Viruses

Comparative Sizes of Microbes

Virus Examples

Viruses occur in a vast array of shapes and configurations. These images provide some information about the shape and structure of the viruses, but the size of their features is generally below the resolution of light microscopes, being in the range of 0.05 - 0.2 μm or 50 - 200 nanometers compared to the range of visible light wavelengths 400-750nm. While the colors used to display the structures are useful for visualization, they are not to be taken literally because the entire virus is smaller than the wavelength of those colors of light. The structural details can be visualized with electron microscope images since the effective electron wavelengths can be made small enough to resolve the details by using high energy electrons.

The Covid-19 pandemic is attributed to a coronavirus named Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2). The virus was first detected in Wuhan, China in 2019. COVID-19 can affect the upper respiratory tract (sinuses, nose, and throat) and the lower respiratory tract (windpipe and lungs).The lungs are the organs most affected by COVID-19 because the virus accesses host cells via the enzyme angiotensin-converting enzyme 2 (ACE2), which is most abundant in type II alveolar cells of the lungs. The virus uses a special surface glycoprotein called a "spike" to connect to ACE2 and enter the host cell. (Wiki) Image created by the CDC.

The herpes virus: Herpes simplex viruses -- more commonly known as herpes -- are categorized into two types: herpes type 1 (HSV-1, or oral herpes) and herpes type 2 (HSV-2, or genital herpes). Most commonly, herpes type 1 causes sores around the mouth and lips (sometimes called fever blisters or cold sores). HSV-1 can cause genital herpes, but most cases of genital herpes are caused by herpes type 2. (WebMD) Image from NativeAntigen site.

The influenza virus: Influenza viruses have a single-stranded segmented RNA genome. The influenza viruses are classified into types A, B, and C on the basis of their core proteins. (WHO) Influenza spreads around the world in yearly outbreaks, resulting in about three to five million cases of severe illness and about 290,000 to 650,000 deaths. (Wiki) Image from CDC

The measles virus: Formally measles morbillivirus, this virus is a single-stranded RNA virus. Humans are the natural hosts of the virus; no animal reservoirs are known to exist. (Wiki). Measles is a highly contagious disease transmitted by respiratory aerosols that triggers a temporary but severe immunosuppression. Symptoms include fever, cough, runny nose, inflamed eyes and the familiar general rash. The virus is spread by coughing and sneezing via close personal contact or direct contact with secretions. Image from CDC

Bacteriophage: Bacteriophages (literally 'bacteria eaters') , about which Karp quips "resemble a landing craft for the moon", infect bacteria by injecting their DNA or RNA into the cells. Within minutes, they are able to begin using the host bacteria's ribosomes to manufacture viral proteins. With a vast number of varieties and wide range of complexity, they are some of the most common biological entities. (Wiki) Some bacteriophages attack bacteria that are harmful to the body, so they provide a medically useful therapy against those bacteria.

Adenovirus: Adenoviruses can cause cold-like symptoms, fever, sore throat, bronchitis, pneumonia, diarrhea, and pink eye (conjunctivitis). CDC They are medium-sized (90-100 nm) viruses that take a crystal-like shape, lack an outer lipid bilayer, and contain a double stranded DNA genome. Their name derives from their initial isolation from human adenoids in 1953. Their DNA enters the nucleus of the host and is able to induce the host to produce mRNAs for needed components to activate viral genes, block host-immune responses, and finally to replicate the viral genome. Wiki.

HIV: The human immunodeficiency virus (HIV) has DNA material which it integrates into the DNA of the host's white blood cells (T-helper cells), creating what is called a provirus. Once inside the T-helper cell, HIV converts its genetic material into HIV DNA, a process called reverse transcription. The new HIV DNA then enters the nucleus of the host cell and takes control of it. (Avert.org). New HIV virions bud out from the cell surfaces of the infected cells. The infected cells may remain alive for a period, acting as a factory for the production of new virions.

The Varied World of Viruses

Karp (the main source of this material) describes viruses as "obligatory intracellular parasites" which cannot reproduce unless they are inside a host cell where they "hijack" its resources.

Outside of a living cell, the virus exists as a particle called a virion, which is little more than a molecular package. Nevertheless, there is a tremendous variety of such packages. They contain genetic material which may be single-stranded or double-stranded and composed of either DNA or RNA. Some viruses have as few as three or four different genes, while others have as many as several hundred. The genetic material is surrounded by a protein capsule, or capsid.

Considering their enormous impact on human life, it is remarkable to note that virions are just molecular aggregates, inanimate particles that by themselves are unable to reproduce, metabolize, or carry out any of the other activities associated with life! Yet each type of virion has on its surface a protein that is able to bind to a particular surface component of its host cell. The size of viruses is so small that it is less than the wavelength of visible light, so they cannot be seen with a light microscope, but they can be "tagged" with a fluorescent molecule, enabling them to be caused to glow when excited by UV light and be tracked by a light microscope. By this technique it was demonstrated that virus particles could penetrate a host cell wall in less than a tenth of a second and move to the cell's nucleus within 15 minutes! Once there, they can take over the cell's manufacturing capacity to induce it to produce components for the virus. All this for a virion which appears as an inanimate particle!

Karp describes viral infections as mostly occurring in two basic types:

1. The virus arrests the normal synthetic activities of the host cell and redirects the cell to use its available materials and energy to manufacture specific viral nucleic acids and proteins. Unlike the process of cell reproduction, these components directly assemble themselves into new virions. Ultimately the cell ruptures or "lyses" to release a new generation of viral particles.

2. The infecting virus does not lead to the death of the host cell, but instead inserts (integrates) its DNA into the DNA of the host cell. The integrated viral DNA is called a provirus. An integrated provirus can have different effects depending upon the type of virus and host cell.