Mycology Quiz: Characteristics and Infections

Course Overview

• The course is titled Microbiology for MD and is offered by Africa Medical College
• The course instructor is Dr. Alem A., PhD in Medical Microbiology
• The course date is December 2024
• The course content covers:
  • Chapter 1: General Microbiology
  • Chapter 2: Systematic Bacteriology
  • Chapter 3: Systematic Virology
  • Chapter 4: Systematic Mycology
  • Chapter 5: System-based microbial infections (Group Seminar)
• The assessment methods include:
  • Tests 1, 2, 3
  • Lab report
  • Attendance
  • Individual presentation
  • Group presentation
  • Final Exam
  • Oral Exam

What is Microbiology?

• Microbiology is the study of living organisms too small to be seen with the naked eye
• These organisms are collectively known as microbes or microorganisms
• Examples include:
  • Bacteria
  • Fungi
  • Protozoa
  • Viruses
  • Prions
  • Archaea
  • Algae
• Many microbes cause serious human diseases

Microbiology Branches

• Medical Microbiology
• Food Microbiology
• Veterinary Microbiology
• Plant Microbiology
• Pharmaceutical Microbiology
• Applied Microbiology
• Industrial Microbiology
• Soil Microbiology

Eukaryotes and Prokaryotes

• Agents of human infectious diseases belong to five major groups (bacteria, fungi, protozoa, helminths, viruses)
• Bacteria, fungi, protozoa, and helminths are cellular
• Viruses are not cellular
• Eukaryotes possess a nucleus, multiple chromosomes, and these components are surrounded by a nuclear membrane
• Eukaryotes use a mitotic apparatus to ensure the equal allocation of chromosomes to progeny cells
• Prokaryotes typically consist of a single circular molecule of loosely organized DNA; they lack a nuclear membrane and mitotic apparatus.
• Eukaryotic cells contain organelles such as mitochondria and lysosomes
• Prokaryotic cells contain no organelles.
• Eukaryotic cells contain larger ribosomes (80S) compared to prokaryotic cells (70S)
• Most prokaryotes possess a rigid external cell wall containing peptidoglycan (a polymer of amino acids and sugars)
• Eukaryotes generally do not contain peptidoglycan.
• Eukaryotic cells have either a flexible cell membrane, or a rigid cell wall with chitin (in the case of fungi).
• A few prokaryotic cells, like Mycoplasma, lack a cell wall
• Most bacteria, protozoa and some, but not all, viruses are motile

Viruses: Structure and Replication

• Viruses consist of an inner core of genetic material (either DNA or RNA)
• Viruses lack cytoplasm and depend on host cells for protein synthesis and energy generation
• Cells replicate by either binary fission (prokaryotes) or mitosis (eukaryotes)
• Viruses assemble, make copies of their nucleic acid and protein, and then reassemble into new viruses
• Viruses replicate within host cells because they lack their own protein-synthesizing and energy-generating systems
• Cells contain both DNA and RNA viruses, while viruses can contain either DNA or RNA but not both

Scientific Nomenclature of Microbes

• Bacteria, fungi, protozoa, and helminths are named according to the binomial Linnaean system
• The system uses genus and species
• Example naming system: Escherichia coli
• Viruses typically have a single name, such as poliovirus or rabies virus.
• Some viruses have two-word names that do not represent genus plus species

Microorganisms

• Microorganisms are small living things too small to see with the naked eye
• Includes bacteria, fungi and protozoa
• Viruses, microscopic but lack cellular structures, are also included

Bacteria

• Bacteria are relatively simple, prokaryotic, unicellular organisms
• They lack a nuclear membrane, mitochondria, Golgi bodies, and endoplasmic reticulum
• They reproduce asexually by dividing into two equal cells (binary fission)
• Bacteria are often classified by size, shape (spherical, rod-shaped, spiral, or filamentous), and arrangement (single, chains, clusters)
• They are found in the environment and in the human body

Fungi

• Fungi have more complex cellular structures than bacteria
• Fungi are eukaryotic
• Fungi contain a nucleus, mitochondria, Golgi bodies, and endoplasmic reticulum
• Fungi can exist as either yeasts (unicellular) or molds (filamentous)
• Many fungi are dimorphic (can exist in both forms)

Parasites

• Parasites are complex eukaryotic microbes, sometimes unicellular, other times multicellular
• They range from tiny protozoa to tapeworms and arthropods
• Parasites can reproduce sexually or asexually

Viruses

• Viruses are the smallest infectious particles, ranging in diameter from 18-600 nanometers
• Viruses contain either DNA or RNA but not both
• Viruses are obligate intracellular parasites, needing a host cell to replicate

Eukaryotes and Prokaryotes - Summary

• Prokaryotic cells (e.g., bacteria) have:
  • No nucleus
  • Usually a single chromosome
  • No membrane-bound organelles
  • 70S Ribosomes
  • Cell walls containing peptidoglycan
• Eukaryotic cells (e.g., fungi, protozoa, and helminths) have:
  • A nucleus
  • More than one chromosome
  • Membrane-bound organelles
  • 80S Ribosomes
  • Cell walls (if present) without peptidoglycan

History of Microbiology

• Robert Hooke (1665): Observed cells in cork using a crude microscope, laying the groundwork for cell theory.
• Antoni van Leeuwenhoek (1673): Observed "animalcules" (microorganisms) using a more powerful microscope, challenging the spontaneous generation theory.
• Francesco Redi (1668): Conducted experiments disproving spontaneous generation for larger organisms using putrifying meat in different environments
• John Needham(1745): Observed microorganisms appeared in sealed vials of gravy after boiling.
• Lazzaro Spallanzani (1765): Repeated Needham's experiment and found no microorganisms in sealed vials that were properly boiled.
• Louis Pasteur (1861): definitively disproved spontaneous generation with his swan-necked flask experiment to demonstrate that microorganisms are present in the air and can contaminate sterile solutions.

