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Chapter 28: Microbiology. 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.
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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.
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.
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.
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.
The Prokaryotes • The bacteria (domain Bacteria) and archaea (domain Archaea) are prokaryotes. • Prokaryotes lack a membrane bounded nucleus and the chromosome is in a nucleoid that lacks an envelope. • There are no membranous organelles but they do have ribosomes; prokaryotes have a cell wall that may be surrounded by a capsule.
Some prokaryotes move by flagella, and some adhere to surfaces by means of small, hairlike appendages called fimbriae. They reproduce by binary fission, and some can form endospores. Three means of genetic recombination (but not sexual reproduction) have been observed in bacteria: conjugation, transformation, and transduction.
Most bacteria are aerobic, requiring a constant oxygen supply for cellular respiration. Obligate anaerobes are unable to grow in the presence of oxygen; facultative anaerobes are able to grow in the presence or absence of oxygen. Bacteria are saprotrophs which externally digest organic compounds and absorb nutrients.
Bacteria are the decomposers in ecosystems. Some bacteria are symbiotic and live in association with other organisms. The bacteria that reside in the human intestinal tract are mutualistic; commensalistic bacteria reside on our skin; and parasitic bacteria cause a wide variety of diseases. The cyanobacteria are photosynthetic in the same manner as plants.
The Bacteria • Bacteria occur in three shapes: rod (bacillus), round (coccus), or spiral (spirillum). • Based on a dye test, bacteria are either Gram-positive or Gram-negative: Gram-positive bacteria have a thick layer of peptidoglycan;Gram-negative bacteria have a thin layer. • A new method to classify bacteria is to use rRNA sequences; previously, endospores, metabolism, growth, and mode of nutrition were considered.
Cyanobacteria • Cyanobacteria are photosynthetic prokaryotes that are rather large (1 µm to 50 µm in length) and may be unicellular, colonial, or filamentous. • Some have heterocysts where nitrogen fixation occurs. • Cyanobacteria are sometimes symbiotic with fungi (forming lichens) and other organisms.
The Archaea • The plasma membrane of archaea have unusual lipids so they can withstand high temperatures and extreme conditions. • Their cell walls lack peptidoglycan typical of bacteria. • Some archaea are methanogens, halophiles, or thermoacidophiles. • None are photosynthetic nor do they cause infectious disease.
Relationships of the Domains Archaea have different rRNA sequences from bacteria. Because archaea and some bacteria are both found in extreme environments, they may have diverged from a common ancestor relatively soon after life began. The eukaryotes appear to have diverged from archaea; thus, eukarya are believed to be more closely related to the archaea than to the bacteria.
The Protists • Protists are eukaryotes of the kingdom Protista that are usually unicellular; if multicellular they lack tissue differentiation. • Protists are grouped according to mode of nutrition, among other traits. • Three life cycles are seen among protists: haplontic cycle, alternation of generations, and diplontic cycle.
Algae • Algae are aquatic autotrophs but they do not need to protect the zygote from drying out. • They produce food for oceanic and fresh water communities. • Algae are grouped according to their color and biochemical differences, such as the chemistry of the cell wall and the way they store reserve food.
Green Algae • Like plants, green algae have a cell wall of cellulose, have both chlorophylls a and b, and store reserve food as starch. • Unicellular green algae include Chlamydomonas which has two whip-like flagella, a chloroplast, a starch-storing pyrenoid,and a haplontic life cycle. • When conditions are favorable, asexual reproduction occurs, otherwise sexual reproduction occurs; gametes from two different strains form a zygote that overwinters.
Colonial green algae include Volvox, in which thousands of flagellated cells are arranged in a single layer surrounding a watery interior. Cells that are specialized for reproduction divide asexually to form a new daughter colony. Sexual reproduction involves heterogametes (gametes that are not identical)—a large nonmotile egg and small flagellated sperm.
Filamentous green algae include Spirogyra with long chains of cells that have spiral chloroplasts. During conjugation, cell contents of one filament move across to cells of other filament, forming diploid zygotes that survive the winter. The zygotes undergo meiosis to produce new haploid adult filaments.
Multicellulargreen algae include Ulva, a “sea lettuce” that is multicellular with a leafy appearance. The thallus is two cells thick and can be a meter long. Ulva has the alternation of generations life cycle but both generations look alike and there are isogametes. Spores are flagellated.
Brown and Golden Brown Algae • Brown algae and golden brown algae both have chlorophylls a and c plus fucoxanthin, a carotenoid pigment that gives them their color. • Some brown algae are familiar seaweeds such as Fucus, which has a diplontic life cycle. • Diatoms are golden brown algae with silica valves that are significant producers in the ocean.
Dinoflagellates • Dinoflagellates are bounded by protective cellulose plates and have two flagella; one is in a transverse groove causes the organism to spin like a top. • They are the cause of “red tides” that kill fish because they contain poisons. • But dinoflagellates are an important source of food and also live symbiotically with corals.
Euglenoids • One-third of euglenoids have chloroplasts and in the absence of sunlight, they become heterotrophic. • They have two flagella, one usually much longer than the other. • A flexible pellicle allows euglenoids to assume different shapes. • Reproduction is by longitudinal cell division.
Red Algae • Red algae are multicellular algae of economic importance. • Seaweeds occur inwarm water that is either shallow or deep. • Filaments may be simple or have a branched or feathery appearance. • Coralline algae are red algae that have walls of calcium carbonate and add to the growth of coral reef.
Protozoans • Protozoans are typically heterotrophic, motile, unicellular organisms. • They have food vacuoles, contractile vacuoles, and many reproduce sexually during part of their life cycle. • Protozoa are not animals because they do not undergo embryonic development. • They are classified according to means of locomotion.
Amoeboids • Amoeboids are protists that move by pseudopods, and eat by phagocytosis. • Amoeba proteus lives in fresh water. • Entamoeba histolytica causes amoebic dysentery. • Foraminifera are amoeboids that have a calcium carbonate shell and are largely marine.
