Chapter 23: Viruses and Prokaryotes

Lecture Outline

  1. Viruses are noncellular infectious agents
  2. A virus particle consists of nucleic acid surrounded by a protein coat
  3. The protein coat is referred to as a capsid
  4. Some viruses also have an outer membranous envelope
  5. Viruses contain either DNA or RNA, not both
  6. The nucleic acid may be single stranded (ss) or double stranded (ds)
  7. Viral capsids are helical or polyhedral, or a combination of both shapes
  8. The International Committee on the Taxonomy of Viruses (ICTV) classifies viruses
  9. They may be classified according to nucleic acids and/or presence or absence of a capsid
  10. This is a non-Linnaean classification system
  11. Viruses may have "escaped" from cells
  12. This hypothesis explains the specificity of viruses and their hosts
  13. Bacteriophages are viruses that attack bacteria
  14. These complex viruses consist of nucleic acid (typically dsDNA) enclosed in a polyhedral head, usually with a tail for attachment to the host
  15. Bacteriophages have been important, both historically and currently, in genetic research
  16. Bacteriophages can be used clinically, as phages can be used to kill bacteria
  17. A lytic reproductive cycle destroys the host cell
  18. Lytic infection follows several steps

a)Attachment (or absorption) is the process by which the phage attaches to the bacterial cell wall

b)Penetration is accomplished by the tail of the virus pushing a hole in the bacterial cell wall and injection of the nucleic material into the host cell

(1)The capsid remains outside of the bacterium

c)In replication, the phage utilizes the host cell’s ribosomes, energy, and enzymes to synthesize raw materials

d)The synthesized macromolecules are assembled into new phages

e)The host cell lyses and releases new phages

  1. The time required for this process is approximately 30 minutes
  1. Temperate viruses can integrate their DNA into the host DNA in a lysogenic cycle
  2. Temperate bacteriophages do not always lyse their host cells
  3. The viral DNA becomes incorporated into the host DNA and is then called a prophage
  4. The prophage then becomes replicated when the host cell replicates
  5. Lysogenic conversion occurs when the host cell (the lysogenic cell) exhibits new or unusual properties

a)The bacterium that causes diphtheria only causes the disease when it contains a certain phage

b)The bacterium that causes botulism is harmless unless it carries a particular prophage

  1. Some viruses infect animal cells
  2. Hundreds of viruses are known that infect animals
  3. Receptor sites are typically species specific and often tissue specific
  4. Viruses may enter animal cells by fusion with the plasma membrane or endocytosis
  5. Animal viruses may contain either DNA or RNA

a)DNA viruses synthesize new nucleic acids and proteins in a manner similar to typical synthesis

b)Some RNA viruses are retroviruses that use an enzyme called reverse transcriptase to transcribe RNA into a DNA intermediate, which is then used to make new viral RNA

(1)HIV and some cancer-causing viruses are retroviruses

  1. A typical human suffers from 2–6 viral infections per year
  2. Examples of human diseases caused by viruses include the common cold, AIDS, Ebola, herpes, measles, hepatitis, and mumps
  3. Other animals are affected by viral diseases such as caninedistemper, feline leukemia, and foot-and-mouth disease
  1. Viruses can infect plant cells
  2. Viruses may be spread to plants by fluid-feeding insects such as aphids, or by infected seeds or asexual propagation
  3. Viruses spread through the plant via the plasmodesmata
  4. Most plant viruses are RNA viruses
  5. Virally infected plants show stunted growth, spots on the leaves, and lower yields
  6. Treatments for viral infections of plants are not well developed
  1. Viroids and prions are smaller than viruses
  2. Viroids contain no proteins nor genes to code for proteins
  3. Prions (proteinaceous infectious particles) are composed of proteins, but contain no nucleic acids
  4. Preusner first described prions in 1982; what was first a controversial finding, is now a well accepted fact
  5. Prions are responsible for transmissible spongiform encephalopathies, including scrapie in sheep and goats, bovine spongiform encephalopathy ("mad cow disease"), and variant Creutzfeldt-Jakob disease in humans (vCJD)
  6. Recently, chronic wasting disease, a disease related to mad cow disease, has spread in deer and elk populations in the U.S.
  1. Prokaryotes are extremely diverse
  2. Prokaryotes are cellular, but compose 2 domains: Archaea and Eubacteria
  3. Prokaryotes are typically one thousandth the volume of small eukaryotic cells
  4. Most prokaryotes are unicells, but some form colonies or filaments
  5. Prokaryotes have several common shapes
  6. Spherical prokaryotes are cocci (singular: coccus)

