA and NA proteins, making the anatomy easier to understand.