Chapter 4

• Microbiology


  The study of microorganisms
• Categories of microorganisms

  • Acellular (viruses, viroids, prions)
  • Cellular (bacteria, archaeans, algae, protozoa, fungi)
• Cellular microorganisms

  • Procaryotic (bacteria, archaeans)
  • Eucaryotic (algae, protozoa, fungi)
• Acellular infectious agents

  Viruses, viroids, prions
• Viruses


  • Complete virus particles are called virions
  • Most viruses are from 10 to 300 nm in diameter
  • Viruses infect humans, animals, plants, fungi, protozoa, algae and bacterial cells
  • Some viruses cause specific types of cancer
• Virion


  A complete virus particle
• Capsid


  Protein coat surrounding the virus genome
• Envelope


  Outer layer of some viruses composed of lipids and polysaccharides
• Properties that distinguish viruses from living cells

  • Possess either DNA or RNA (living cells possess both)
  • Unable to replicate on their own
  • Do not divide by binary fission, mitosis or meiosis
  • Lack genes and enzymes for energy production
  • Depend on host cell for protein and nucleic acid production
• Criteria for virus classification

  • Type of genetic material (DNA or RNA)
  • Shape and size of capsid
  • Number of capsomeres
  • Presence or absence of an envelope
  • Type of host it infects
  • Disease it produces
  • Target cell
  • Immunologic/antigenic properties
• Categories of viruses based on nucleic acid

  • Double-stranded DNA
  • Single-stranded RNA
  • Single-stranded DNA
  • Double-stranded RNA
• Bacteriophages


  Viruses that infect bacteria
• Lytic cycle


  Cycle of virulent bacteriophages that ends with destruction of the bacterial cell
• Multiplication of animal viruses

  1. Attachment

  2. Penetration

  3. Uncoating

  4. Biosynthesis

  5. Assembly
• Inclusion bodies


  Remnants or collections of viruses, often seen in infected cells and used as a diagnostic tool
• Latent virus infections


  Viral infections where the virus hides from the host's immune system by entering cells and remaining dormant
• Antibiotics are not effective against viral infections
• Antiviral agents


  Drugs used to treat viral infections, interfere with virus-specific enzymes and virus production
• Oncogenic viruses or oncoviruses


  Viruses that cause cancer
• Human immunodeficiency virus (HIV)


  The cause of acquired immunodeficiency syndrome (AIDS), an enveloped, double-stranded RNA virus
• Viroids


  Short, naked fragments of single-stranded RNA that can interfere with plant cell metabolism
• Prions


  Small infectious proteins that cause fatal neurologic diseases in animals
• Characteristics used to classify and identify bacteria

  • Cell morphology
  • Staining reactions
  • Motility
  • Colony morphology
  • Atmospheric requirements
  • Nutritional requirements
  • Biochemical and metabolic activities
  • Enzymes produced
  • Pathogenicity
  • Genetic composition
• Basic bacterial cell shapes

  • Cocci (round)
  • Bacilli (rods)
  • Curved and spiral-shaped
• Cocci


  Round bacteria, may be seen singly, in pairs, chains, clusters, packets of 4, or packets of 8
• Bacilli


  Rod-shaped bacteria, may be short or long, thick or thin, pointed or blunt
• Coccobacilli


  Extremely short bacilli
• Medically important bacilli

  • Escherichia
  • Klebsiella
  • Proteus
  • Pseudomonas
  • Haemophilus
  • Bacillus
• Curved bacteria


  • Vibrio
  • Campylobacter
  • Helicobacter
• Spiral-shaped bacteria

  • Treponema
  • Borrelia
• Staining procedures

  • Simple stains
  • Structural staining (capsule, spore, flagella)
  • Differential staining (Gram, acid-fast)
• Gram staining


  Divides bacteria into Gram-positive (blue-to-purple) and Gram-negative (pink-to-red)
• Gram-positive


  Bacteria with a thick layer of peptidoglycan in their cell wall
• Gram-negative


  Bacteria with a thin layer of peptidoglycan in their cell wall
• Gram-variable


  Bacteria that are neither consistently Gram-positive nor Gram-negative
• Acid-fast stain


  Stain used to identify Mycobacterium species
• Streptococcus pneumoniae


  • in blood culture
• Clostridium perfringens


  • in a smear from a broth culture
• Clostridium tetani


  • in a smear from a broth culture (note terminal spores on some cells)
• Gram-negative bacilli


  • in a smear from a bacterial colony