Pathogens

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Microorganisms that cause Disease

Bacteria

• Majority of bacteria are non-pathogenic (harmless to humans)
• Some are pathogenic (cause disease)
• Essential to life on Earth
  • Decomposition of organic material and cycling of elements
• In/on the human body, bacteria colonise the gut, mouth, skin, genitals and airways
• Single-celled microscopic (0.1 - 5.0 μm) organisms
• Visible under light microscope

Prokaryotic

• No membrane-bound organelles
• No nucleus
• DNA
• Single circular molecule: bacterial chromosome
• Bacterial cell envelope
• Capsule
• Cell wall
  • Generally comprised of peptidoglycan
    • Sugars and amino acids
• Cell membrane
  • Similar to that of other cells

Types of Bacteria

• Bacteria are classified into groups based on their basic shapes
• Spherical shape: coccus
• Rod shaped: bacillus
• Curved:
  • Spiral: spirilla
  • Comma: vibrio

Cocci - NOTE: NOT NEEDED

• Spherical cells
• Can occur
  • Singly
  • In pairs (diplococci)
  • In clusters (staphylococci)
  • In chains (streptococci)

Bacilli - NOTE: NOT NEEDED

• Rod-shaped cells
• Have flagella for movement
• Can occur
  • Singly (bacillus)
  • In chains (streptobacillus)
  • Oval (coccobacillus)

Curved - NOTE: NOT NEEDED

• Spirilla
  • Thick, rigid spiral
• Vibrio
  • Curved or comma shaped rod
• Spirochaetes
  • Thin, flexible spiral
  • Corkscrew

Cholera

• Vibrio cholera
• Lives in water and human hosts
• Halophilic: salt loving
• Highly motile(not a typo, i thought it was though ;-; ): moves very well using flagella (tail-like structure)
• Causes acute diarrhoeal infection when ingested through contaminated food or water

Bacteria DNA Fishing

• Bacteria use pili to harness DNA from dead bacteria to incorporate into their own DNA
  • Horizontal gene transfer
• Evolutionary advantage such as obtaining antibiotic resistance

Viruses

• Individual virus is called a virion
• Sub-microscopic particle (20-300 nm)
• Visible under electron microscope
• Non-cellular
• Doesn’t have cell membranes, cytoplasm, ribosomes and other cell organelles
• Unable to make proteins or even replicate on their own 
• Instead they must depend on a host cell to synthesis their proteins and replicate
• Can contain either DNA or RNA
• A virion consists of
  • Nucleic acid core (RNA or DNA) surrounded by a protective protein coat (Capsid)
  • Sometimes they have a lipid envelope covering the protein capsid (enveloped virus)

Types of virus

• RNA virus
  • Usually single stranded
  • Can be double stranded
• DNA virus
  • Usually double stranded
  • Can be single stranded

Virus Replication

• As viruses are non-cellular, they don’t reproduce on their own like living organisms, instead they infect a host cell to replicate (make multiple copies of themselves)
• They infect a wide variety of organisms, including eukaryotes and prokaryotes
• Once inside the host cell, they use the cell’s ATP(Adenosine Triphosphate), ribosomes, enzymes and other cellular parts to replicate

Fungi

• Most are multicellular

  • Yeast: uni-cellular fungi

• Heterotrophs

• Cell walls made of chitin (fibrous substance consisting of polysaccharides)

• Cell membrane has phospholipid bilayer

• Eukaryotic

  • Membrane-bound organelles with DNA
  • Membrane-bound organelles

• Microscopic to macroscopic

• Reproduce by forming spores

• Fungal disease: pneumocystis pneumonia caused by Pneumocystis jirovecii

  

Protist

• Large diverse group of microorganisms that aren’t plants, animals or fungi
• Between 60000-200000 different species
• Microscopic to macroscopic
  • 10 µm (unicellular protists) to several meters or more (multicellular seaweeds)
• Unicellular, multicellular or colonial
• Eukaryotic with nuclear membranes and membrane-bound organelles
• Most protists are aquatic organisms
• Nucleus with DNA
• Animal-like protists: Protozoa
  • Single-celled eukaryotes unlike animals that are multi-cellular
  • Like animals, they can move, and are heterotrophs
  • Size: 0.01-0.5 mm
  • Animal-like protists include the flagellates, ciliates, amoeba and apicomplexa

Type of Protazoa	Movement	Example
Flagellates	Long flagela/tails that rotate in a propeller-like fashion	Trypanosoma: African sleeping sickness
Pseudopodia (temporary feet.) Amoebae	Extends projections of cytoplasm out to form feet-like structures that propel the cell forward	Naegleria fowleri, deadly brain infections
Ciliates	Cillia = tiny tail-like projections that extend outward from the cell body and beat back and forth	Paramecium
Apicomplexa	They don’t move, are intracellular parasites. They use special enzymes to penetrate the host’s tissues. Complex life cycle involving transmission between several host species	Plasmodium: spread by mosquitoes as malaria

• Plant-like protists
  • Algae are plant-like protists
  • Large and diverse group
  • Autotrophs
  • Don’t have true stems, roots or leaves unlike plants
  • Most plant-like protists live in oceans, ponds and lakes
  • Can be uni-cellular or multi-cellular
  • Plants are multi-cellular
  • Example: Kelp and seaweed
• Fungi-like protists
  • Share many characteristics with fungi
  • Heterotrophs
  • Reproduce by forming spores
  • Differ from fungi as their cell walls are made of cellulose, fungi’s cell walls are made of chitin
  • 2 major types of fungus-like protists
    • Slime moulds
      • Grow as slimy masses on decaying matter
      • Commonly found on items such as rotting logs
    • Water moulds
      • Live in moist soil and surface water
      • Exist as parasites or feed on decaying organisms