Chapter 6 Acellular Pathogens PDF

## 6 Acellular Pathogens ### 6.1 Viruses #### Top For Section Objectives - Despite their small size, which prevented them from being seen with light microscopes, the discovery of a filterable component smaller than a bacterium that causes tobacco mosaic disease (TMD) dates back to 1892. - At that...

## 6 Acellular Pathogens ### 6.1 Viruses #### Top For Section Objectives - Despite their small size, which prevented them from being seen with light microscopes, the discovery of a filterable component smaller than a bacterium that causes tobacco mosaic disease (TMD) dates back to 1892. - At that time, Dmitri Ivanovski, a Russian botanist, discovered the source of TMD by using a porcelain filtering device first invented by Charles Chamberland and Louis Pasteur in Paris in 1884. - Porcelain Chamberland filters have a pore size of $0.1 \mu m$, which is small enough to remove all bacteria $\geq 0.2 \mu m$ from any liquids passed through the device. - An extract obtained from TMD-infected tobacco plants was made to determine the cause of the disease. Initially, the source of the disease was thought to be bacterial. It was surprising to everyone when Ivanovski, using a Chamberland filter, found that the cause of TMD was not removed after passing the extract through the porcelain filter. - So if a bacterium was not the cause of TMD, what could be causing the disease? Ivanovski concluded the cause of TMD must be an extremely small bacterium or bacterial spore. - Other scientists, including Martinus Beijerinck, continued investigating the cause of TMD. It was Beijerinck, in 1899, who eventually concluded the causative agent was not a bacterium but, instead, possibly a chemical, like a biological poison we would describe today as a toxin. - As a result, the word virus, Latin for poison, was used to describe the cause of TMD a few years after Ivanovski's initial discovery. - Even though he was not able to see the virus that caused TMD, and did not realize the cause was not a bacterium, Ivanovski is credited as the original discoverer of viruses and a founder of the field of virology. #### Tap To Explore #### Clinical Focus ##### Part 1 ###### Check Your Understanding - Why was the first virus investigated mistaken for a toxin? ## Chapter Outline 1. Viruses 2. The Viral Life Cycle 3. Isolation, Culture, and Identification of Viruses 4. Viroids, Virusoids, and Prions ### Introduction - Public health measures in the developed world have dramatically reduced mortality from viral epidemics. - But when epidemics do occur, they can spread quickly with global air travel. - In 2009, an outbreak of H1N1 influenza spread across various continents. - In early 2014, cases of Ebola in Guinea led to a massive epidemic in western Africa. - This included the case of an infected man who traveled to the United States, sparking fears the epidemic might spread beyond Africa. - Until the late 1930s and the advent of the electron microscope, no one had seen a virus. - Yet treatments for preventing or curing viral infections were used and developed long before that. - Historical records suggest that by the 17th century, and perhaps earlier, inoculation (also known as variolation) was being used to prevent the viral disease smallpox in various parts of the world. - By the late 18th century, Englishman Edward Jenner was inoculating patients with cowpox to prevent smallpox, a technique he coined vaccination. ### 2014 Ebola virus epidemic in West Africa - Total cases: 17,145 - Total deaths: 6,070 - Senegal: cases: 1, deaths: 0 - Mauritania: cases: 1 - Mali: deaths: 0 - Gambia: cases: 8, deaths: 6 - Guinea: cases: 7,312, deaths: 1,327 - Guinea-Bissau: cases: 2,164, deaths: 1,583 - Sierra Leone: cases: 7,635, deaths: 3,145 - Liberia: cases: 20, deaths: 8 - Benin: cases: 8 - Togo: - Nigeria: deaths: 8 - Ghana: - Côte d'Ivoire: - Cameroon: - 30 November 2014 - Figure 6.1 The year 2014 saw the first large-scale outbreak of Ebola virus (electron micrograph, left) in human populations in West Africa (right). Such epidemics are now widely reported and documented, but viral epidemics are sure to have plagued human populations since the origin of our species. (credit left: modification of work by Thomas W. Geisbert) ### 6.2 The Viral Life Cycle ### 6.3 Isolation, Culture, and Identification of Viruses ### 6.4 Viroids, Virusoids, and Prions ##### Figure 6.2 Tobacco mosaic virus (TMV) viewed with transmission electron microscope. - (a) Tobacco mosaic virus (TMV) viewed with transmission electron microscope. - (b) Plants infected with tobacco mosaic disease (TMD), caused by TMV. (credit a: modification of work by USDA Agricultural Research Service-scale-bar