The Germ Theory of Disease

• First proposed criteria for proving that microorganisms cause disease (Friedrich Henle, 1840)
• Robert Koch (1843-1910) conducted definitive experiments on anthrax, providing strong evidence for the germ theory and establishing Koch's postulates.
• Koch's postulates are criteria for establishing a causal link between a specific microorganism and a particular infectious disease.

Bacterial Exceptions

• Mycobacteria and mycoplasma are two bacterial exceptions
• Mycobacteria have cell walls containing peptidoglycan surrounded by a wax-like lipid coat of mycolic acid
• Mycoplasmas have no cell wall and incorporate sterols from the host into their membranes.

Bacterial Growth

• Bacteria reproduce by binary fission, resulting in exponential (logarithmic) growth.
• Physical requirements for bacterial growth include temperature, pH, and osmotic pressure.
• Chemical requirements include sources of carbon, nitrogen, sulfur, phosphorus, and oxygen, trace elements, and organic growth factors
• Generation time is the time required for a bacterial population to double in number

Growth Curve

• A typical growth curve has four phases:
  • Lag phase: adaptation to the new environment
  • Log phase: exponential growth
  • Stationary phase: growth rate slows, replication equals death
  • Death phase: nutrients are depleted; cell death exceeds population growth

Aerobic and Anaerobic Growth

• Aerobic bacteria can survive and thrive in the presence of oxygen, using oxygen as electron acceptors

• Anaerobic bacteria cannot survive in the presence of oxygen because they do not possess the defense mechanisms to deal with toxic byproducts of respiration like superoxides or hydrogen peroxide

Cultivation of Bacteria

• The isolation and identification of microorganisms involve techniques to grow bacteria in a culture medium, providing proper environmental conditions
• Culture media are artificial media containing the necessary nutrients for bacterial growth.

Forms of Culture Media

• Solid culture media (e.g., agar plates)
• Semisolid culture media (e.g., for motility tests)
• Liquid culture media (e.g., broths)
• Types of culture media include
  • Basal media
  • Enriched media
  • Selective Media
  • Differential media
  • Transport media

Anti-microbial Sensitivity Testing (AST)

• AST methods determine the susceptibility of bacteria to certain antimicrobials
• Methods include
  • Disk diffusion (Kirby-Bauer method), identifying the susceptibility or resistance of bacteria to different antimicrobials
  • Broth dilution method, determines the minimal inhibitory concentration (MIC) or the lowest drug concentration that inhibits the growth of bacteria.

Molecular Diagnostic Methods

• These methods involve detecting genes that code for resistance to antimicrobials, and involve - Whole-genome sequencing - PCR - MALDI TOF MS

PCR

• Polymerase chain reaction (PCR) is a laboratory technique used to amplify specific DNA sequences, which allows the rapid detection of microorganisms

Real-time PCR

• Real-time PCR is a technique that detects the amplified DNA product as the reaction progresses, which is faster and more accurate compared to conventional PCR

Serologic Diagnosis

• Serologic assays detect or quantify antigens or antibodies in clinical samples

• The tests can evaluate the immune response to infection.

• Quantitative measurements and tests for antigens and antibodies include, but are not limited to

  • Precipitation techniques
    • ELISA (enzyme-linked immunosorbent assay)

• Agglutination

• Complement fixation

• Immunofluorescence

Antimicrobial Susceptibility Testing (AST)

• AST is done to determine the effectiveness of antimicrobials, which allows clinicians to select appropriate and effective treatment strategies for patients.

Microscopy

• Microscopy allows for detection of microorganisms in samples using a microscope
• Sample types include
  • Wet mounts
  • Stained slides (e.g., gram stain, acid-fast stain)

Stains

• Stains are a means to facilitate and improve the visualization of microorganisms inside and outside of cells

  • Gram stain helps in differentiating gram-positive from gram-negative bacteria
  • Acid-fast stain helps distinguish acid-fast organisms (e.g. Mycobacteria) from non-acid-fast organisms

Types of Microbial Pathogens, Toxins and Viral Replication

• Pathogenic bacteria - can cause disease
• Opportunistic pathogens can cause disease under specific conditions, like a weakened immune system
• Virulence factors are certain traits that increase pathogenicity
• A toxin is an active biological molecule, with a toxic effect on host organisms or systems.
  • Exotoxins are proteins produced and secreted by bacteria.
  • Endotoxins are associated with bacterial cell walls.
• Replication of viruses in a host system is a complex series of steps including attachment to the host cell, entry into the host cell, replication of the viral nucleic acid and/or proteins, assembly or packaging of new viral particles, and release of the new viral particles

Typical stages of infection

• Incubation period: the time between exposure to a pathogen and the onset of symptoms.
• Prodromal period: the period when the first nonspecific symptoms of infection (e.g., fever, malaise) appear
• Illness period: overt, characteristic signs and symptoms of infection
• Convalescence period: illness abates, the person returns to a healthy state

Outcomes of Viral infection

1. Death of the cell
2. Fusion of cells or the formation of multinucleated cells
3. Maligned transformation
4. No observable change in cell structure or function

Principles of Infection Prevention

• Principles of infection prevention include sterilization and disinfection
• Sterilization kills all microorganisms, including bacterial spores.
• Disinfection kills many but not all microorganisms. Antisepsis reduces or eliminates microorganisms on living tissue (e.g., skin)

Additional Topics

• Probiotics
• Host-parasite relationships
• Commensalism
• Mutualism
• Parasitism