Chapter 28.1: Microbiology-VIRUS! Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Viruses Viruses are noncellular, nonliving particles and therefore are not included in the classification scheme. Comparable in size to a large protein macromolecule, many viruses can be purified, crystallized, and stored as chemicals.

Structure of Viruses A virus has an outer capsid composed of protein subunits, and an inner core of nucleic acid. An outer membranous envelope may be acquired when the virus buds from the cell. It may also include enzymes for nucleic acid replication. Viruses are classified by type of nucleic acid, viral shape and size, and by presence of an outer envelope.

Adenovirus

Parasitic Nature Viruses are obligate intracellular parasites. Viruses are very specific for the type of cells they infect (e.g., HIV only infects certain kinds of blood cells). Viruses are likely derived from the very host they infect, and therefore evolved after cells evolved. Some viruses, such as the flu virus, can mutate rapidly.

Replication of Viruses Viruses are specific to a particular host cell because they bind to a particular plasma membrane receptor. After viral nucleic acid enters the host cell, it takes over the metabolic machinery of the host cell so that more viruses are produced.

Replication of Bacteriophages Bacteriophages are viruses that parasitize bacteria. Some undergo two cycles, a lytic cycle and a lysogenic cycle. The lytic cycle is divided into five phases: attachment, penetration, biosynthesis, maturation, and release. In the lysogenic cycle, the infected bacterium does not immediately produce viruses but may do so sometime in the future; the phage has a latent period and is called a prophage during this time.

Lytic and lysogenic cycles

Replication of Animal Viruses Entire animal virus penetrates host cell by endocytosis. Once inside, the virus is uncoated to remove the envelope and capsid. The viral genome, either DNA or RNA, is now free and biosynthesis proceeds. The assembled viruses bud from the cell and acquire envelopes.

Retroviruses are RNA animal viruses that have a DNA stage. They have an enzyme called reverse transcriptase that carries out RNA → cDNA transcription. Following replication, cDNA integrates into the host genome until viral reproduction occurs.

Reproduction of HIV, a retrovirus

Viral Infections Viruses cause infectious diseases of plants and animals, including humans. Some crop diseases are attributed not to viruses but to naked strands of RNA called viriods. Some diseases in humans and animals are attributed to prions which are protein particles. Mad cow disease (BSE) in Britain is believed to be a prion disease.

Lab Ex 19: Platyhelminthes (flatworms) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Flatworms Flatworms are characterized by the tissue level of organization and a sac body plan. These acoelomates (NO coelom) have three germ layers, and have all organs except respiratory and circulatory organs. The flat body facilitates diffusion of oxygen and other molecules from cell to cell.

Planarians Planarians are freshwater, free living, flatworms. Flame cells function in excretion. The small brain extends to a ladder arrangement of nerves. Light-sensitive organs (eyespots) are in the head; planarians exhibit cephalization. One organism has both male and female sex organs – they are hermaphroditic.

Planarian

Parasitic Flatworms Flukes and tapeworms are two classes of parasitic flatworms; both have intermediate hosts. Flukes are oval to elongate and have suckers at the anterior end. Blood flukes cause schistosomiasis; other flukes infect the digestive tract, bile duct, and lungs. A tapeworm has an anterior scolex with hooks and suckers to hold itself inside the gut.

Schistosomiasis

Lab Ex 20: Aschelminthes (roundworms) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Roundworms Roundworms have the tube-within-a-tube plan; they are prevalent in soil and some parasitize animals and plants. The pseudocoelom is a body cavity incompletely lined with mesoderm. The fluid-filled interior forms a hydrostatic skeleton. Most species of roundworms have separate males and females.

Coelom structure and function

Ascaris Ascaris larvae are swallowed and burrow through the intestinal wall and make their way through various organs until they reach the lungs. In the lungs, they grow in size for 10 days, then move up to the throat, and are then swallowed. After they mature in the intestine, females produce eggs that pass out with feces.

Roundworm anatomy

Other Roundworms Trichinosis is a roundworm infection from eating undercooked pork containing encysted Trichinella larvae. The filarial worm is carried by mosquitoes and causes elephantiasis by blocking lymphatic drainage. Pinworms are common infections in children. Hookworm is a more serious infection seen in the southern United States.