data from Matt Russell; credit b: modification of work by USDA Forest Service, Department of Plant Pathology Archive North Carolina State University) ##### Figure 6.3 In this transmission electron micrograph, a bacteriophage (a virus that infects bacteria) is dwarfed by the bacterial cell it infects. - (a) In this transmission electron micrograph, a bacteriophage (a virus that infects bacteria) is dwarfed by the bacterial cell it infects. - (b) An illustration of the bacteriophage in the micrograph. (credit a: modification of work by U.S. Department of Energy, Office of Science, LBL, PBD) #### Table 6.1 Characteristics of Viruses: - Infectious, acellular pathogens - Obligate intracellular parasites with host and cell-type specificity - DNA or RNA genome (never both) - Genome is surrounded by a protein capsid and, in some cases, a phospholipid membrane studded with viral glycoproteins - Lack genes for many products needed for successful reproduction, requiring exploitation of host-cell genomes to reproduce #### Hosts and Viral Transmission - Viruses can infect every type of host cell, including those of plants, animals, fungi, protists, bacteria, and archaea. - Most viruses will only be able to infect the cells of one or a few species of organism. - This is called the host range. - However, having a wide host range is not common and viruses will typically only infect specific hosts and only specific cell types within those hosts. - The viruses that infect bacteria are called bacteriophages, or simply phages. - The word phage comes from the Greek word for devour. - Other viruses are just identified by their host group, such as animal or plant viruses. - Once a cell is infected, the effects of the virus can vary depending on the type of virus. - Viruses may cause abnormal growth of the cell or cell death, alter the cell's genome, or cause little noticeable effect in the cell. - Viruses can be transmitted through direct contact, indirect contact with fomites, or through a vector: an animal that transmits a pathogen from one host to another. - Arthropods such as mosquitoes, ticks, and flies, are typical vectors for viral diseases, and they may act as mechanical vectors or biological vectors. - Mechanical transmission occurs when the arthropod carries a viral pathogen on the outside of its body and transmits it to a new host by physical contact. - Biological transmission occurs when the arthropod carries the viral pathogen inside its body and transmits it to the new host through biting. - In humans, a wide variety of viruses are capable of causing various infections and diseases. - Some of the deadliest emerging pathogens in humans are viruses, yet we have few treatments or drugs to deal with viral infections, making them difficult to eradicate. - Viruses that can be transmitted from an animal host to a human host can cause zoonoses. - For example, the avian influenza virus originates in birds, but can cause disease in humans. - Reverse zoonoses are caused by infection of an animal by a virus that originated in a human. #### Micro Connections #### Fighting Bacteria with Viruses ###### Check Your Understanding - Why do humans not have to be concerned about the presence of bacteriophages in their food? - What are three ways that viruses can be transmitted between hosts? #### Viral Structures - In general, virions (viral particles) are small and cannot be observed using a regular light microscope. - They are much smaller than prokaryotic and eukaryotic cells; this is an adaptation allowing viruses to infect these larger cells. - The size of a virion can range from $20nm$ for small viruses up to $900nm$ for typical, large viruses. - Recent discoveries, however, have identified new giant viral species, such as Pandoravirus salinus and Pithovirus sibericum, with sizes approaching that of a bacterial cell. ##### Figure 6.4 The size of a virus is small relative to the size of most bacterial and eukaryotic cells and their organelles. - The size of a virus is small relative to the size of most bacterial and eukaryotic cells and their organelles. - In 1935, after the development of the electron microscope, Wendell Stanley was the first scientist to crystallize the structure of the tobacco mosaic virus and discovered that it is composed of RNA and protein. - In 1943, he isolated Influenza B virus, which contributed to the development of an influenza (flu) vaccine. - Stanley's discoveries unlocked the mystery of the nature of viruses that had been puzzling scientists for over 40 years and his contributions to the field of virology led to him being awarded the Nobel Prize in 1946. - As a result of continuing research of viruses, we now know they con