a)Groups of 2 cocci are diplococci

b)Chains of cocci are streptococci

c)Groups of cocci are staphylococci

  1. Rod-shaped prokaryotes are bacilli (singular: bacillus)

a)Bacilli can occur singly or in chains as well

  1. Short spiral-shaped forms are vibrios
  2. Long spiral-shaped rigid forms are spirilla (singular: spirillum)
  3. Long spiral-shaped forms that are flexible are spirochetes
  1. Prokaryotic cells lack membrane-enclosed organelles
  2. Prokaryotes have ribosomes, but lack all membrane-enclosed organelles such as the Golgi complexes and ER
  3. The plasma membrane may be infolded to increase the surface area for enzymes needed for respiration and photosynthesis
  4. A cell wall generally covers the cell surface
  5. The cell wall of prokaryotes is very strong and resists bursting, as prokaryotes are adapted for living in hypotonic media

a)Prokaryotes do not grow well in hypertonic media; this is why foods can be preserved by immersion in solutions with a high sugar or salt content

b)The prokaryotic cell wall is homoplastic with the cell wall of plants (i.e., not homologous)

  1. In the eubacteria, peptidoglycan is an interlinked organic molecule that interlinks sugars and peptides to form a single molecule that composes the entire cell wall
  2. In 1888, Christian Gram developed the Gram stain

a)Bacteria that stain purple, due to thick peptidoglycan walls, are classified as Gram positive

b)Bacteria that do not retain the purple stain (and remain red in color), due to a thin peptidoglycan wall with a thick outer layer of lipids and carbohydrates, are classified as Gram negative

c)Penicillin interferes with peptidoglycan synthesis and therefore is most effective against Gram-positive bacteria

  1. Some bacteria produce a capsule that may protect against phagocytosis or attach to substrates
  2. Some bacteria have pili, used for attachment or transmission of DNA between bacteria

  1. Many types of prokaryotes are motile
  2. Bacteria may secrete mucus and glide, or move via flagella

a)Prokaryotic flagella are very different from eukaryotic flagella, having a basal body, hook, and filament

  1. Prokaryotes have a single DNA molecule
  2. The DNA of a prokaryotes is located in the cytoplasm, and is not surrounded by a membrane
  3. The DNA of prokaryotes is circular and highly folded

a)Little protein is associated with bacterial DNA

  1. Plasmids are extraneous DNA circles that replicate independently of the primary DNA

a)Plasmids typically have genes that allow resistance to antibiotics

  1. Most prokaryotes reproduce by binary fission
  2. Binary fission is an asexual process
  3. The circular DNA replicates, and then a new cell wall grows between the DNA molecules
  4. Binary fission can occur as often as every 20 minutes under optimal conditions
  5. Prokaryotes can reproduce asexually by budding or fragmentation
  6. Although not considered sexual reproduction, genetic exchange between bacteria occurs via 3 mechanisms:

a)In transformation, fragments of DNA from a damaged cell may be taken up by another cell

b)In transduction, bacteriophages transfer prokaryotic genes

c)In conjugation, genetic material may be transferred via pili

(1)DNA is transferred between different mating strains

(2)Exchange occurs between donor and recipient cells

  1. Some prokaryotes form endospores
  2. Endospores are formed during periods of unfavorable environmental conditions
  3. Endospores allow prokaryotes to resist freezing, drying, or boiling
  4. Endospores are different from reproductive spores of fungi and plants as prokaryotic endospores are not reproductive, and only one is formed per original cell
  5. Metabolic diversity is evident among prokaryotes
  6. About 5000 different compounds are found in a single prokaryotic cell
  7. Prokaryotes are either heterotrophic or autotrophic

a)Most heterotrophs are free-living saprobes (saprotrophs)

b)Other heterotrophs are disease causing or symbiotic

c)Autotrophs are photosynthetic or chemosynthetic

(1)Photosynthetic autotrophs obtain their energy from light

(2)Chemotrophs obtain their energy from inorganic chemicals

  1. Most prokaryotes are aerobic; others are facultative or obligate anaerobes

  1. The 2 domains, Archaea and Eubacteria, are fundamentally distinct
  2. Molecular evidence has shown that the two types of prokaryotes are taxonomically distinct
  3. Prokaryotes may be divided into groups based on their ribosomal RNA (signature sequences)
  4. Woese and colleagues, using sequence analysis of small subunit ribosomal RNA (SSU rRNA), showed that these 2 lineages split early in the history of life
  5. The definitive reference for bacterial classification is Bergey’s Manual of Systematic Bacteriology
  6. The domain Archaea lack peptidoglycan in their cell walls and have unusual biochemistry

a)They have unique isoprene units and ether linkages in their cell membranes that are similar to those in eukaryotes

b)They have no simple RNA polymerases as is found in eubacteria

c)Transcription is not inhibited by the antibiotic rafamycin, like eukaryotes (eubacteria are inhibited by this)

d)Their translation mechanism is very similar to that of eukaryotes

e) Members of this group appear to be more closely related to eukaryotes than other prokaryotes

f)Bergey’s Manual divides them into 2 phyla

(1)Phylum Crenarchaeota is found in hot pools

(2)Phylum Euryarchaeota are found in a number of extreme environments

(3)Phylum Kararchaeota is proposed

  1. The domain Eubacteria (Bacteria) includes the typical bacteria
  2. In this classification scheme, eukaryotes are classified in the domain Eukarya
  1. The domain Archaea (kingdom Archaebacteria) includes methanogens, extreme halophiles, and extreme hermophiles
  2. The Archaea are adapted for extreme conditions that are rare on earth today, but may have been more common on the primitive Earth
  3. Methanogens are obligate anaerobes, found in sewage ponds and swamps and the gastrointestinal tract of some animals
  4. Extreme halophiles live in hypersaline environments
  5. Extreme thermophiles grow in hot, sometimes acidic, environments such as hot springs and deep sea vents
  6. Eubacteria are the most familiar prokaryotes
  7. Eubacteria are of great ecological importance
  8. Some bacteriafix nitrogen
  9. Cyanobacteria (blue-green algae) are phototrophs that may fix nitrogen
  10. Bergey’s Manual divides them into 21 phyla
  11. Some prokaryotes (mostly eubacteria) cause disease
  12. Most bacteria do not cause disease; they are normal inhabitants of the body of their "host"
  13. Pathogenic bacteria can enter the body via food, dust, wounds, or bites
  14. Exotoxins are poisons that are secreted by bacteria or leak out when the bacterium dies

a)Botulism is caused by an exotoxin

b)Exotoxins are typically destroyed by heating

  1. Endotoxins are components of the cell wall of most Gram-positive bacteria and are not destroyed by heat
  2. Koch’s postulates in 1876 demonstrated the process by which one can show that a bacterium truly caused a disease
  1. Eubacteria are used in many commercial processes
  2. Lactic acid bacteria ferment sugar and produce lactic acid, used in the production of yogurt, pickles, and olives
  3. Many antibiotics are derived from bacteria, as well as fungi
  4. Bacteria are used in the production of many vitamins and other commercial products such as citric acid
  5. Bioremediation involves bacterial decontamination of a site
  6. Archae have achieved economic importance by producing enzymes that are added to detergents and use in the food industry to convert corn starch to dextrins

Research and Discussion Topics

  • Discuss the pros and cons of the recent discussion concerning the division of the prokaryotes into 2 groups: the Archaea and the Eubacteria. Should these be 1 kingdom, 2 kingdoms, or larger groups, such as two domains?
  • Investigate the wide variety of commercial processes that involve the use of bacteria. Compile a list of products made using bacteria.
  • Investigate the use of recombinantly produced bacteria in the manufacture of various hormones and drugs.
  • Investigate the contributions by early scientists, such as Ignaz Semmelweiss, Oliver Wendell Homes, and Joseph Lister, in the field of microbiology and hygiene.
  • What is Bacillus turingiensis? What commercial products utilize this bacterium?
  • What is "germ" warfare? What bacteria are utilized? How are they utilized? Relate this to current international events.

Investigate the virus or bacteria that has been in the news most recently. As I write this, SARS appears to be a past issue, having been in the news and scaring many people in the spring and summer of 2002. Right now I am taking care of 2 kids with the flu. I had my flu shot a few months ago, and so far, I’m OK. What is "going around" when you are studying this chapter?